Chapter 15. Expressions

Table of Contents

15.1. Evaluation, Denotation, and Result
15.2. Forms of Expressions
15.3. Type of an Expression
15.4. FP-strict Expressions
15.5. Expressions and Run-Time Checks
15.6. Normal and Abrupt Completion of Evaluation
15.7. Evaluation Order
15.7.1. Evaluate Left-Hand Operand First
15.7.2. Evaluate Operands before Operation
15.7.3. Evaluation Respects Parentheses and Precedence
15.7.4. Argument Lists are Evaluated Left-to-Right
15.7.5. Evaluation Order for Other Expressions
15.8. Primary Expressions
15.8.1. Lexical Literals
15.8.2. Class Literals
15.8.3. this
15.8.4. Qualified this
15.8.5. Parenthesized Expressions
15.9. Class Instance Creation Expressions
15.9.1. Determining the Class being Instantiated
15.9.2. Determining Enclosing Instances
15.9.3. Choosing the Constructor and its Arguments
15.9.4. Run-Time Evaluation of Class Instance Creation Expressions
15.9.5. Anonymous Class Declarations
15.9.5.1. Anonymous Constructors
15.10. Array Creation and Access Expressions
15.10.1. Array Creation Expressions
15.10.2. Run-Time Evaluation of Array Creation Expressions
15.10.3. Array Access Expressions
15.10.4. Run-Time Evaluation of Array Access Expressions
15.11. Field Access Expressions
15.11.1. Field Access Using a Primary
15.11.2. Accessing Superclass Members using super
15.12. Method Invocation Expressions
15.12.1. Compile-Time Step 1: Determine Class or Interface to Search
15.12.2. Compile-Time Step 2: Determine Method Signature
15.12.2.1. Identify Potentially Applicable Methods
15.12.2.2. Phase 1: Identify Matching Arity Methods Applicable by Strict Invocation
15.12.2.3. Phase 2: Identify Matching Arity Methods Applicable by Loose Invocation
15.12.2.4. Phase 3: Identify Methods Applicable by Variable Arity Invocation
15.12.2.5. Choosing the Most Specific Method
15.12.2.6. Method Invocation Type
15.12.3. Compile-Time Step 3: Is the Chosen Method Appropriate?
15.12.4. Run-Time Evaluation of Method Invocation
15.12.4.1. Compute Target Reference (If Necessary)
15.12.4.2. Evaluate Arguments
15.12.4.3. Check Accessibility of Type and Method
15.12.4.4. Locate Method to Invoke
15.12.4.5. Create Frame, Synchronize, Transfer Control
15.13. Method Reference Expressions
15.13.1. Compile-Time Declaration of a Method Reference
15.13.2. Type of a Method Reference
15.13.3. Run-time Evaluation of Method References
15.14. Postfix Expressions
15.14.1. Expression Names
15.14.2. Postfix Increment Operator ++
15.14.3. Postfix Decrement Operator --
15.15. Unary Operators
15.15.1. Prefix Increment Operator ++
15.15.2. Prefix Decrement Operator --
15.15.3. Unary Plus Operator +
15.15.4. Unary Minus Operator -
15.15.5. Bitwise Complement Operator ~
15.15.6. Logical Complement Operator !
15.16. Cast Expressions
15.17. Multiplicative Operators
15.17.1. Multiplication Operator *
15.17.2. Division Operator /
15.17.3. Remainder Operator %
15.18. Additive Operators
15.18.1. String Concatenation Operator +
15.18.2. Additive Operators (+ and -) for Numeric Types
15.19. Shift Operators
15.20. Relational Operators
15.20.1. Numerical Comparison Operators <, <=, >, and >=
15.20.2. Type Comparison Operator instanceof
15.21. Equality Operators
15.21.1. Numerical Equality Operators == and !=
15.21.2. Boolean Equality Operators == and !=
15.21.3. Reference Equality Operators == and !=
15.22. Bitwise and Logical Operators
15.22.1. Integer Bitwise Operators &, ^, and |
15.22.2. Boolean Logical Operators &, ^, and |
15.23. Conditional-And Operator &&
15.24. Conditional-Or Operator ||
15.25. Conditional Operator ? :
15.25.1. Boolean Conditional Expressions
15.25.2. Numeric Conditional Expressions
15.25.3. Reference Conditional Expressions
15.26. Assignment Operators
15.26.1. Simple Assignment Operator =
15.26.2. Compound Assignment Operators
15.27. Lambda Expressions
15.27.1. Lambda Parameters
15.27.2. Lambda Body
15.27.3. Type of a Lambda Expression
15.27.4. Run-time Evaluation of Lambda Expressions
15.28. Constant Expressions

Much of the work in a program is done by evaluating expressions, either for their side effects, such as assignments to variables, or for their values, which can be used as arguments or operands in larger expressions, or to affect the execution sequence in statements, or both.

This chapter specifies the meanings of expressions and the rules for their evaluation.

15.1. Evaluation, Denotation, and Result

When an expression in a program is evaluated (executed), the result denotes one of three things:

  • A variable (§4.12) (in C, this would be called an lvalue)

  • A value (§4.2, §4.3)

  • Nothing (the expression is said to be void)

If an expression denotes a variable, and a value is required for use in further evaluation, then the value of that variable is used. In this context, if the expression denotes a variable or a value, we may speak simply of the value of the expression.

Value set conversion (§5.1.13) is applied to the result of every expression that produces a value, including when the value of a variable of type float or double is used.

An expression denotes nothing if and only if it is a method invocation (§15.12) that invokes a method that does not return a value, that is, a method declared void (§8.4). Such an expression can be used only as an expression statement (§14.8), because every other context in which an expression can appear requires the expression to denote something. An expression statement that is a method invocation may also invoke a method that produces a result; in this case the value returned by the method is quietly discarded.

Evaluation of an expression can produce side effects, because expressions may contain embedded assignments, increment operators, decrement operators, and method invocations.

An expression occurs in either:

  • The declaration of some (class or interface) type that is being declared: in a field initializer, in a static initializer, in an instance initializer, in a constructor declaration, in a method declaration, or in an annotation.

  • An annotation on a package declaration or on a top level type declaration.

15.2. Forms of Expressions

Expressions can be broadly categorized into one of the following syntactic forms:

Precedence among operators is managed by a hierarchy of grammar productions. The lowest precedence operator is the arrow of a lambda expression (->), followed by the assignment operators. Thus, all expressions are syntactically included in the LambdaExpression and AssignmentExpression nonterminals:

When some expressions appear in certain contexts, they are considered poly expressions. The following forms of expressions may be poly expressions:

  • Parenthesized expressions (§15.8.5)

  • Class instance creation expressions (§15.9)

  • Method invocation expressions (§15.12)

  • Method reference expressions (§15.13)

  • Conditional expressions (§15.25)

  • Lambda expressions (§15.27)

The rules determining whether an expression of one of these forms is a poly expression are given in the individual sections that specify these forms of expressions.

Expressions that are not poly expressions are standalone expressions. Standalone expressions are expressions of the forms above when determined not to be poly expressions, as well as all expressions of all other forms. Expressions of all other forms are said to have a standalone form.

Some expressions have a value that can be determined at compile time. These are constant expressions (§15.28).

15.3. Type of an Expression

If an expression denotes a variable or a value, then the expression has a type known at compile time. The type of a standalone expression can be determined entirely from the contents of the expression; in contrast, the type of a poly expression may be influenced by the expression's target type (§5 (Conversions and Contexts)). The rules for determining the type of an expression are explained separately below for each kind of expression.

The value of an expression is assignment compatible (§5.2) with the type of the expression, unless heap pollution occurs (§4.12.2).

Likewise, the value stored in a variable is always compatible with the type of the variable, unless heap pollution occurs.

In other words, the value of an expression whose type is T is always suitable for assignment to a variable of type T.

Note that an expression whose type is a class type F that is declared final is guaranteed to have a value that is either a null reference or an object whose class is F itself, because final types have no subclasses.

15.4. FP-strict Expressions

If the type of an expression is float or double, then there is a question as to what value set (§4.2.3) the value of the expression is drawn from. This is governed by the rules of value set conversion (§5.1.13); these rules in turn depend on whether or not the expression is FP-strict.

Every constant expression (§15.28) is FP-strict.

If an expression is not a constant expression, then consider all the class declarations, interface declarations, and method declarations that contain the expression. If any such declaration bears the strictfp modifier (§8.1.1.3, §8.4.3.5, §9.1.1.2), then the expression is FP-strict.

If a class, interface, or method, X, is declared strictfp, then X and any class, interface, method, constructor, instance initializer, static initializer, or variable initializer within X is said to be FP-strict.

Note that an annotation's element value (§9.7) is always FP-strict, because it is always a constant expression.

It follows that an expression is not FP-strict if and only if it is not a constant expression and it does not appear within any declaration that has the strictfp modifier.

Within an FP-strict expression, all intermediate values must be elements of the float value set or the double value set, implying that the results of all FP-strict expressions must be those predicted by IEEE 754 arithmetic on operands represented using single and double formats.

Within an expression that is not FP-strict, some leeway is granted for an implementation to use an extended exponent range to represent intermediate results; the net effect, roughly speaking, is that a calculation might produce "the correct answer" in situations where exclusive use of the float value set or double value set might result in overflow or underflow.

15.5. Expressions and Run-Time Checks

If the type of an expression is a primitive type, then the value of the expression is of that same primitive type.

If the type of an expression is a reference type, then the class of the referenced object, or even whether the value is a reference to an object rather than null, is not necessarily known at compile time. There are a few places in the Java programming language where the actual class of a referenced object affects program execution in a manner that cannot be deduced from the type of the expression. They are as follows:

  • Method invocation (§15.12). The particular method used for an invocation o.m(...) is chosen based on the methods that are part of the class or interface that is the type of o. For instance methods, the class of the object referenced by the run-time value of o participates because a subclass may override a specific method already declared in a parent class so that this overriding method is invoked. (The overriding method may or may not choose to further invoke the original overridden m method.)

  • The instanceof operator (§15.20.2). An expression whose type is a reference type may be tested using instanceof to find out whether the class of the object referenced by the run-time value of the expression is assignment compatible (§5.2) with some other reference type.

  • Casting (§5.5, §15.16). The class of the object referenced by the run-time value of the operand expression might not be compatible with the type specified by the cast. For reference types, this may require a run-time check that throws an exception if the class of the referenced object, as determined at run time, is not assignment compatible (§5.2) with the target type.

  • Assignment to an array component of reference type (§10.5, §15.13, §15.26.1). The type-checking rules allow the array type S[] to be treated as a subtype of T[] if S is a subtype of T, but this requires a run-time check for assignment to an array component, similar to the check performed for a cast.

  • Exception handling (§14.20). An exception is caught by a catch clause only if the class of the thrown exception object is an instanceof the type of the formal parameter of the catch clause.

Situations where the class of an object is not statically known may lead to run-time type errors.

In addition, there are situations where the statically known type may not be accurate at run time. Such situations can arise in a program that gives rise to compile-time unchecked warnings. Such warnings are given in response to operations that cannot be statically guaranteed to be safe, and cannot immediately be subjected to dynamic checking because they involve non-reifiable types (§4.7). As a result, dynamic checks later in the course of program execution may detect inconsistencies and result in run-time type errors.

A run-time type error can occur only in these situations:

  • In a cast, when the actual class of the object referenced by the value of the operand expression is not compatible with the target type specified by the cast operator (§5.5, §15.16); in this case a ClassCastException is thrown.

  • In an automatically generated cast introduced to ensure the validity of an operation on a non-reifiable type (§4.7).

  • In an assignment to an array component of reference type, when the actual class of the object referenced by the value to be assigned is not compatible with the actual run-time component type of the array (§10.5, §15.13, §15.26.1); in this case an ArrayStoreException is thrown.

  • When an exception is not caught by any catch clause of a try statement (§14.20); in this case the thread of control that encountered the exception first attempts to invoke an uncaught exception handler (§11.3) and then terminates.

15.6. Normal and Abrupt Completion of Evaluation

Every expression has a normal mode of evaluation in which certain computational steps are carried out. The following sections describe the normal mode of evaluation for each kind of expression.

If all the steps are carried out without an exception being thrown, the expression is said to complete normally.

If, however, evaluation of an expression throws an exception, then the expression is said to complete abruptly. An abrupt completion always has an associated reason, which is always a throw with a given value.

Run-time exceptions are thrown by the predefined operators as follows:

  • A class instance creation expression (§15.9.4), array creation expression (§15.10.2), method reference expression (§15.13.3), array initializer expression (§10.6), string concatenation operator expression (§15.18.1), or lambda expression (§15.27.4) throws an OutOfMemoryError if there is insufficient memory available.

  • An array creation expression (§15.10.2) throws a NegativeArraySizeException if the value of any dimension expression is less than zero.

  • An array access expression (§15.10.4) throws a NullPointerException if the value of the array reference expression is null.

  • An array access expression (§15.10.4) throws an ArrayIndexOutOfBoundsException if the value of the array index expression is negative or greater than or equal to the length of the array.

  • A field access expression (§15.11) throws a NullPointerException if the value of the object reference expression is null.

  • A method invocation expression (§15.12) that invokes an instance method throws a NullPointerException if the target reference is null.

  • A cast expression (§15.16) throws a ClassCastException if a cast is found to be impermissible at run time.

  • An integer division (§15.17.2) or integer remainder (§15.17.3) operator throws an ArithmeticException if the value of the right-hand operand expression is zero.

  • An assignment to an array component of reference type (§15.26.1), a method invocation expression (§15.12), or a prefix or postfix increment (§15.14.2, §15.15.1) or decrement operator (§15.14.3, §15.15.2) may all throw an OutOfMemoryError as a result of boxing conversion (§5.1.7).

  • An assignment to an array component of reference type (§15.26.1) throws an ArrayStoreException when the value to be assigned is not compatible with the component type of the array (§10.5).

A method invocation expression can also result in an exception being thrown if an exception occurs that causes execution of the method body to complete abruptly.

A class instance creation expression can also result in an exception being thrown if an exception occurs that causes execution of the constructor to complete abruptly.

Various linkage and virtual machine errors may also occur during the evaluation of an expression. By their nature, such errors are difficult to predict and difficult to handle.

If an exception occurs, then evaluation of one or more expressions may be terminated before all steps of their normal mode of evaluation are complete; such expressions are said to complete abruptly.

If evaluation of an expression requires evaluation of a subexpression, then abrupt completion of the subexpression always causes the immediate abrupt completion of the expression itself, with the same reason, and all succeeding steps in the normal mode of evaluation are not performed.

The terms "complete normally" and "complete abruptly" are also applied to the execution of statements (§14.1). A statement may complete abruptly for a variety of reasons, not just because an exception is thrown.

15.7. Evaluation Order

The Java programming language guarantees that the operands of operators appear to be evaluated in a specific evaluation order, namely, from left to right.

It is recommended that code not rely crucially on this specification. Code is usually clearer when each expression contains at most one side effect, as its outermost operation, and when code does not depend on exactly which exception arises as a consequence of the left-to-right evaluation of expressions.

15.7.1. Evaluate Left-Hand Operand First

The left-hand operand of a binary operator appears to be fully evaluated before any part of the right-hand operand is evaluated.

If the operator is a compound-assignment operator (§15.26.2), then evaluation of the left-hand operand includes both remembering the variable that the left-hand operand denotes and fetching and saving that variable's value for use in the implied binary operation.

If evaluation of the left-hand operand of a binary operator completes abruptly, no part of the right-hand operand appears to have been evaluated.

Example 15.7.1-1. Left-Hand Operand Is Evaluated First

In the following program, the * operator has a left-hand operand that contains an assignment to a variable and a right-hand operand that contains a reference to the same variable. The value produced by the reference will reflect the fact that the assignment occurred first.

class Test1 {
    public static void main(String[] args) {
        int i = 2;
        int j = (i=3) * i;
        System.out.println(j);
    }
}

This program produces the output:

9

It is not permitted for evaluation of the * operator to produce 6 instead of 9.


Example 15.7.1-2. Implicit Left-Hand Operand In Operator Of Compound Assigment

In the following program, the two assignment statements both fetch and remember the value of the left-hand operand, which is 9, before the right-hand operand of the addition operator is evaluated, at which point the variable is set to 3.

class Test2 {
    public static void main(String[] args) {
        int a = 9;
        a += (a = 3);  // first example
        System.out.println(a);
        int b = 9;
        b = b + (b = 3);  // second example
        System.out.println(b);
    }
}

This program produces the output:

12
12

It is not permitted for either assignment (compound for a, simple for b) to produce the result 6.

See also the example in §15.26.2.


Example 15.7.1-3. Abrupt Completion of Evaluation of the Left-Hand Operand

class Test3 {
    public static void main(String[] args) {
        int j = 1;
        try {
            int i = forgetIt() / (j = 2);
        } catch (Exception e) {
            System.out.println(e);
            System.out.println("Now j = " + j);
        }
    }
    static int forgetIt() throws Exception {
        throw new Exception("I'm outta here!");
    }
}

This program produces the output:

java.lang.Exception: I'm outta here!
Now j = 1

That is, the left-hand operand forgetIt() of the operator / throws an exception before the right-hand operand is evaluated and its embedded assignment of 2 to j occurs.


15.7.2. Evaluate Operands before Operation

The Java programming language guarantees that every operand of an operator (except the conditional operators &&, ||, and ? :) appears to be fully evaluated before any part of the operation itself is performed.

If the binary operator is an integer division / (§15.17.2) or integer remainder % (§15.17.3), then its execution may raise an ArithmeticException, but this exception is thrown only after both operands of the binary operator have been evaluated and only if these evaluations completed normally.

Example 15.7.2-1. Evaluation of Operands Before Operation

class Test {
    public static void main(String[] args) {
        int divisor = 0;
        try {
            int i = 1 / (divisor * loseBig());
        } catch (Exception e) {
            System.out.println(e);
        }
    }
    static int loseBig() throws Exception {
        throw new Exception("Shuffle off to Buffalo!");
    }
}

This program produces the output:

java.lang.Exception: Shuffle off to Buffalo!

and not:

java.lang.ArithmeticException: / by zero

since no part of the division operation, including signaling of a divide-by-zero exception, may appear to occur before the invocation of loseBig completes, even though the implementation may be able to detect or infer that the division operation would certainly result in a divide-by-zero exception.


15.7.3. Evaluation Respects Parentheses and Precedence

The Java programming language respects the order of evaluation indicated explicitly by parentheses and implicitly by operator precedence.

An implementation of the Java programming language may not take advantage of algebraic identities such as the associative law to rewrite expressions into a more convenient computational order unless it can be proven that the replacement expression is equivalent in value and in its observable side effects, even in the presence of multiple threads of execution (using the thread execution model in §17 (Threads and Locks)), for all possible computational values that might be involved.

In the case of floating-point calculations, this rule applies also for infinity and not-a-number (NaN) values.

For example, !(x<y) may not be rewritten as x>=y, because these expressions have different values if either x or y is NaN or both are NaN.

Specifically, floating-point calculations that appear to be mathematically associative are unlikely to be computationally associative. Such computations must not be naively reordered.

For example, it is not correct for a Java compiler to rewrite 4.0*x*0.5 as 2.0*x; while roundoff happens not to be an issue here, there are large values of x for which the first expression produces infinity (because of overflow) but the second expression produces a finite result.

So, for example, the test program:

strictfp class Test {
    public static void main(String[] args) {
        double d = 8e+307;
        System.out.println(4.0 * d * 0.5);
        System.out.println(2.0 * d);
    }
}

prints:

Infinity
1.6e+308

because the first expression overflows and the second does not.

In contrast, integer addition and multiplication are provably associative in the Java programming language.

For example a+b+c, where a, b, and c are local variables (this simplifying assumption avoids issues involving multiple threads and volatile variables), will always produce the same answer whether evaluated as (a+b)+c or a+(b+c); if the expression b+c occurs nearby in the code, a smart Java compiler may be able to use this common subexpression.

15.7.4. Argument Lists are Evaluated Left-to-Right

In a method or constructor invocation or class instance creation expression, argument expressions may appear within the parentheses, separated by commas. Each argument expression appears to be fully evaluated before any part of any argument expression to its right.

If evaluation of an argument expression completes abruptly, no part of any argument expression to its right appears to have been evaluated.

Example 15.7.4-1. Evaluation Order At Method Invocation

class Test1 {
    public static void main(String[] args) {
        String s = "going, ";
        print3(s, s, s = "gone");
    }
    static void print3(String a, String b, String c) {
        System.out.println(a + b + c);
    }
}

This program produces the output:

going, going, gone

because the assignment of the string "gone" to s occurs after the first two arguments to print3 have been evaluated.


Example 15.7.4-2. Abrupt Completion of Argument Expression

class Test2 {
    static int id;
    public static void main(String[] args) {
        try {
            test(id = 1, oops(), id = 3);
        } catch (Exception e) {
            System.out.println(e + ", id=" + id);
        }
    }
    static int test(int a, int b, int c) {
        return a + b + c;
    }
    static int oops() throws Exception {
        throw new Exception("oops");
    }
}

This program produces the output:

java.lang.Exception: oops, id=1

because the assignment of 3 to id is not executed.


15.7.5. Evaluation Order for Other Expressions

The order of evaluation for some expressions is not completely covered by these general rules, because these expressions may raise exceptional conditions at times that must be specified. See the detailed explanations of evaluation order for the following kinds of expressions:

15.8. Primary Expressions

Primary expressions include most of the simplest kinds of expressions, from which all others are constructed: literals, object creations, field accesses, method invocations, method references, and array accesses. A parenthesized expression is also treated syntactically as a primary expression.

This part of the grammar of the Java programming language is unusual, in two ways. First, one might expect simple names, such as names of local variables and method parameters, to be primary expressions. For technical reasons, names are grouped together with primary expressions a little later when postfix expressions are introduced (§15.14).

The technical reasons have to do with allowing left-to-right parsing of Java programs with only one-token lookahead. Consider the expressions (z[3]) and (z[]). The first is a parenthesized array access (§15.10.3) and the second is the start of a cast (§15.16). At the point that the look-ahead symbol is [, a left-to-right parse will have reduced the z to the nonterminal Name. In the context of a cast we prefer not to have to reduce the name to a Primary, but if Name were one of the alternatives for Primary, then we could not tell whether to do the reduction (that is, we could not determine whether the current situation would turn out to be a parenthesized array access or a cast) without looking ahead two tokens, to the token following the [. The grammar presented here avoids the problem by keeping Name and Primary separate and allowing either in certain other syntax rules (those for ClassInstanceCreationExpression, MethodInvocation, ArrayAccess, and PostfixExpression, but not for FieldAccess because this uses an identifier directly). This strategy effectively defers the question of whether a Name should be treated as a Primary until more context can be examined.

The second unusual feature avoids a potential grammatical ambiguity in the expression "new int[3][3]" which in Java always means a single creation of a multidimensional array, but which, without appropriate grammatical finesse, might also be interpreted as meaning the same as "(new int[3])[3]".

This ambiguity is eliminated by splitting the expected definition of Primary into Primary and PrimaryNoNewArray. (This may be compared to the splitting of Statement into Statement and StatementNoShortIf (§14.5) to avoid the "dangling else" problem.)

15.8.1. Lexical Literals

A literal (§3.10) denotes a fixed, unchanging value.

The following production from §3.10 is shown here for convenience:

The type of a literal is determined as follows:

  • The type of an integer literal (§3.10.1) that ends with L or l is long (§4.2.1).

    The type of any other integer literal is int (§4.2.1).

  • The type of a floating-point literal (§3.10.2) that ends with F or f is float and its value must be an element of the float value set (§4.2.3).

    The type of any other floating-point literal is double and its value must be an element of the double value set (§4.2.3).

  • The type of a boolean literal (§3.10.3) is boolean (§4.2.5).

  • The type of a character literal (§3.10.4) is char (§4.2.1).

  • The type of a string literal (§3.10.5) is String (§4.3.3).

  • The type of the null literal null (§3.10.7) is the null type (§4.1); its value is the null reference.

Evaluation of a lexical literal always completes normally.

15.8.2. Class Literals

A class literal is an expression consisting of the name of a class, interface, array, or primitive type, or the pseudo-type void, followed by a '.' and the token class.

The type of C.class, where C is the name of a class, interface, or array type (§4.3), is Class<C>.

The type of p.class, where p is the name of a primitive type (§4.2), is Class<B>, where B is the type of an expression of type p after boxing conversion (§5.1.7).

The type of void.class (§8.4.5) is Class<Void>.

It is a compile-time error if the named type is a type variable (§4.4) or a parameterized type (§4.5) or an array whose element type is a type variable or parameterized type.

It is a compile-time error if the named type does not denote a type that is accessible (§6.6) and in scope (§6.3) at the point where the class literal appears.

A class literal evaluates to the Class object for the named type (or for void) as defined by the defining class loader (§12.2) of the class of the current instance.

15.8.3. this

The keyword this may be used only in the body of an instance method or default method, or in the body of a constructor of a class, or in an instance initializer of a class, or in the initializer of an instance variable of a class. If it appears anywhere else, a compile-time error occurs.

The keyword this may be used in a lambda expression only if it is allowed in the context in which the lambda expression appears. Otherwise, a compile-time error occurs.

When used as a primary expression, the keyword this denotes a value that is a reference to the object for which the instance method or default method was invoked (§15.12), or to the object being constructed. The value denoted by this in a lambda body is the same as the value denoted by this in the surrounding context.

The keyword this is also used in explicit constructor invocation statements (§8.8.7.1).

The type of this is the class or interface type T within which the keyword this occurs.

Default methods provide the unique ability to access this inside an interface. (All other interface methods are either abstract or static, so provide no access to this.) As a result, it is possible for this to have an interface type.

At run time, the class of the actual object referred to may be T, if T is a class type, or a class that is a subtype of T.

Example 15.8.3-1. The this Expression

class IntVector {
    int[] v;
    boolean equals(IntVector other) {
        if (this == other)
            return true;
        if (v.length != other.v.length)
            return false;
        for (int i = 0; i < v.length; i++) {
            if (v[i] != other.v[i]) return false;
        }
        return true;
    }
}

Here, the class IntVector implements a method equals, which compares two vectors. If the other vector is the same vector object as the one for which the equals method was invoked, then the check can skip the length and value comparisons. The equals method implements this check by comparing the reference to the other object to this.


15.8.4. Qualified this

Any lexically enclosing instance (§8.1.3) can be referred to by explicitly qualifying the keyword this.

Let T be the type denoted by TypeName. Let n be an integer such that T is the n'th lexically enclosing type declaration of the class or interface in which the qualified this expression appears.

The value of an expression of the form TypeName.this is the n'th lexically enclosing instance of this.

The type of the expression is T.

It is a compile-time error if the expression occurs in a class or interface which is not an inner class of class T or T itself.

15.8.5. Parenthesized Expressions

A parenthesized expression is a primary expression whose type is the type of the contained expression and whose value at run time is the value of the contained expression. If the contained expression denotes a variable then the parenthesized expression also denotes that variable.

The use of parentheses affects only the order of evaluation, except for a corner case whereby (-2147483648) and (-9223372036854775808L) are legal but -(2147483648) and -(9223372036854775808L) are illegal.

This is because the decimal literals 2147483648 and 9223372036854775808L are allowed only as an operand of the unary minus operator (§3.10.1).

In particular, the presence or absence of parentheses around an expression does not (except for the case noted above) affect in any way:

  • the choice of value set (§4.2.3) for the value of an expression of type float or double.

  • whether a variable is definitely assigned, definitely assigned when true, definitely assigned when false, definitely unassigned, definitely unassigned when true, or definitely unassigned when false (§16 (Definite Assignment)).

If a parenthesized expression appears in a context of a particular kind with target type T (§5 (Conversions and Contexts)), its contained expression similarly appears in a context of the same kind with target type T.

If the contained expression is a poly expression (§15.2), the parenthesized expression is also a poly expression. Otherwise, it is a standalone expression.

15.9. Class Instance Creation Expressions

A class instance creation expression is used to create new objects that are instances of classes.

The following production from §15.12 is shown here for convenience:

ArgumentList:

A class instance creation expression specifies a class to be instantiated, possibly followed by type arguments (§4.5.1) or a diamond (<>) if the class being instantiated is generic (§8.1.2), followed by (a possibly empty) list of actual value arguments to the constructor.

If the type argument list to the class is empty — the diamond form <> — the type arguments of the class are inferred. It is legal, though strongly discouraged as a matter of style, to have white space between the "<" and ">" of a diamond.

If the constructor is generic (§8.8.4), the type arguments to the constructor may similarly either be inferred or passed explicitly. If passed explicitly, the type arguments to the constructor immediately follow the keyword new.

It is a compile-time error if a class instance creation expression provides type arguments to a constructor but uses the diamond form for type arguments to the class.

This rule is introduced because inference of a generic class's type arguments may influence the constraints on a generic constructor's type arguments.

If TypeArguments is present between new and Identifier, or between Identifier and (, then it is a compile-time error if any of the type arguments are wildcards (§4.5.1).

The exception types that a class instance creation expression can throw are specified in §11.2.1.

Class instance creation expressions have two forms:

  • Unqualified class instance creation expressions begin with the keyword new.

    An unqualified class instance creation expression may be used to create an instance of a class, regardless of whether the class is a top level (§7.6), member (§8.5, §9.5), local (§14.3), or anonymous class (§15.9.5).

  • Qualified class instance creation expressions begin with a Primary expression or an ExpressionName.

    A qualified class instance creation expression enables the creation of instances of inner member classes and their anonymous subclasses.

Both unqualified and qualified class instance creation expressions may optionally end with a class body. Such a class instance creation expression declares an anonymous class (§15.9.5) and creates an instance of it.

A class instance creation expression is a poly expression (§15.2) if it uses the diamond form for type arguments to the class, and it appears in an assignment context or an invocation context (§5.2, §5.3). Otherwise, it is a standalone expression.

We say that a class is instantiated when an instance of the class is created by a class instance creation expression. Class instantiation involves determining the class to be instantiated (§15.9.1), the enclosing instances (if any) of the newly created instance (§15.9.2), and the constructor to be invoked to create the new instance (§15.9.3).

15.9.1. Determining the Class being Instantiated

If the class instance creation expression ends in a class body, then the class being instantiated is an anonymous class. Then:

  • If the class instance creation expression is unqualified:

    The Identifier after the new token must denote a class that is accessible, non-final, and not an enum type; or denote an interface that is accessible. Otherwise a compile-time error occurs.

    If TypeArgumentsOrDiamond is present after the Identifier, and denotes <>, then a compile-time error occurs

    Let T be the type denoted by Identifier [TypeArgumentsOrDiamond] after the new token. If TypeArguments is present after the Identifier, then T must denote a well-formed parameterized type (§4.5), or a compile-time error occurs.

    If T denotes a class, then an anonymous direct subclass of T is declared. If T denotes an interface, then an anonymous direct subclass of Object that implements T is declared. In either case, the body of the subclass is the ClassBody given in the class instance creation expression.

    The class being instantiated is the anonymous subclass.

  • If the class instance creation expression is qualified:

    The Identifier after the new token must unambiguously denote an inner class that is accessible, non-final, not an enum type, and a member of the compile-time type of the Primary expression or the ExpressionName. Otherwise, a compile-time error occurs.

    If TypeArgumentsOrDiamond is present after the Identifier, and denotes <>, then a compile-time error occurs.

    Let T be the type denoted by Identifier [TypeArgumentsOrDiamond] after the new token. If TypeArguments is present after the Identifier, then T must denote a well-formed parameterized type, or a compile-time error occurs.

    An anonymous direct subclass of T is declared. The body of the subclass is the ClassBody given in the class instance creation expression.

    The class being instantiated is the anonymous subclass.

If a class instance creation expression does not declare an anonymous class, then:

  • If the class instance creation expression is unqualified:

    The Identifier after the new token must denote a class that is accessible, non-abstract, and not an enum type. Otherwise, a compile-time error occurs.

    It TypeArgumentsOrDiamond is present after the Identifier, and denotes <>, and the class denoted by Identifier is not generic, then a compile-time error occurs.

    Let T be the type denoted by Identifier [TypeArgumentsOrDiamond] after the new token. If TypeArguments is present after the Identifier, then T must denote a well-formed parameterized type, or a compile-time error occurs.

    The class being instantiated is the class denoted by Identifier.

  • If the class instance creation expression is qualified:

    The Identifier after the new token must unambiguously denote an inner class that is accessible, non-abstract, not an enum type, and a member of the compile-time type of the Primary expression or the ExpressionName.

    If TypeArgumentsOrDiamond is present after the Identifier, and denotes <>, and the class denoted by Identifier is not generic, then a compile-time error occurs.

    Let T be the type denoted by Identifier [TypeArgumentsOrDiamond] after the new token. If TypeArguments is present after the Identifier, then T must denote a well-formed parameterized type, or a compile-time error occurs.

    The class being instantiated is the class denoted by Identifier.

15.9.2. Determining Enclosing Instances

Let C be the class being instantiated, and let i be the instance being created. If C is an inner class, then i may have an immediately enclosing instance (§8.1.3), determined as follows:

  • If C is an anonymous class, then:

    • If the class instance creation expression occurs in a static context, then i has no immediately enclosing instance.

    • Otherwise, the immediately enclosing instance of i is this.

  • If C is a local class, then:

    • If C occurs in a static context, then i has no immediately enclosing instance.

    • Otherwise, if the class instance creation expression occurs in a static context, then a compile-time error occurs.

    • Otherwise, let O be the immediately enclosing class of C. Let n be an integer such that O is the n'th lexically enclosing type declaration of the class in which the class instance creation expression appears.

      The immediately enclosing instance of i is the n'th lexically enclosing instance of this.

  • If C is an inner member class, then:

    • If the class instance creation expression is unqualified, then:

      • If the class instance creation expression occurs in a static context, then a compile-time error occurs.

      • Otherwise, if C is a member of a class enclosing the class in which the class instance creation expression appears, then let O be the immediately enclosing class of which C is a member. Let n be an integer such that O is the n'th lexically enclosing type declaration of the class in which the class instance creation expression appears.

        The immediately enclosing instance of i is the n'th lexically enclosing instance of this.

      • Otherwise, a compile-time error occurs.

    • If the class instance creation expression is qualified, then the immediately enclosing instance of i is the object that is the value of the Primary expression or the ExpressionName.

If C is an anonymous class, and its direct superclass S is an inner class, then i may have an immediately enclosing instance with respect to S, determined as follows:

  • If S is a local class, then:

    • If S occurs in a static context, then i has no immediately enclosing instance with respect to S.

    • Otherwise, if the class instance creation expression occurs in a static context, then a compile-time error occurs.

    • Otherwise, let O be the immediately enclosing class of S. Let n be an integer such that O is the n'th lexically enclosing type declaration of the class in which the class instance creation expression appears.

      The immediately enclosing instance of i with respect to S is the n'th lexically enclosing instance of this.

  • If S is an inner member class, then:

    • If the class instance creation expression is unqualified, then:

      • If the class instance creation expression occurs in a static context, then a compile-time error occurs.

      • Otherwise, if S is a member of a class enclosing the class in which the class instance creation expression appears, then let O be the immediately enclosing class of which S is a member. Let n be an integer such that O is the n'th lexically enclosing type declaration of the class in which the class instance creation expression appears.

        The immediately enclosing instance of i with respect to S is the n'th lexically enclosing instance of this.

      • Otherwise, a compile-time error occurs.

    • If the class instance creation expression is qualified, then the immediately enclosing instance of i with respect to S is the object that is the value of the Primary expression or the ExpressionName.

15.9.3. Choosing the Constructor and its Arguments

Let C be the class being instantiated. To create an instance of C, i, a constructor of C is chosen at compile time by the following rules:

First, the actual arguments to the constructor invocation are determined:

  • If C is an anonymous class with direct superclass S, then:

    • If S is not an inner class, or if S is a local class that occurs in a static context, then the arguments to the constructor are the arguments in the argument list of the class instance creation expression, if any, in the order they appear in the expression.

    • Otherwise, the first argument to the constructor is the immediately enclosing instance of i with respect to S (§15.9.2), and the subsequent arguments to the constructor are the arguments in the argument list of the class instance creation expression, if any, in the order they appear in the class instance creation expression.

  • If C is a local class or a private inner member class, then the arguments to the constructor are the arguments in the argument list of the class instance creation expression, if any, in the order they appear in the class instance creation expression.

  • If C is a non-private inner member class, then the first argument to the constructor is the immediately enclosing instance of i (§8.8.1, §15.9.2), and the subsequent arguments to its constructor are the arguments in the argument list of the class instance creation expression, if any, in the order they appear in the class instance creation expression.

  • Otherwise, the arguments to the constructor are the arguments in the argument list of the class instance creation expression, if any, in the order they appear in the expression.

Second, a constructor of C and corresponding return type and throws clause are determined:

  • If the class instance creation expression uses <> to elide class type arguments, a list of methods m1...mn is defined for the purpose of overload resolution and type argument inference.

    Let c1...cn be the constructors of class C. Let #m be an automatically generated name that is distinct from all constructor and method names in C. For all j (1 j n), mj is defined in terms of cj as follows:

    • A substitution θj is first defined to instantiate the types in cj.

      Let F1...Fp be the type parameters of C, and let G1...Gq be the type parameters (if any) of cj. Let X1...Xp and Y1...Yq be type variables with distinct names that are not in scope in the body of C.

      θj is [F1:=X1, ..., Fp:=Xp, G1:=Y1, ..., Gq:=Yq].

    • The modifiers of mj are those of cj.

    • The type parameters of mj are X1...Xp,Y1...Yq. The bound of each parameter, if any, is θj applied to the corresponding parameter bound in C or cj.

    • The return type of mj is θj applied to C<F1,...,Fp>.

    • The name of mj is #m.

    • The (possibly empty) list of argument types of mj is θj applied to the argument types of cj.

    • The (possibly empty) list of thrown types of mj is θj applied to the thrown types of cj.

    • The body of mj is irrelevant.

    To choose a constructor, we temporarily consider m1...mn to be members of C. Then one of m1...mn is selected, as determined by the class instance creation's argument expressions, using the process specified in §15.12.2.

    If there is no unique most specific method that is both applicable and accessible, then a compile-time error occurs.

    Otherwise, where mj is the selected method, cj is the chosen constructor. The return type and throws clause of cj are the same as the return type and throws clause determined for mj (§15.12.2.6).

  • Otherwise, the class instance creation expression does not use <> to elide class type arguments.

    Let T be the type denoted by C followed by any class type arguments in the expression. The process specified in §15.12.2, modified to handle constructors, is used to select one of the constructors of T and determine its throws clause.

    If there is no unique most-specific constructor that is both applicable and accessible, then a compile-time error occurs (as in method invocations).

    Otherwise, the return type is T.

It is a compile-time error if an argument to a class instance creation expression is not compatible with its target type, as derived from the invocation type (§15.12.2.6).

If the compile-time declaration is applicable by variable arity invocation (§15.12.2.4), then where the last formal parameter type of the invocation type of the constructor is Fn[], it is a compile-time error if the type which is the erasure of Fn is not accessible at the point of invocation.

The type of the class instance creation expression is the return type of the chosen constructor, as defined above.

Note that the type of the class instance creation expression may be an anonymous class type, in which case the constructor being invoked is an anonymous constructor (§15.9.5.1).

15.9.4. Run-Time Evaluation of Class Instance Creation Expressions

At run time, evaluation of a class instance creation expression is as follows.

First, if the class instance creation expression is a qualified class instance creation expression, the qualifying primary expression is evaluated. If the qualifying expression evaluates to null, a NullPointerException is raised, and the class instance creation expression completes abruptly. If the qualifying expression completes abruptly, the class instance creation expression completes abruptly for the same reason.

Next, space is allocated for the new class instance. If there is insufficient space to allocate the object, evaluation of the class instance creation expression completes abruptly by throwing an OutOfMemoryError.

The new object contains new instances of all the fields declared in the specified class type and all its superclasses. As each new field instance is created, it is initialized to its default value (§4.12.5).

Next, the actual arguments to the constructor are evaluated, left-to-right. If any of the argument evaluations completes abruptly, any argument expressions to its right are not evaluated, and the class instance creation expression completes abruptly for the same reason.

Next, the selected constructor of the specified class type is invoked. This results in invoking at least one constructor for each superclass of the class type. This process can be directed by explicit constructor invocation statements (§8.8) and is specified in detail in §12.5.

The value of a class instance creation expression is a reference to the newly created object of the specified class. Every time the expression is evaluated, a fresh object is created.

Example 15.9.4-1. Evaluation Order and Out-Of-Memory Detection

If evaluation of a class instance creation expression finds there is insufficient memory to perform the creation operation, then an OutOfMemoryError is thrown. This check occurs before any argument expressions are evaluated.

So, for example, the test program:

class List {
    int value;
    List next;
    static List head = new List(0);
    List(int n) { value = n; next = head; head = this; }
}
class Test {
    public static void main(String[] args) {
        int id = 0, oldid = 0;
        try {
            for (;;) {
                ++id;
                new List(oldid = id);
            }
        } catch (Error e) {
            List.head = null;
            System.out.println(e.getClass() + ", " + (oldid==id));
        }
    }
}

prints:

class java.lang.OutOfMemoryError, false

because the out-of-memory condition is detected before the argument expression oldid = id is evaluated.

Compare this to the treatment of array creation expressions, for which the out-of-memory condition is detected after evaluation of the dimension expressions (§15.10.2).


15.9.5. Anonymous Class Declarations

An anonymous class declaration is automatically derived from a class instance creation expression by the Java compiler.

An anonymous class is never abstract (§8.1.1.1).

An anonymous class is always implicitly final (§8.1.1.2).

An anonymous class is always an inner class (§8.1.3); it is never static (§8.1.1, §8.5.1).

15.9.5.1. Anonymous Constructors

An anonymous class cannot have an explicitly declared constructor. Instead, an anonymous constructor is implicitly declared for an anonymous class. The form of the anonymous constructor for an anonymous class C with direct superclass S is as follows:

  • If S is not an inner class, or if S is a local class that occurs in a static context, then the anonymous constructor has one formal parameter for each actual argument to the class instance creation expression in which C is declared.

    The actual arguments to the class instance creation expression are used to determine a constructor cs of S, using the same rules as for method invocations (§15.12). The type of each formal parameter of the anonymous constructor must be identical to the corresponding formal parameter of cs.

    The constructor body consists of an explicit constructor invocation (§8.8.7.1) of the form super(...), where the actual arguments are the formal parameters of the constructor, in the order they were declared.

  • Otherwise, the first formal parameter of the constructor of C represents the value of the immediately enclosing instance of i with respect to S (§15.9.2, §15.9.3). The type of this parameter is the class type that immediately encloses the declaration of S.

    The constructor has an additional formal parameter for each actual argument to the class instance creation expression that declared the anonymous class. The n'th formal parameter e corresponds to the n-1'th actual argument.

    The actual arguments to the class instance creation expression are used to determine a constructor cs of S, using the same rules as for method invocations (§15.12). The type of each formal parameter of the anonymous constructor must be identical to the corresponding formal parameter of cs.

    The constructor body consists of an explicit constructor invocation (§8.8.7.1) of the form o.super(...), where o is the first formal parameter of the constructor, and the actual arguments are the subsequent formal parameters of the constructor, in the order they were declared.

In all cases, the throws clause of an anonymous constructor must list all the checked exceptions thrown by the explicit superclass constructor invocation statement contained within the anonymous constructor, and all checked exceptions thrown by any instance initializers or instance variable initializers of the anonymous class.

Note that it is possible for the signature of the anonymous constructor to refer to an inaccessible type (for example, if such a type occurred in the signature of the superclass constructor cs). This does not, in itself, cause any errors at either compile-time or run-time.

15.10. Array Creation and Access Expressions

15.10.1. Array Creation Expressions

An array creation expression is used to create new arrays (§10 (Arrays)).

The following production from §4.3 is shown here for convenience:

Dims:
{Annotation} [ ] {{Annotation} [ ]}

An array creation expression creates an object that is a new array whose elements are of the type specified by the PrimitiveType or ClassOrInterfaceType.

It is a compile-time error if the ClassOrInterfaceType does not denote a reifiable type (§4.7). Otherwise, the ClassOrInterfaceType may name any named reference type, even an abstract class type (§8.1.1.1) or an interface type.

The rules above imply that the element type in an array creation expression cannot be a parameterized type, unless all type arguments to the parameterized type are unbounded wildcards.

The type of each dimension expression within a DimExpr must be a type that is convertible (§5.1.8) to an integral type, or a compile-time error occurs.

Each dimension expression undergoes unary numeric promotion (§5.6.1). The promoted type must be int, or a compile-time error occurs.

The type of the array creation expression is an array type that can denoted by a copy of the array creation expression from which the new keyword and every DimExpr expression and array initializer have been deleted.

For example, the type of the creation expression:

new double[3][3][]

is:

double[][][]

15.10.2. Run-Time Evaluation of Array Creation Expressions

At run time, evaluation of an array creation expression behaves as follows:

  • If there are no dimension expressions, then there must be an array initializer. A newly allocated array will be initialized with the values provided by the array initializer as described in §10.6. The value of the array initializer becomes the value of the array creation expression.

  • Otherwise, there is no array initializer, and:

    • First, the dimension expressions are evaluated, left-to-right. If any of the expression evaluations completes abruptly, the expressions to the right of it are not evaluated.

    • Next, the values of the dimension expressions are checked. If the value of any DimExpr expression is less than zero, then a NegativeArraySizeException is thrown.

    • Next, space is allocated for the new array. If there is insufficient space to allocate the array, evaluation of the array creation expression completes abruptly by throwing an OutOfMemoryError.

    • Then, if a single DimExpr appears, a one-dimensional array is created of the specified length, and each component of the array is initialized to its default value (§4.12.5).

    • Otherwise, if n DimExpr expressions appear, then array creation effectively executes a set of nested loops of depth n-1 to create the implied arrays of arrays.

      A multidimensional array need not have arrays of the same length at each level.

Example 15.10.2-1. Array Creation Evaluation

In an array creation expression with one or more dimension expressions, each dimension expression is fully evaluated before any part of any dimension expression to its right. Thus:

class Test1 {
    public static void main(String[] args) {
        int i = 4;
        int ia[][] = new int[i][i=3];
        System.out.println(
            "[" + ia.length + "," + ia[0].length + "]");
    }
}

prints:

[4,3]

because the first dimension is calculated as 4 before the second dimension expression sets i to 3.

If evaluation of a dimension expression completes abruptly, no part of any dimension expression to its right will appear to have been evaluated. Thus:

class Test2 {
    public static void main(String[] args) {
        int[][] a = { { 00, 01 }, { 10, 11 } };
        int i = 99;
        try {
            a[val()][i = 1]++;
        } catch (Exception e) {
            System.out.println(e + ", i=" + i);
        }
    }
    static int val() throws Exception {
        throw new Exception("unimplemented");
    }
}

prints:

java.lang.Exception: unimplemented, i=99

because the embedded assignment that sets i to 1 is never executed.


Example 15.10.2-2. Multi-Dimensional Array Creation

The declaration:

float[][] matrix = new float[3][3];

is equivalent in behavior to:

float[][] matrix = new float[3][];
for (int d = 0; d < matrix.length; d++)
    matrix[d] = new float[3];

and:

Age[][][][][] Aquarius = new Age[6][10][8][12][];

is equivalent to:

Age[][][][][] Aquarius = new Age[6][][][][];
for (int d1 = 0; d1 < Aquarius.length; d1++) {
    Aquarius[d1] = new Age[10][][][];
    for (int d2 = 0; d2 < Aquarius[d1].length; d2++) {
        Aquarius[d1][d2] = new Age[8][][];
        for (int d3 = 0; d3 < Aquarius[d1][d2].length; d3++) {
            Aquarius[d1][d2][d3] = new Age[12][];
        }
    }
}

with d, d1, d2, and d3 replaced by names that are not already locally declared. Thus, a single new expression actually creates one array of length 6, 6 arrays of length 10, 6x10 = 60 arrays of length 8, and 6x10x8 = 480 arrays of length 12. This example leaves the fifth dimension, which would be arrays containing the actual array elements (references to Age objects), initialized only to null references. These arrays can be filled in later by other code, such as:

Age[] Hair = { new Age("quartz"), new Age("topaz") };
Aquarius[1][9][6][9] = Hair;

A triangular matrix may be created by:

float triang[][] = new float[100][];
for (int i = 0; i < triang.length; i++)
    triang[i] = new float[i+1];

If evaluation of an array creation expression finds there is insufficient memory to perform the creation operation, then an OutOfMemoryError is thrown. If the array creation expression does not have an array initializer, then this check occurs only after evaluation of all dimension expressions has completed normally. If the array creation expression does have an array initializer, then an OutOfMemoryError can occur when an object of reference type is allocated during evaluation of a variable initializer expression, or when space is allocated for an array to hold the values of a (possibly nested) array initializer.

Example 15.10.2-3. OutOfMemoryError and Dimension Expression Evaluation

class Test3 {
    public static void main(String[] args) {
        int len = 0, oldlen = 0;
        Object[] a = new Object[0];
        try {
            for (;;) {
                ++len;
                Object[] temp = new Object[oldlen = len];
                temp[0] = a;
                a = temp;
            }
        } catch (Error e) {
            System.out.println(e + ", " + (oldlen==len));
        }
    }
}

This program produces the output:

java.lang.OutOfMemoryError, true

because the out-of-memory condition is detected after the dimension expression oldlen = len is evaluated.

Compare this to class instance creation expressions (§15.9), which detect the out-of-memory condition before evaluating argument expressions (§15.9.4).


15.10.3. Array Access Expressions

An array access expression refers to a variable that is a component of an array.

An array access expression contains two subexpressions, the array reference expression (before the left bracket) and the index expression (within the brackets).

Note that the array reference expression may be a name or any primary expression that is not an array creation expression (§15.10).

The type of the array reference expression must be an array type (call it T[], an array whose components are of type T), or a compile-time error occurs.

The index expression undergoes unary numeric promotion (§5.6.1). The promoted type must be int, or a compile-time error occurs.

The type of the array access expression is the result of applying capture conversion (§5.1.10) to T.

The result of an array access expression is a variable of type T, namely the variable within the array selected by the value of the index expression.

This resulting variable, which is a component of the array, is never considered final, even if the array reference expression denoted a final variable.

15.10.4. Run-Time Evaluation of Array Access Expressions

At run time, evaluation of an array access expression behaves as follows:

  • First, the array reference expression is evaluated. If this evaluation completes abruptly, then the array access completes abruptly for the same reason and the index expression is not evaluated.

  • Otherwise, the index expression is evaluated. If this evaluation completes abruptly, then the array access completes abruptly for the same reason.

  • Otherwise, if the value of the array reference expression is null, then a NullPointerException is thrown.

  • Otherwise, the value of the array reference expression indeed refers to an array. If the value of the index expression is less than zero, or greater than or equal to the array's length, then an ArrayIndexOutOfBoundsException is thrown.

  • Otherwise, the result of the array access is the variable of type T, within the array, selected by the value of the index expression.

Example 15.10.4-1. Array Reference Is Evaluated First

In an array access, the expression to the left of the brackets appears to be fully evaluated before any part of the expression within the brackets is evaluated. For example, in the (admittedly monstrous) expression a[(a=b)[3]], the expression a is fully evaluated before the expression (a=b)[3]; this means that the original value of a is fetched and remembered while the expression (a=b)[3] is evaluated. This array referenced by the original value of a is then subscripted by a value that is element 3 of another array (possibly the same array) that was referenced by b and is now also referenced by a.

Thus, the program:

class Test1 {
    public static void main(String[] args) {
        int[] a = { 11, 12, 13, 14 };
        int[] b = { 0, 1, 2, 3 };
        System.out.println(a[(a=b)[3]]);
    }
}

prints:

14

because the monstrous expression's value is equivalent to a[b[3]] or a[3] or 14.


Example 15.10.4-2. Abrupt Completion of Array Reference Evaluation

If evaluation of the expression to the left of the brackets completes abruptly, no part of the expression within the brackets will appear to have been evaluated. Thus, the program:

class Test2 {
    public static void main(String[] args) {
        int index = 1;
        try {
            skedaddle()[index=2]++;
        } catch (Exception e) {
            System.out.println(e + ", index=" + index);
        }
    }
    static int[] skedaddle() throws Exception {
        throw new Exception("Ciao");
    }
}

prints:

java.lang.Exception: Ciao, index=1

because the embedded assignment of 2 to index never occurs.


Example 15.10.4-3. null Array Reference

If the array reference expression produces null instead of a reference to an array, then a NullPointerException is thrown at run time, but only after all parts of the array access expression have been evaluated and only if these evaluations completed normally. Thus, the program:

class Test3 {
    public static void main(String[] args) {
        int index = 1;
        try {
            nada()[index=2]++;
        } catch (Exception e) {
            System.out.println(e + ", index=" + index);
        }
    }
    static int[] nada() { return null; }
}

prints:

java.lang.NullPointerException, index=2

because the embedded assignment of 2 to index occurs before the check for a null array reference expression. As a related example, the program:

class Test4 {
    public static void main(String[] args) {
        int[] a = null;
        try {
            int i = a[vamoose()];
            System.out.println(i);
        } catch (Exception e) {
            System.out.println(e);
        }
    }
    static int vamoose() throws Exception {
        throw new Exception("Twenty-three skidoo!");
    }
}

always prints:

java.lang.Exception: Twenty-three skidoo!

A NullPointerException never occurs, because the index expression must be completely evaluated before any further part of the array access occurs, and that includes the check as to whether the value of the array reference expression is null.


15.11. Field Access Expressions

A field access expression may access a field of an object or array, a reference to which is the value of either an expression or the special keyword super.

FieldAccess:

The meaning of a field access expression is determined using the same rules as for qualified names (§6.5.6.2), but limited by the fact that an expression cannot denote a package, class type, or interface type.

It is also possible to refer to a field of the current instance or current class by using a simple name (§6.5.6.1).

15.11.1. Field Access Using a Primary

The type of the Primary must be a reference type T, or a compile-time error occurs.

The meaning of the field access expression is determined as follows:

  • If the identifier names several accessible (§6.6) member fields in type T, then the field access is ambiguous and a compile-time error occurs.

  • If the identifier does not name an accessible member field in type T, then the field access is undefined and a compile-time error occurs.

  • Otherwise, the identifier names a single accessible member field in type T, and the type of the field access expression is the type of the member field after capture conversion (§5.1.10).

At run time, the result of the field access expression is computed as follows: (assuming that the program is correct with respect to definite assignment analysis, i.e. every blank final variable is definitely assigned before access)

  • If the field is static:

    • The Primary expression is evaluated, and the result is discarded. If evaluation of the Primary expression completes abruptly, the field access expression completes abruptly for the same reason.

    • If the field is a non-blank final field, then the result is the value of the specified class variable in the class or interface that is the type of the Primary expression.

    • If the field is not final, or is a blank final and the field access occurs in a static initializer or class variable initializer, then the result is a variable, namely, the specified class variable in the class that is the type of the Primary expression.

  • If the field is not static:

    • The Primary expression is evaluated. If evaluation of the Primary expression completes abruptly, the field access expression completes abruptly for the same reason.

    • If the value of the Primary is null, then a NullPointerException is thrown.

    • If the field is a non-blank final, then the result is the value of the named member field in type T found in the object referenced by the value of the Primary.

    • If the field is not final, or is a blank final and the field access occurs in a constructor or instance variable initializer, then the result is a variable, namely the named member field in type T found in the object referenced by the value of the Primary.

Note that only the type of the Primary expression, not the class of the actual object referred to at run time, is used in determining which field to use.

Example 15.11.1-1. Static Binding for Field Access

class S           { int x = 0; }
class T extends S { int x = 1; }
class Test1 {
    public static void main(String[] args) {
        T t = new T();
        System.out.println("t.x=" + t.x + when("t", t));
        S s = new S();
        System.out.println("s.x=" + s.x + when("s", s));
        s = t;
        System.out.println("s.x=" + s.x + when("s", s));
    }
    static String when(String name, Object t) {
        return " when " + name + " holds a "
                        + t.getClass() + " at run time.";
    }
}

This program produces the output:

t.x=1 when t holds a class T at run time.
s.x=0 when s holds a class S at run time.
s.x=0 when s holds a class T at run time.

The last line shows that, indeed, the field that is accessed does not depend on the run-time class of the referenced object; even if s holds a reference to an object of class T, the expression s.x refers to the x field of class S, because the type of the expression s is S. Objects of class T contain two fields named x, one for class T and one for its superclass S.

This lack of dynamic lookup for field accesses allows programs to be run efficiently with straightforward implementations. The power of late binding and overriding is available, but only when instance methods are used. Consider the same example using instance methods to access the fields:

class S           { int x = 0; int z() { return x; } }
class T extends S { int x = 1; int z() { return x; } }
class Test2 {
    public static void main(String[] args) {
        T t = new T();
        System.out.println("t.z()=" + t.z() + when("t", t));
        S s = new S();
        System.out.println("s.z()=" + s.z() + when("s", s));
        s = t;
        System.out.println("s.z()=" + s.z() + when("s", s));
    }
    static String when(String name, Object t) {
        return " when " + name + " holds a "
                        + t.getClass() + " at run time.";
    }
}

Now the output is:

t.z()=1 when t holds a class T at run time.
s.z()=0 when s holds a class S at run time.
s.z()=1 when s holds a class T at run time.

The last line shows that, indeed, the method that is accessed does depend on the run-time class of the referenced object; when s holds a reference to an object of class T, the expression s.z() refers to the z method of class T, despite the fact that the type of the expression s is S. Method z of class T overrides method z of class S.


Example 15.11.1-2. Receiver Variable Is Irrelevant For static Field Access

The following program demonstrates that a null reference may be used to access a class (static) variable without causing an exception:

class Test3 {
    static String mountain = "Chocorua";
    static Test3 favorite(){
        System.out.print("Mount ");
        return null;
    }
    public static void main(String[] args) {
        System.out.println(favorite().mountain);
    }
}

It compiles, executes, and prints:

Mount Chocorua

Even though the result of favorite() is null, a NullPointerException is not thrown. That "Mount " is printed demonstrates that the Primary expression is indeed fully evaluated at run time, despite the fact that only its type, not its value, is used to determine which field to access (because the field mountain is static).


15.11.2. Accessing Superclass Members using super

The form super.Identifier refers to the field named Identifier of the current object, but with the current object viewed as an instance of the superclass of the current class.

The form T.super.Identifier refers to the field named Identifier of the lexically enclosing instance corresponding to T, but with that instance viewed as an instance of the superclass of T.

The forms using the keyword super are valid only in an instance method, instance initializer, or constructor of a class, or in the initializer of an instance variable of a class. If they appear anywhere else, a compile-time error occurs.

These are exactly the same situations in which the keyword this may be used in a class declaration (§15.8.3).

It is a compile-time error if the forms using the keyword super appear in the declaration of class Object, since Object has no superclass.

Suppose that a field access expression super.f appears within class C, and the immediate superclass of C is class S. If f in S is accessible from class C (§6.6), then super.f is treated as if it had been the expression this.f in the body of class S. Otherwise, a compile-time error occurs.

Thus, super.f can access the field f that is accessible in class S, even if that field is hidden by a declaration of a field f in class C.

Suppose that a field access expression T.super.f appears within class C, and the immediate superclass of the class denoted by T is a class whose fully qualified name is S. If f in S is accessible from C, then T.super.f is treated as if it had been the expression this.f in the body of class S. Otherwise, a compile-time error occurs.

Thus, T.super.f can access the field f that is accessible in class S, even if that field is hidden by a declaration of a field f in class T.

It is a compile-time error if the current class is not an inner class of class T or T itself.

Example 15.11.2-1. The super Expression

interface I           { int x = 0; }
class T1 implements I { int x = 1; }
class T2 extends T1   { int x = 2; }
class T3 extends T2 {
    int x = 3;
    void test() {
        System.out.println("x=\t\t"          + x);
        System.out.println("super.x=\t\t"    + super.x);
        System.out.println("((T2)this).x=\t" + ((T2)this).x);
        System.out.println("((T1)this).x=\t" + ((T1)this).x);
        System.out.println("((I)this).x=\t"  + ((I)this).x);
    }
}
class Test {
    public static void main(String[] args) {
        new T3().test();
    }
}

This program produces the output:

x=              3
super.x=        2
((T2)this).x=   2
((T1)this).x=   1
((I)this).x=    0

Within class T3, the expression super.x has the same effect as ((T2)this).x when x has package access. Note that super.x is not specified in terms of a cast, due to difficulties around access to protected members of the superclass.


15.12. Method Invocation Expressions

A method invocation expression is used to invoke a class or instance method.

Resolving a method name at compile time is more complicated than resolving a field name because of the possibility of method overloading. Invoking a method at run time is also more complicated than accessing a field because of the possibility of instance method overriding.

Determining the method that will be invoked by a method invocation expression involves several steps. The following three sections describe the compile-time processing of a method invocation. The determination of the type of the method invocation expression is specified in §15.12.3.

The exception types that a method invocation expression can throw are specified in §11.2.1.

It is a compile-time error if the name to the left of the rightmost "." that occurs before the ( in a MethodInvocation cannot be classified as a TypeName or an ExpressionName (§6.5.2).

If TypeArguments is present to the left of Identifier, then it is a compile-time error if any of the type arguments are wildcards (§4.5.1).

A method invocation expression is a poly expression if all of the following are true:

  • The invocation appears in an assignment context or an invocation context (§5.2, §5.3).

  • If the invocation is qualified (that is, any form of MethodInvocation except for the first), then the invocation elides TypeArguments to the left of the Identifier.

  • The method to be invoked, as determined by the following subsections, is generic (§8.4.4) and has a return type that mentions at least one of the method's type parameters.

Otherwise, the method invocation expression is a standalone expression.

15.12.1. Compile-Time Step 1: Determine Class or Interface to Search

The first step in processing a method invocation at compile time is to figure out the name of the method to be invoked and which class or interface to search for definitions of methods of that name.

The name of the method is specified by the MethodName or Identifier which immediately precedes the left parenthesis of the MethodInvocation.

For the class or interface to search, there are six cases to consider, depending on the form that precedes the left parenthesis of the MethodInvocation:

  • If the form is MethodName, that is, just an Identifier, then:

    If the Identifier appears in the scope of a visible method declaration with that name (§6.3, §6.4.1), then:

    • If there is an enclosing type declaration of which that method is a member, let T be the innermost such type declaration. The class or interface to search is T.

      This search policy is called the "comb rule". It effectively looks for methods in a nested class's superclass hierarchy before looking for methods in an enclosing class and its superclass hierarchy. See §6.5.7.1 for an example.

    • Otherwise, the visible method declaration may be in scope due to one or more single-static-import or static-import-on-demand declarations. There is no class or interface to search, as the method to be invoked is determined later (§15.12.2.1).

  • If the form is TypeName . [TypeArguments] Identifier, then the type to search is the type denoted by TypeName.

  • If the form is ExpressionName . [TypeArguments] Identifier, then the class or interface to search is the declared type T of the variable denoted by ExpressionName if T is a class or interface type, or the upper bound of T if T is a type variable.

  • If the form is Primary . [TypeArguments] Identifier, then let T be the type of the Primary expression. The class or interface to search is T if T is a class or interface type, or the upper bound of T if T is a type variable.

    It is a compile-time error if T is not a reference type.

  • If the form is super . [TypeArguments] Identifier, then the class to search is the superclass of the class whose declaration contains the method invocation.

    Let T be the type declaration immediately enclosing the method invocation. It is a compile-time error if T is the class Object or T is an interface.

  • If the form is TypeName . super . [TypeArguments] Identifier, then:

    • It is a compile-time error if TypeName denotes neither a class nor an interface.

    • If TypeName denote a class, C, then the class to search is the superclass of C.

      It is a compile-time error if C is not a lexically enclosing type declaration of the current class, or if C is the class Object.

      Let T be the type declaration immediately enclosing the method invocation. It is a compile-time error if T is the class Object.

    • Otherwise, TypeName denotes the interface to be searched, I.

      Let T be the type declaration immediately enclosing the method invocation. It is a compile-time error if I is not a direct superinterface of T, or if there exists some other direct superclass or direct superinterface of T, J, such that J is a subtype of I.

The TypeName . super syntax is overloaded: traditionally, the TypeName refers to a lexically enclosing type declaration which is a class, and the target is the superclass of this class, as if the invocation were an unqualified super in the lexically enclosing type declaration.

class Superclass {
    void foo() { System.out.println("Hi"); }
}

class Subclass1 extends Superclass {
    void foo() { throw new UnsupportedOperationException(); }

    Runnable tweak = new Runnable() {
        void run() {
            Subclass1.super.foo();  // Gets the 'println' behavior
        }
    };
}

To support invocation of default methods in superinterfaces, the TypeName may also refer to a direct superinterface of the current class or interface, and the target is that superinterface.

interface Superinterface {
    default void foo() { System.out.println("Hi"); }
}

class Subclass2 implements Superinterface {
    void foo() { throw new UnsupportedOperationException(); }

    void tweak() {
        Superinterface.super.foo();  // Gets the 'println' behavior
    }
}

No syntax supports a combination of these forms, that is, invoking a superinterface method of a lexically enclosing type declaration which is a class, as if the invocation were of the form InterfaceName . super in the lexically enclosing type declaration.

class Subclass3 implements Superinterface {
    void foo() { throw new UnsupportedOperationException(); }

    Runnable tweak = new Runnable() {
        void run() {
            Subclass3.Superinterface.super.foo();  // Illegal
        }
    };
}

A workaround is to introduce a private method in the lexically enclosing type declaration, that performs the interface super call.

15.12.2. Compile-Time Step 2: Determine Method Signature

The second step searches the type determined in the previous step for member methods. This step uses the name of the method and the argument expressions to locate methods that are both accessible and applicable, that is, declarations that can be correctly invoked on the given arguments.

There may be more than one such method, in which case the most specific one is chosen. The descriptor (signature plus return type) of the most specific method is the one used at run time to perform the method dispatch.

A method is applicable if it is applicable by one of strict invocation (§15.12.2.2), loose invocation (§15.12.2.3), or variable arity invocation (§15.12.2.4).

Certain argument expressions that contain implicitly typed lambda expressions (§15.27.1) or inexact method references (§15.13.1) are ignored by the applicability tests, because their meaning cannot be determined until a target type is selected.

Although the method invocation may be a poly expression, only its argument expressions - not the invocation's target type - influence the selection of applicable methods.

The process of determining applicability begins by determining the potentially applicable methods (§15.12.2.1).

The remainder of the process is split into three phases, to ensure compatibility with versions of the Java programming language prior to Java SE 5.0. The phases are:

  1. The first phase (§15.12.2.2) performs overload resolution without permitting boxing or unboxing conversion, or the use of variable arity method invocation. If no applicable method is found during this phase then processing continues to the second phase.

    This guarantees that any calls that were valid in the Java programming language before Java SE 5.0 are not considered ambiguous as the result of the introduction of variable arity methods, implicit boxing and/or unboxing. However, the declaration of a variable arity method (§8.4.1) can change the method chosen for a given method method invocation expression, because a variable arity method is treated as a fixed arity method in the first phase. For example, declaring m(Object...) in a class which already declares m(Object) causes m(Object) to no longer be chosen for some invocation expressions (such as m(null)), as m(Object[]) is more specific.

  2. The second phase (§15.12.2.3) performs overload resolution while allowing boxing and unboxing, but still precludes the use of variable arity method invocation. If no applicable method is found during this phase then processing continues to the third phase.

    This ensures that a method is never chosen through variable arity method invocation if it is applicable through fixed arity method invocation.

  3. The third phase (§15.12.2.4) allows overloading to be combined with variable arity methods, boxing, and unboxing.

Deciding whether a method is applicable will, in the case of generic methods (§8.4.4), require an analysis of the type arguments. Type arguments may be passed explicitly or implicitly. If they are passed implicitly, bounds of the type arguments must be inferred (§18 (Type Inference)) from the argument expressions.

If several applicable methods have been identified during one of the three phases of applicability testing, then the most specific one is chosen, as specified in section §15.12.2.5.

To check for applicability, the types of an invocation's arguments cannot, in general, be inputs to the analysis. This is because:

  • The arguments to a method invocation may be poly expressions

  • Poly expressions cannot be typed in the absence of a target type

  • Overload resolution has to be completed before the arguments' target types will be known

Instead, the input to the applicability check is a list of argument expressions, which can be checked for compatibility with potential target types, even if the ultimate types of the expressions are unknown.

Note that overload resolution is independent of a target type. This is for two reasons:

  • First, it makes the user model more accessible and less error-prone. The meaning of a method name (i.e., the declaration corresponding to the name) is too fundamental to the meaning of a program to depend on subtle contextual hints. (In contrast, other poly expressions may have different behavior depending on a target type; but the variation in behavior is always limited and essentially equivalent, while no such guarantees can be made about the behavior of an arbitrary set of methods that share a name and arity.)

  • Second, it allows other properties - such as whether or not the method is a poly expression (§15.12) or how to categorize a conditional expression (§15.25) - to depend on the meaning of the method name, even before a target type is known.

Example 15.12.2-1. Method Applicability

class Doubler {
            static int two()      { return two(1); }
    private static int two(int i) { return 2*i;    }
}
class Test extends Doubler {	
    static long two(long j) { return j+j; }

    public static void main(String[] args) {
        System.out.println(two(3));
        System.out.println(Doubler.two(3)); // compile-time error
    }
}

For the method invocation two(1) within class Doubler, there are two accessible methods named two, but only the second one is applicable, and so that is the one invoked at run time.

For the method invocation two(3) within class Test, there are two applicable methods, but only the one in class Test is accessible, and so that is the one to be invoked at run time (the argument 3 is converted to type long).

For the method invocation Doubler.two(3), the class Doubler, not class Test, is searched for methods named two; the only applicable method is not accessible, and so this method invocation causes a compile-time error.

Another example is:

class ColoredPoint {
    int x, y;
    byte color;
    void setColor(byte color) { this.color = color; }
}
class Test {
    public static void main(String[] args) {
        ColoredPoint cp = new ColoredPoint();
        byte color = 37;
        cp.setColor(color);
        cp.setColor(37);  // compile-time error
    }
}

Here, a compile-time error occurs for the second invocation of setColor, because no applicable method can be found at compile time. The type of the literal 37 is int, and int cannot be converted to byte by invocation conversion. Assignment conversion, which is used in the initialization of the variable color, performs an implicit conversion of the constant from type int to byte, which is permitted because the value 37 is small enough to be represented in type byte; but such a conversion is not allowed for invocation conversion.

If the method setColor had, however, been declared to take an int instead of a byte, then both method invocations would be correct; the first invocation would be allowed because invocation conversion does permit a widening conversion from byte to int. However, a narrowing cast would then be required in the body of setColor:

void setColor(int color) { this.color = (byte)color; }

Here is an example of overloading ambiguity. Consider the program:

class Point { int x, y; }
class ColoredPoint extends Point { int color; }
class Test {
    static void test(ColoredPoint p, Point q) {
        System.out.println("(ColoredPoint, Point)");
    }
    static void test(Point p, ColoredPoint q) {
        System.out.println("(Point, ColoredPoint)");
    }
    public static void main(String[] args) {
        ColoredPoint cp = new ColoredPoint();
        test(cp, cp);  // compile-time error
    }
}

This example produces an error at compile time. The problem is that there are two declarations of test that are applicable and accessible, and neither is more specific than the other. Therefore, the method invocation is ambiguous.

If a third definition of test were added:

static void test(ColoredPoint p, ColoredPoint q) {
    System.out.println("(ColoredPoint, ColoredPoint)");
}

then it would be more specific than the other two, and the method invocation would no longer be ambiguous.


Example 15.12.2-2. Return Type Not Considered During Method Selection

class Point { int x, y; }
class ColoredPoint extends Point { int color; }
class Test {
    static int test(ColoredPoint p) {
        return p.color;
    }
    static String test(Point p) {
        return "Point";
    }
    public static void main(String[] args) {
        ColoredPoint cp = new ColoredPoint();
        String s = test(cp);  // compile-time error
    }
}

Here, the most specific declaration of method test is the one taking a parameter of type ColoredPoint. Because the result type of the method is int, a compile-time error occurs because an int cannot be converted to a String by assignment conversion. This example shows that the result types of methods do not participate in resolving overloaded methods, so that the second test method, which returns a String, is not chosen, even though it has a result type that would allow the example program to compile without error.


Example 15.12.2-3. Choosing The Most Specific Method

The most specific method is chosen at compile time; its descriptor determines what method is actually executed at run time. If a new method is added to a class, then source code that was compiled with the old definition of the class might not use the new method, even if a recompilation would cause this method to be chosen.

So, for example, consider two compilation units, one for class Point:

package points;
public class Point {
    public int x, y;
    public Point(int x, int y) { this.x = x; this.y = y; }
    public String toString() { return toString(""); }
    public String toString(String s) {
        return "(" + x + "," + y + s + ")";
    }
}

and one for class ColoredPoint:

package points;
public class ColoredPoint extends Point {
    public static final int
        RED = 0, GREEN = 1, BLUE = 2;
    public static String[] COLORS =
        { "red", "green", "blue" };

    public byte color;
    public ColoredPoint(int x, int y, int color) {
        super(x, y);
        this.color = (byte)color;
    }

    /** Copy all relevant fields of the argument into
        this ColoredPoint object. */
    public void adopt(Point p) { x = p.x; y = p.y; }

    public String toString() {
        String s = "," + COLORS[color];
        return super.toString(s);
    }
}

Now consider a third compilation unit that uses ColoredPoint:

import points.*;
class Test {
    public static void main(String[] args) {
        ColoredPoint cp =
            new ColoredPoint(6, 6, ColoredPoint.RED);
        ColoredPoint cp2 =
            new ColoredPoint(3, 3, ColoredPoint.GREEN);
        cp.adopt(cp2);
        System.out.println("cp: " + cp);
    }
}

The output is:

cp: (3,3,red)

The programmer who coded class Test has expected to see the word green, because the actual argument, a ColoredPoint, has a color field, and color would seem to be a "relevant field". (Of course, the documentation for the package points ought to have been much more precise!)

Notice, by the way, that the most specific method (indeed, the only applicable method) for the method invocation of adopt has a signature that indicates a method of one parameter, and the parameter is of type Point. This signature becomes part of the binary representation of class Test produced by the Java compiler and is used by the method invocation at run time.

Suppose the programmer reported this software error and the maintainer of the points package decided, after due deliberation, to correct it by adding a method to class ColoredPoint:


public void adopt(ColoredPoint p) {
    adopt((Point)p);
    color = p.color;
}

If the programmer then runs the old binary file for Test with the new binary file for ColoredPoint, the output is still:

cp: (3,3,red)

because the old binary file for Test still has the descriptor "one parameter, whose type is Point; void" associated with the method call cp.adopt(cp2). If the source code for Test is recompiled, the Java compiler will then discover that there are now two applicable adopt methods, and that the signature for the more specific one is "one parameter, whose type is ColoredPoint; void"; running the program will then produce the desired output:

cp: (3,3,green)

With forethought about such problems, the maintainer of the points package could fix the ColoredPoint class to work with both newly compiled and old code, by adding defensive code to the old adopt method for the sake of old code that still invokes it on ColoredPoint arguments:


public void adopt(Point p) {
    if (p instanceof ColoredPoint)
        color = ((ColoredPoint)p).color;
    x = p.x; y = p.y;
}

Ideally, source code should be recompiled whenever code that it depends on is changed. However, in an environment where different classes are maintained by different organizations, this is not always feasible. Defensive programming with careful attention to the problems of class evolution can make upgraded code much more robust. See §13 (Binary Compatibility) for a detailed discussion of binary compatibility and type evolution.


15.12.2.1. Identify Potentially Applicable Methods

The class or interface determined by compile-time step 1 (§15.12.1) is searched for all member methods that are potentially applicable to this method invocation; members inherited from superclasses and superinterfaces are included in this search.

In addition, if the form of the method invocation expression is MethodName - that is, a single Identifier - then the search for potentially applicable methods also examines all member methods that are imported by single-static-import declarations and static-import-on-demand declarations of the compilation unit where the method invocation occurs (§7.5.3, §7.5.4) and that are not shadowed at the point where the method invocation appears.

A member method is potentially applicable to a method invocation if and only if all of the following are true:

  • The name of the member is identical to the name of the method in the method invocation.

  • The member is accessible (§6.6) to the class or interface in which the method invocation appears.

    Whether a member method is accessible at a method invocation depends on the access modifier (public, protected, no modifier (package access), or private) in the member's declaration and on where the method invocation appears.

  • If the member is a fixed arity method with arity n, the arity of the method invocation is equal to n, and for all i (1 i n), the i'th argument of the method invocation is potentially compatible, as defined below, with the type of the i'th parameter of the method.

  • If the member is a variable arity method with arity n, then for all i (1 i n-1), the i'th argument of the method invocation is potentially compatible with the type of the i'th parameter of the method; and, where the nth parameter of the method has type T[], one of the following is true:

    • The arity of the method invocation is equal to n-1.

    • The arity of the method invocation is equal to n, and the nth argument of the method invocation is potentially compatible with either T or T[].

    • The arity of the method invocation is m, where m > n, and for all i (n i m), the i'th argument of the method invocation is potentially compatible with T.

  • If the method invocation includes explicit type arguments, and the member is a generic method, then the number of type arguments is equal to the number of type parameters of the method.

    This clause implies that a non-generic method may be potentially applicable to an invocation that supplies explicit type arguments. Indeed, it may turn out to be applicable. In such a case, the type arguments will simply be ignored.

    This rule stems from issues of compatibility and principles of substitutability. Since interfaces or superclasses may be generified independently of their subtypes, we may override a generic method with a non-generic one. However, the overriding (non-generic) method must be applicable to calls to the generic method, including calls that explicitly pass type arguments. Otherwise the subtype would not be substitutable for its generified supertype.

If the search does not yield at least one method that is potentially applicable, then a compile-time error occurs.

An expression is potentially compatible with a target type according to the following rules:

  • A lambda expression (§15.27) is potentially compatible with a functional interface type (§9.8) if all of the following are true:

    • The arity of the target type's function type is the same as the arity of the lambda expression.

    • If the target type's function type has a void return, then the lambda body is either a statement expression (§14.8) or a void-compatible block (§15.27.2).

    • If the target type's function type has a (non-void) return type, then the lambda body is either an expression or a value-compatible block (§15.27.2).

  • A method reference expression (§15.13) is potentially compatible with a functional interface type if, where the type's function type arity is n, there exists at least one potentially applicable method for the method reference expression with arity n (§15.13.1), and one of the following is true:

    • The method reference expression has the form ReferenceType :: [TypeArguments] Identifier and at least one potentially applicable method is i) static and supports arity n, or ii) not static and supports arity n-1.

    • The method reference expression has some other form and at least one potentially applicable method is not static.

  • A lambda expression or a method reference expression is potentially compatible with a type variable if the type variable is a type parameter of the candidate method.

  • A parenthesized expression (§15.8.5) is potentially compatible with a type if its contained expression is potentially compatible with that type.

  • A conditional expression (§15.25) is potentially compatible with a type if each of its second and third operand expressions are potentially compatible with that type.

  • A class instance creation expression, a method invocation expression, or an expression of a standalone form (§15.2) is potentially compatible with any type.

The definition of potential applicability goes beyond a basic arity check to also take into account the presence and "shape" of functional interface target types. In some cases involving type argument inference, a lambda expression appearing as a method invocation argument cannot be properly typed until after overload resolution. These rules allow the form of the lambda expression to still be taken into account, discarding obviously incorrect target types that might otherwise cause ambiguity errors.

15.12.2.2. Phase 1: Identify Matching Arity Methods Applicable by Strict Invocation

An argument expression is considered pertinent to applicability for a potentially applicable method m unless it has one of the following forms:

  • An implicitly typed lambda expression (§15.27.1).

  • An inexact method reference expression (§15.13.1).

  • If m is a generic method and the method invocation does not provide explicit type arguments, an explicitly typed lambda expression or an exact method reference expression for which the corresponding target type (as derived from the signature of m) is a type parameter of m.

  • An explicitly typed lambda expression whose body is an expression that is not pertinent to applicability.

  • An explicitly typed lambda expression whose body is a block, where at least one result expression is not pertinent to applicability.

  • A parenthesized expression (§15.8.5) whose contained expression is not pertinent to applicability.

  • A conditional expression (§15.25) whose second or third operand is not pertinent to applicability.

Let m be a potentially applicable method (§15.12.2.1) with arity n and formal parameter types F1 ... Fn, and let e1, ..., en be the actual argument expressions of the method invocation. Then:

  • If m is a generic method and the method invocation does not provide explicit type arguments, then the applicability of the method is inferred as specified in §18.5.1.

  • If m is a generic method and the method invocation provides explicit type arguments, then let R1 ... Rp (p 1) be the type parameters of m, let Bl be the declared bound of Rl (1 l p), and let U1, ..., Up be the explicit type arguments given in the method invocation. Then m is applicable by strict invocation if both of the following are true:

    • For 1 i n, if ei is pertinent to applicability then ei is compatible in a strict invocation context with Fi[R1:=U1, ..., Rp:=Up].

    • For 1 l p, Ul <: Bl[R1:=U1, ..., Rp:=Up].

  • If m is not a generic method, then m is applicable by strict invocation if, for 1 i n, either ei is compatible in a strict invocation context with Fi or ei is not pertinent to applicability.

If no method applicable by strict invocation is found, the search for applicable methods continues with phase 2 (§15.12.2.3).

Otherwise, the most specific method (§15.12.2.5) is chosen among the methods that are applicable by strict invocation.

The meaning of an implicitly typed lambda expression or an inexact method reference expression is sufficiently vague prior to resolving a target type that arguments containing these expressions are not considered pertinent to applicability; they are simply ignored (except for their expected arity) until overload resolution is finished.

15.12.2.3. Phase 2: Identify Matching Arity Methods Applicable by Loose Invocation

Let m be a potentially applicable method (§15.12.2.1) with arity n and formal parameter types F1, ..., Fn, and let e1, ..., en be the actual argument expressions of the method invocation. Then:

  • If m is a generic method and the method invocation does not provide explicit type arguments, then the applicability of the method is inferred as specified in §18.5.1.

  • If m is a generic method and the method invocation provides explicit type arguments, then let R1 ... Rp (p 1) be the type parameters of m, let Bl be the declared bound of Rl (1 l p), and let U1 ... Up be the explicit type arguments given in the method invocation. Then m is applicable by loose invocation if both of the following are true:

    • For 1 i n, if ei is pertinent to applicability (§15.12.2.2) then ei is compatible in a loose invocation context with Fi[R1:=U1, ..., Rp:=Up].

    • For 1 l p, Ul <: Bl[R1:=U1, ..., Rp:=Up].

  • If m is not a generic method, then m is applicable by loose invocation if, for 1 i n, either ei is compatible in a loose invocation context with Fi or ei is not pertinent to applicability.

If no method applicable by loose invocation is found, the search for applicable methods continues with phase 3 (§15.12.2.4).

Otherwise, the most specific method (§15.12.2.5) is chosen among the methods that are applicable by loose invocation.

15.12.2.4. Phase 3: Identify Methods Applicable by Variable Arity Invocation

Where a variable arity method has formal parameter types F1, ..., Fn-1, Fn[], let the i'th variable arity parameter type of the method be defined as follows:

  • For i n-1, the i'th variable arity parameter type is Fi.

  • For i n, the i'th variable arity parameter type is Fn.

Let m be a potentially applicable method (§15.12.2.1) with variable arity, let T1, ..., Tk be the first k variable arity parameter types of m, and let e1, ..., ek be the actual argument expressions of the method invocation. Then:

  • If m is a generic method and the method invocation does not provide explicit type arguments, then the applicability of the method is inferred as specified in §18.5.1.

  • If m is a generic method and the method invocation provides explicit type arguments, then let R1 ... Rp (p 1) be the type parameters of m, let Bl be the declared bound of Rl (1 l p), and let U1 ... Up be the explicit type arguments given in the method invocation. Then m is applicable by variable arity invocation if:

    • For 1 i k, if ei is pertinent to applicability (§15.12.2.2) then ei is compatible in a loose invocation context with Ti[R1:=U1, ..., Rp:=Up].

    • For 1 l p, Ul <: Bl[R1:=U1, ..., Rp:=Up].

  • If m is not a generic method, then m is applicable by variable arity invocation if, for 1 i k, either ei is compatible in a loose invocation context with Ti or ei is not pertinent to applicability (§15.12.2.2).

If no method applicable by variable arity invocation is found, a compile-time error occurs.

Otherwise, the most specific method (§15.12.2.5) is chosen among the methods applicable by variable arity invocation.

15.12.2.5. Choosing the Most Specific Method

If more than one member method is both accessible and applicable to a method invocation, it is necessary to choose one to provide the descriptor for the run-time method dispatch. The Java programming language uses the rule that the most specific method is chosen.

The informal intuition is that one method is more specific than another if any invocation handled by the first method could be passed on to the other one without a compile-time error. In cases such as an explicitly typed lambda expression argument (§15.27.1) or a variable arity invocation (§15.12.2.4), some flexibility is allowed to adapt one signature to the other.

One applicable method m1 is more specific than another applicable method m2, for an invocation with argument expressions e1, ..., ek, if any of the following are true:

  • m2 is generic and m1 is inferred to be more specific than m2 for argument expressions e1, ..., ek by §18.5.4.

  • m2 is not generic, m1 and m2 are applicable by strict or loose invocation, and where m1 has formal parameter types S1, ..., Sn and m2 has formal parameter types T1, ..., Tn, the type Si is more specific than Ti for argument ei for all i (1 i n, n = k).

  • m2 is not generic, m1 and m2 are applicable by variable arity invocation, and where the first k variable arity parameter types of m1 are S1, ..., Sk and the first k variable arity parameter types of m2 are T1, ..., Tk, the type Si is more specific than Ti for argument ei for all i (1 i k). Additionally, if m2 has k+1 parameters, then the k+1'th variable arity parameter type of m1 is a subtype of the k+1'th variable arity parameter type of m2.

The above conditions are the only circumstances under which one method may be more specific than another.

A type S is more specific than a type T for any expression if S <: T (§4.10).

A functional interface type S is more specific than a functional interface type T for an expression e if T is not a subtype of S and one of the following is true (where U1 ... Uk and R1 are the parameter types and return type of the function type of the capture of S, and V1 ... Vk and R2 are the parameter types and return type of the function type of T):

  • If e is an explicitly typed lambda expression (§15.27.1), then one of the following is true:

    • R2 is void.

    • R1 <: R2.

    • R1 and R2 are functional interface types, and R1 is more specific than R2 for each result expression of e.

      The result expression of a lambda expression with a block body is defined in §15.27.2; the result expression of a lambda expression with an expression body is simply the body itself.

    • R1 is a primitive type, R2 is a reference type, and each result expression of e is a standalone expression (§15.2) of a primitive type.

    • R1 is a reference type, R2 is a primitive type, and each result expression of e is either a standalone expression of a reference type or a poly expression.

  • If e is an exact method reference expression (§15.13.1), then i) for all i (1 i k), Ui is the same as Vi, and ii) one of the following is true:

    • R2 is void.

    • R1 <: R2.

    • R1 is a primitive type, R2 is a reference type, and the compile-time declaration for the method reference has a return type which is a primitive type.

    • R1 is a reference type, R2 is a primitive type, and the compile-time declaration for the method reference has a return type which is a reference type.

  • If e is a parenthesized expression, then one of these conditions applies recursively to the contained expression.

  • If e is a conditional expression, then for each of the second and third operands, one of these conditions applies recursively.

A method m1 is strictly more specific than another method m2 if and only if m1 is more specific than m2 and m2 is not more specific than m1.

A method is said to be maximally specific for a method invocation if it is accessible and applicable and there is no other method that is applicable and accessible that is strictly more specific.

If there is exactly one maximally specific method, then that method is in fact the most specific method; it is necessarily more specific than any other accessible method that is applicable. It is then subjected to some further compile-time checks as specified in §15.12.3.

It is possible that no method is the most specific, because there are two or more methods that are maximally specific. In this case:

  • If all the maximally specific methods have override-equivalent signatures (§8.4.2), then:

    • If exactly one of the maximally specific methods is concrete (that is, non-abstract or default), it is the most specific method.

    • Otherwise, if all the maximally specific methods are abstract or default, and the signatures of all of the maximally specific methods have the same erasure (§4.6), then the most specific method is chosen arbitrarily among the subset of the maximally specific methods that have the most specific return type.

      In this case, the most specific method is considered to be abstract. Also, the most specific method is considered to throw a checked exception if and only if that exception or its erasure is declared in the throws clauses of each of the maximally specific methods.

  • Otherwise, the method invocation is ambiguous, and a compile-time error occurs.

15.12.2.6. Method Invocation Type

The invocation type of a most specific accessible and applicable method is a method type (§8.2) expressing the target types of the invocation arguments, the result (return type or void) of the invocation, and the exception types of the invocation. It is determined as follows:

  • If the chosen method is generic and the method invocation does not provide explicit type arguments, the invocation type is inferred as specified in §18.5.2.

  • If the chosen method is generic and the method invocation provides explicit type arguments, let Pi be the type parameters of the method and let Ti be the explicit type arguments provided for the method invocation (1 i p). Then:

    • If unchecked conversion was necessary for the method to be applicable, then the invocation type's parameter types are obtained by applying the substitution [P1:=T1, ..., Pp:=Tp] to the parameter types of the method's type, and the invocation type's return type and thrown types are given by the erasure of the return type and thrown types of the method's type.

    • If unchecked conversion was not necessary for the method to be applicable, then the invocation type is obtained by applying the substitution [P1:=T1, ..., Pp:=Tp] to the method's type.

  • If the chosen method is not generic, then:

    • If unchecked conversion was necessary for the method to be applicable, the parameter types of the invocation type are the parameter types of the method's type, and the return type and thrown types are given by the erasures of the return type and thrown types of the method's type.

    • Otherwise, if the chosen method is the getClass method of the class Object (§4.3.2), the invocation type is the same as the method's type, except that the return type is Class<? extends T>, where T is the type that was searched, as determined by §15.12.1.

    • Otherwise, the invocation type is the same as the method's type.

15.12.3. Compile-Time Step 3: Is the Chosen Method Appropriate?

If there is a most specific method declaration for a method invocation, it is called the compile-time declaration for the method invocation.

It is a compile-time error if an argument to a method invocation is not compatible with its target type, as derived from the invocation type of the compile-time declaration.

If the compile-time declaration is applicable by variable arity invocation, then where the last formal parameter type of the invocation type of the method is Fn[], it is a compile-time error if the type which is the erasure of Fn is not accessible at the point of invocation.

If the compile-time declaration is void, then the method invocation must be a top level expression (that is, the Expression in an expression statement or in the ForInit or ForUpdate part of a for statement), or a compile-time error occurs. Such a method invocation produces no value and so must be used only in a situation where a value is not needed.

In addition, whether the compile-time declaration is appropriate may depend on the form of the method invocation expression before the left parenthesis, as follows:

  • If the form is MethodName - that is, just an Identifier - and the compile-time declaration is an instance method, then:

    • It is a compile-time error if the method invocation occurs in a static context (§8.1.3).

    • Otherwise, let C be the immediately enclosing class of which the compile-time declaration is a member. If the method invocation is not directly enclosed by C or an inner class of C, then a compile-time error occurs.

  • If the form is TypeName . [TypeArguments] Identifier, then the compile-time declaration must be static, or a compile-time error occurs.

  • If the form is ExpressionName . [TypeArguments] Identifier or Primary . [TypeArguments] Identifier, then the compile-time declaration must not be a static method declared in an interface, or a compile-time error occurs.

  • If the form is super . [TypeArguments] Identifier, then:

    • It is a compile-time error if the compile-time declaration is abstract.

    • It is a compile-time error if the method invocation occurs in a static context.

  • If the form is TypeName . super . [TypeArguments] Identifier, then:

    • It is a compile-time error if the compile-time declaration is abstract.

    • It is a compile-time error if the method invocation occurs in a static context.

    • If TypeName denotes a class C, then if the method invocation is not directly enclosed by C or an inner class of C, a compile-time error occurs.

    • If TypeName denotes an interface, let T be the type declaration immediately enclosing the method invocation. A compile-time error occurs if there exists a method, distinct from the compile-time declaration, that overrides (§9.4.1) the compile-time declaration from a direct superclass or direct superinterface of T.

      In the case that a superinterface overrides a method declared in a grandparent interface, this rule prevents the child interface from "skipping" the override by simply adding the grandparent to its list of direct superinterfaces. The appropriate way to access functionality of a grandparent is through the direct superinterface, and only if that interface chooses to expose the desired behavior. (Alternately, the developer is free to define his own additional superinterface that exposes the desired behavior with a super method invocation.)

The compile-time parameter types and compile-time result are determined as follows:

  • If the compile-time declaration for the method invocation is not a signature polymorphic method, then the compile-time parameter types are the types of the formal parameters of the compile-time declaration, and the compile-time result is the result chosen for the compile-time declaration (§15.12.2.6).

  • If the compile-time declaration for the method invocation is a signature polymorphic method, then:

    • The compile-time parameter types are the static types of the actual argument expressions. An argument expression which is the null literal null (§3.10.7) is treated as having the static type Void.

    • The compile-time result is determined as follows:

      • If the method invocation expression is an expression statement, the compile-time result is void.

      • Otherwise, if the method invocation expression is the operand of a cast expression (§15.16), the compile-time result is the erasure of the type of the cast expression (§4.6).

      • Otherwise, the compile-time result is the signature polymorphic method's declared return type, Object.

A method is signature polymorphic if all of the following are true:

  • It is declared in the java.lang.invoke.MethodHandle class.

  • It takes a single variable arity parameter (§8.4.1) whose declared type is Object[].

  • It has a return type of Object.

  • It is native.

In Java SE 8, the only signature polymorphic methods are the invoke and invokeExact methods of the class java.lang.invoke.MethodHandle.

The following compile-time information is then associated with the method invocation for use at run time:

  • The name of the method.

  • The qualifying type of the method invocation (§13.1).

  • The number of parameters and the compile-time parameter types, in order.

  • The compile-time result, or void.

  • The invocation mode, computed as follows:

    • If the qualifying type of the method declaration is a class, then:

      • If the compile-time declaration has the static modifier, then the invocation mode is static.

      • Otherwise, if the compile-time declaration has the private modifier, then the invocation mode is nonvirtual.

      • Otherwise, if the part of the method invocation before the left parenthesis is of the form super . Identifier or of the form TypeName . super . Identifier, then the invocation mode is super.

      • Otherwise, the invocation mode is virtual.

    • If the qualifying type of the method invocation is an interface, then the invocation mode is interface.

If the result of the invocation type of the compile-time declaration is not void, then the type of the method invocation expression is obtained by applying capture conversion (§5.1.10) to the return type of the invocation type of the compile-time declaration.

15.12.4. Run-Time Evaluation of Method Invocation

At run time, method invocation requires five steps. First, a target reference may be computed. Second, the argument expressions are evaluated. Third, the accessibility of the method to be invoked is checked. Fourth, the actual code for the method to be executed is located. Fifth, a new activation frame is created, synchronization is performed if necessary, and control is transferred to the method code.

15.12.4.1. Compute Target Reference (If Necessary)

There are six cases to consider, depending on the form of the method invocation:

  • If the form is MethodName - that is, just an Identifier - then:

    • If the invocation mode is static, then there is no target reference.

    • Otherwise, let T be the enclosing type declaration of which the method is a member, and let n be an integer such that T is the n'th lexically enclosing type declaration of the class whose declaration immediately contains the method invocation. The target reference is the n'th lexically enclosing instance of this.

      It is a compile-time error if the n'th lexically enclosing instance of this does not exist.

  • If the form is TypeName . [TypeArguments] Identifier, then there is no target reference.

  • If form is ExpressionName . [TypeArguments] Identifier, then:

    • If the invocation mode is static, then there is no target reference. The ExpressionName is evaluated, but the result is then discarded.

    • Otherwise, the target reference is the value denoted by ExpressionName.

  • If the form is Primary . [TypeArguments] Identifier involved, then:

    • If the invocation mode is static, then there is no target reference. The Primary expression is evaluated, but the result is then discarded.

    • Otherwise, the Primary expression is evaluated and the result is used as the target reference.

    In either case, if the evaluation of the Primary expression completes abruptly, then no part of any argument expression appears to have been evaluated, and the method invocation completes abruptly for the same reason.

  • If the form is super . [TypeArguments] Identifier, then the target reference is the value of this.

  • If the form is TypeName . super . [TypeArguments] Identifier, then if TypeName denotes a class, the target reference is the value of TypeName.this; otherwise, the target reference is the value of this.

Example 15.12.4.1-1. Target References and static Methods

When a target reference is computed and then discarded because the invocation mode is static, the reference is not examined to see whether it is null:

class Test1 {
    static void mountain() {
        System.out.println("Monadnock");
    }
    static Test1 favorite(){
        System.out.print("Mount ");
        return null;
    }
    public static void main(String[] args) {
        favorite().mountain();
    }
}

which prints:

Mount Monadnock

Here favorite() returns null, yet no NullPointerException is thrown.


Example 15.12.4.1-2. Evaluation Order During Method Invocation

As part of an instance method invocation (§15.12), there is an expression that denotes the object to be invoked. This expression appears to be fully evaluated before any part of any argument expression to the method invocation is evaluated.

So, for example, in:

class Test2 {
    public static void main(String[] args) {
        String s = "one";
        if (s.startsWith(s = "two"))
            System.out.println("oops");
    }
}

the occurrence of s before ".startsWith" is evaluated first, before the argument expression s = "two". Therefore, a reference to the string "one" is remembered as the target reference before the local variable s is changed to refer to the string "two". As a result, the startsWith method is invoked for target object "one" with argument "two", so the result of the invocation is false, as the string "one" does not start with "two". It follows that the test program does not print "oops".


15.12.4.2. Evaluate Arguments

The process of evaluating the argument list differs, depending on whether the method being invoked is a fixed arity method or a variable arity method (§8.4.1).

If the method being invoked is a variable arity method m, it necessarily has n > 0 formal parameters. The final formal parameter of m necessarily has type T[] for some T, and m is necessarily being invoked with k 0 actual argument expressions.

If m is being invoked with k n actual argument expressions, or, if m is being invoked with k = n actual argument expressions and the type of the k'th argument expression is not assignment compatible with T[], then the argument list (e1, ..., en-1, en, ..., ek) is evaluated as if it were written as (e1, ..., en-1, new |T[]| { en, ..., ek }), where |T[]| denotes the erasure (§4.6) of T[].

The preceding paragraphs are crafted to handle the interaction of parameterized types and array types that occurs in a Java Virtual Machine with erased generics. Namely, if the element type T of the variable array parameter is non-reifiable, e.g. List<String>, then special care must be taken with the array creation expression (§15.10) because the created array's element type must be reifiable. By erasing the array type of the final expression in the argument list, we are guaranteed to obtain a reifiable element type. Then, since the array creation expression appears in an invocation context (§5.3), an unchecked conversion is possible from the array type with reifiable element type to an array type with non-reifiable element type, specifically that of the variable arity parameter. A Java compiler is required to give a compile-time unchecked warning at this conversion. Oracle's reference implementation of a Java compiler identifies the unchecked warning here as a more informative unchecked generic array creation.

The argument expressions (possibly rewritten as described above) are now evaluated to yield argument values. Each argument value corresponds to exactly one of the method's n formal parameters.

The argument expressions, if any, are evaluated in order, from left to right. If the evaluation of any argument expression completes abruptly, then no part of any argument expression to its right appears to have been evaluated, and the method invocation completes abruptly for the same reason. The result of evaluating the j'th argument expression is the j'th argument value, for 1 j n. Evaluation then continues, using the argument values, as described below.

15.12.4.3. Check Accessibility of Type and Method

Let C be the class containing the method invocation, and let T be the qualifying type of the method invocation (§13.1), and let m be the name of the method as determined at compile time (§15.12.3).

An implementation of the Java programming language must ensure, as part of linkage, that the method m still exists in the type T. If this is not true, then a NoSuchMethodError (which is a subclass of IncompatibleClassChangeError) occurs.

If the invocation mode is interface, then the implementation must also check that the target reference type still implements the specified interface. If the target reference type does not still implement the interface, then an IncompatibleClassChangeError occurs.

The implementation must also ensure, during linkage, that the type T and the method m are accessible:

  • For the type T:

    • If T is in the same package as C, then T is accessible.

    • If T is in a different package than C, and T is public, then T is accessible.

    • If T is in a different package than C, and T is protected, then T is accessible if and only if C is a subclass of T.

  • For the method m:

    • If m is public, then m is accessible. (All members of interfaces are public (§9.2).)

    • If m is protected, then m is accessible if and only if either T is in the same package as C, or C is T or a subclass of T.

    • If m has package access, then m is accessible if and only if T is in the same package as C.

    • If m is private, then m is accessible if and only if C is T, or C encloses T, or T encloses C, or T and C are both enclosed by a third class.

If either T or m is not accessible, then an IllegalAccessError occurs (§12.3).

15.12.4.4. Locate Method to Invoke

The strategy for method lookup depends on the invocation mode.

If the invocation mode is static, no target reference is needed and overriding is not allowed. Method m of class T is the one to be invoked.

Otherwise, an instance method is to be invoked and there is a target reference. If the target reference is null, a NullPointerException is thrown at this point. Otherwise, the target reference is said to refer to a target object and will be used as the value of the keyword this in the invoked method. The other four possibilities for the invocation mode are then considered.

If the invocation mode is nonvirtual, overriding is not allowed. Method m of class T is the one to be invoked.

Otherwise, if the invocation mode is virtual, and T and m jointly indicate a signature polymorphic method (§15.12.3), then the target object is an instance of java.lang.invoke.MethodHandle. The method handle encapsulates a type which is matched against the information associated with the method invocation at compile time (§15.12.3). Details of this matching are given in The Java Virtual Machine Specification, Java SE 8 Edition and the Java SE platform API. If matching succeeds, the target method encapsulated by the method handle is directly and immediately invoked, and the procedure in §15.12.4.5 is not executed.

Otherwise, the invocation mode is interface, virtual, or super, and overriding may occur. A dynamic method lookup is used. The dynamic lookup process starts from a class S, determined as follows:

  • If the invocation mode is interface or virtual, then S is initially the actual run-time class R of the target object.

    This is true even if the target object is an array instance. (Note that for invocation mode interface, R necessarily implements T; for invocation mode virtual, R is necessarily either T or a subclass of T.)

  • If the invocation mode is super, then S is initially the qualifying type (§13.1) of the method invocation.

The dynamic method lookup uses the following procedure to search class S, and then the superclasses and superinterfaces of class S, as necessary, for method m.

Let X be the compile-time type of the target reference of the method invocation. Then:

  1. If class S contains a declaration for a method named m with the same descriptor (same number of parameters, the same parameter types, and the same return type) required by the method invocation as determined at compile time (§15.12.3), then:

    • If the invocation mode is super or interface, then this is the method to be invoked, and the procedure terminates.

    • If the invocation mode is virtual, and the declaration in S overrides X.m (§8.4.8.1), then the method declared in S is the method to be invoked, and the procedure terminates.

  2. Otherwise, if S has a superclass, the lookup procedure of steps 1 and 2 is performed recursively using the direct superclass of S in place of S; the method to be invoked, if any, is the result of the recursive invocation of this lookup procedure.

  3. If no method is found by the previous two steps, the superinterfaces of S are searched for a suitable method.

    A set of candidate methods is considered with the following properties: i) each method is declared in a (direct or indirect) superinterface of S; ii) each method has the name and descriptor required by the method invocation; iii) each method is non-static; iv) for each method, where the method's declaring interface is I, there is no other method satisfying (i) through (iii) that is declared in a subinterface of I.

    If this set contains a default method, one such method is the method to be invoked. Otherwise, an abstract method in the set is selected as the method to be invoked.

Dynamic method lookup may cause the following errors to occur:

  • If the method to be invoked is abstract, an AbstractMethodError is thrown.

  • If the method to be invoked is default, and more than one default method appears in the set of candidates in step 3 above, an IncompatibleClassChangeError is thrown.

  • If the invocation mode is interface and the selected method is not public, an IllegalAccessError is thrown.

The above procedure (if it terminates without error) will find a non-abstract, accessible method to invoke, provided that all classes and interfaces in the program have been consistently compiled. However, if this is not the case, then various errors may occur, as specified above; additional details about the behavior of the Java Virtual Machine under these circumstances are given by The Java Virtual Machine Specification, Java SE 8 Edition.

The dynamic lookup process, while described here explicitly, will often be implemented implicitly, for example as a side-effect of the construction and use of per-class method dispatch tables, or the construction of other per-class structures used for efficient dispatch.

Example 15.12.4.4-1. Overriding and Method Invocation

class Point {
    final int EDGE = 20;
    int x, y;
    void move(int dx, int dy) {
        x += dx; y += dy;
        if (Math.abs(x) >= EDGE || Math.abs(y) >= EDGE)
            clear();
    }
    void clear() {
        System.out.println("\tPoint clear");
        x = 0; y = 0;
    }
}
class ColoredPoint extends Point {
    int color;
    void clear() {
        System.out.println("\tColoredPoint clear");
        super.clear();
        color = 0;
    }
}

Here, the subclass ColoredPoint extends the clear abstraction defined by its superclass Point. It does so by overriding the clear method with its own method, which invokes the clear method of its superclass, using the form super.clear().

This method is then invoked whenever the target object for an invocation of clear is a ColoredPoint. Even the method move in Point invokes the clear method of class ColoredPoint when the class of this is ColoredPoint, as shown by the output of this test program:

class Test1 {
    public static void main(String[] args) {
        Point p = new Point();
        System.out.println("p.move(20,20):");
        p.move(20, 20);

        ColoredPoint cp = new ColoredPoint();
        System.out.println("cp.move(20,20):");
        cp.move(20, 20);

        p = new ColoredPoint();
        System.out.println("p.move(20,20), p colored:");
        p.move(20, 20);
    }
}

which is:

p.move(20,20):
        Point clear
cp.move(20,20):
        ColoredPoint clear
        Point clear
p.move(20,20), p colored:
        ColoredPoint clear
        Point clear

Overriding is sometimes called "late-bound self-reference"; in this example it means that the reference to clear in the body of Point.move (which is really syntactic shorthand for this.clear) invokes a method chosen "late" (at run time, based on the run-time class of the object referenced by this) rather than a method chosen "early" (at compile time, based only on the type of this). This provides the programmer a powerful way of extending abstractions and is a key idea in object-oriented programming.


Example 15.12.4.4-2. Method Invocation Using super

An overridden instance method of a superclass may be accessed by using the keyword super to access the members of the immediate superclass, bypassing any overriding declaration in the class that contains the method invocation.

When accessing an instance variable, super means the same as a cast of this (§15.11.2), but this equivalence does not hold true for method invocation. This is demonstrated by the example:

class T1 {
    String s() { return "1"; }
}
class T2 extends T1 {
    String s() { return "2"; }
}
class T3 extends T2 {
    String s() { return "3"; }
    void test() {
        System.out.println("s()=\t\t"          + s());
        System.out.println("super.s()=\t"      + super.s());
        System.out.println("((T2)this).s()=\t" + ((T2)this).s());
        System.out.println("((T1)this).s()=\t" + ((T1)this).s());
    }
}
class Test2 {
    public static void main(String[] args) {
        T3 t3 = new T3();
        t3.test();
    }
}

which produces the output:

s()=            3
super.s()=      2
((T2)this).s()= 3
((T1)this).s()= 3

The casts to types T1 and T2 do not change the method that is invoked, because the instance method to be invoked is chosen according to the run-time class of the object referred to by this. A cast does not change the class of an object; it only checks that the class is compatible with the specified type.


15.12.4.5. Create Frame, Synchronize, Transfer Control

A method m in some class S has been identified as the one to be invoked.

Now a new activation frame is created, containing the target reference (if any) and the argument values (if any), as well as enough space for the local variables and stack for the method to be invoked and any other bookkeeping information that may be required by the implementation (stack pointer, program counter, reference to previous activation frame, and the like). If there is not sufficient memory available to create such an activation frame, a StackOverflowError is thrown.

The newly created activation frame becomes the current activation frame. The effect of this is to assign the argument values to corresponding freshly created parameter variables of the method, and to make the target reference available as this, if there is a target reference. Before each argument value is assigned to its corresponding parameter variable, it is subjected to invocation conversion (§5.3), which includes any required value set conversion (§5.1.13).

If the erasure (§4.6) of the type of the method being invoked differs in its signature from the erasure of the type of the compile-time declaration for the method invocation (§15.12.3), then if any of the argument values is an object which is not an instance of a subclass or subinterface of the erasure of the corresponding formal parameter type in the compile-time declaration for the method invocation, then a ClassCastException is thrown.

If the method m is a native method but the necessary native, implementation-dependent binary code has not been loaded or otherwise cannot be dynamically linked, then an UnsatisfiedLinkError is thrown.

If the method m is not synchronized, control is transferred to the body of the method m to be invoked.

If the method m is synchronized, then an object must be locked before the transfer of control. No further progress can be made until the current thread can obtain the lock. If there is a target reference, then the target object must be locked; otherwise the Class object for class S, the class of the method m, must be locked. Control is then transferred to the body of the method m to be invoked. The object is automatically unlocked when execution of the body of the method has completed, whether normally or abruptly. The locking and unlocking behavior is exactly as if the body of the method were embedded in a synchronized statement (§14.19).

Example 15.12.4.5-1. Invoked Method Signature Has Different Erasure Than Compile-Time Method Signature

Consider the declarations:

abstract class C<T> {
    abstract T id(T x);
}
class D extends C<String> {
    String id(String x) { return x; }
}

Now, given an invocation:

C c = new D();
c.id(new Object());  // fails with a ClassCastException

The erasure of the actual method being invoked, D.id(), differs in its signature from that of the compile-time method declaration, C.id(). The former takes an argument of type String while the latter takes an argument of type Object. The invocation fails with a ClassCastException before the body of the method is executed.

Such situations can only arise if the program gives rise to a compile-time unchecked warning (§4.8, §5.1.9, §5.5.2, §8.4.1, §8.4.8.3, §8.4.8.4, §9.4.1.2, §15.12.4.2).

Implementations can enforce these semantics by creating bridge methods. In the above example, the following bridge method would be created in class D:

Object id(Object x) { return id((String) x); }

This is the method that would actually be invoked by the Java Virtual Machine in response to the call c.id(new Object()) shown above, and it will execute the cast and fail, as required.


15.13. Method Reference Expressions

A method reference expression is used to refer to the invocation of a method without actually performing the invocation. Certain forms of method reference expression also allow class instance creation (§15.9) or array creation (§15.10) to be treated as if it were a method invocation.

If TypeArguments is present to the right of ::, then it is a compile-time error if any of the type arguments are wildcards (§4.5.1).

If a method reference expression has the form ExpressionName :: [TypeArguments] Identifier or Primary :: [TypeArguments] Identifier, it is a compile-time error if the type of the ExpressionName or Primary is not a reference type.

If a method reference expression has the form super :: [TypeArguments] Identifier, let T be the type declaration immediately enclosing the method reference expression. It is a compile-time error if T is the class Object or T is an interface.

If a method reference expression has the form TypeName . super :: [TypeArguments] Identifier, then:

  • If TypeName denotes a class, C, then it is a compile-time error if C is not a lexically enclosing class of the current class, or if C is the class Object.

  • If TypeName denotes an interface, I, then let T be the type declaration immediately enclosing the method reference expression. It is a compile-time error if I is not a direct superinterface of T, or if there exists some other direct superclass or direct superinterface of T, J, such that J is a subtype of I.

  • If TypeName denotes a type variable, then a compile-time error occurs.

If a method reference expression has the form super :: [TypeArguments] Identifier or TypeName . super :: [TypeArguments] Identifier, it is a compile-time error if the expression occurs in a static context.

If a method reference expression has the form ClassType :: [TypeArguments] new, then:

  • ClassType must denote a class that is accessible, non-abstract, and not an enum type, or a compile-time error occurs.

  • If ClassType denotes a parameterized type (§4.5), then it is a compile-time error if any of its type arguments are wildcards.

  • If ClassType denotes a raw type (§4.8), then it is a compile-time error if TypeArguments is present after the ::.

If a method reference expression has the form ArrayType :: new, then ArrayType must denote a type that is reifiable (§4.7), or a compile-time error occurs.

The target reference of an instance method (§15.12.4.1) may be provided by the method reference expression using an ExpressionName, a Primary, or super, or it may be provided later when the method is invoked. The immediately enclosing instance of a new inner class instance (§15.9.2) must be provided by a lexically enclosing instance of this (§8.1.3).

When more than one member method of a type has the same name, or when a class has more than one constructor, the appropriate method or constructor is selected based on the functional interface type targeted by the expression, as specified in §15.13.1.

If a method or constructor is generic, the appropriate type arguments may either be inferred or provided explicitly. Similarly, the type arguments of a generic type mentioned by the method reference expression may be provided explicitly or inferred.

Method reference expressions are always poly expressions (§15.2).

It is a compile-time error if a method reference expression occurs in a program in someplace other than an assignment context (§5.2), an invocation context (§5.3), or a casting context (§5.5).

Evaluation of a method reference expression produces an instance of a functional interface type (§9.8). Method reference evaluation does not cause the execution of the corresponding method; instead, this may occur at a later time when an appropriate method of the functional interface is invoked.

Here are some method reference expressions, first with no target reference and then with a target reference:

String::length             // instance method
System::currentTimeMillis  // static method
List<String>::size  // explicit type arguments for generic type
List::size          // inferred type arguments for generic type
int[]::clone
T::tvarMember

System.out::println
"abc"::length
foo[x]::bar
(test ? list.replaceAll(String::trim) : list) :: iterator
super::toString

Here are some more method reference expressions:

String::valueOf       // overload resolution needed     
Arrays::sort          // type arguments inferred from context
Arrays::<String>sort  // explicit type arguments

Here are some method reference expressions that represent a deferred creation of an object or an array:

ArrayList<String>::new     // constructor for parameterized type
ArrayList::new             // inferred type arguments
                           // for generic class
Foo::<Integer>new          // explicit type arguments
                           // for generic constructor
Bar<String>::<Integer>new  // generic class, generic constructor
Outer.Inner::new           // inner class constructor
int[]::new                 // array creation

It is not possible to specify a particular signature to be matched, for example, Arrays::sort(int[]). Instead, the functional interface provides argument types that are used as input to the overload resolution algorithm (§15.12.2). This should satisfy the vast majority of use cases; when the rare need arises for more precise control, a lambda expression can be used.

The use of type argument syntax in the class name before a delimiter (List<String>::size) raises the parsing problem of distinguishing between < as a type argument bracket and < as a less-than operator. In theory, this is no worse than allowing type arguments in cast expressions; however, the difference is that the cast case only comes up when a ( token is encountered; with the addition of method reference expressions, the start of every expression is potentially a parameterized type.

15.13.1. Compile-Time Declaration of a Method Reference

The compile-time declaration of a method reference is the method to which the expression refers. In special cases, the compile-time declaration does not actually exist, but is a notional method that represents a class instance creation or an array creation. The choice of compile-time declaration depends on a function type targeted by the expression, just as the compile-time declaration of a method invocation depends on the invocation's arguments (§15.12).

The search for a compile-time declaration mirrors the process for method invocations in §15.12.1 and §15.12.2, as follows:

  • First, a type to search is determined:

    • If the method reference expression has the form ExpressionName :: [TypeArguments] Identifier or Primary :: [TypeArguments] Identifier, the type to search is the type of the expression preceding the :: token.

    • If the method reference expression has the form ReferenceType :: [TypeArguments] Identifier, the type to search is the result of capture conversion (§5.1.10) applied to ReferenceType.

    • If the method reference expression has the form super :: [TypeArguments] Identifier, the type to search is the superclass type of the class whose declaration contains the method reference.

    • If the method reference expression has the form TypeName . super :: [TypeArguments] Identifier, then if TypeName denotes a class, the type to search is the superclass type of the named class; otherwise, TypeName denotes an interface, and the corresponding superinterface type of the class or interface whose declaration contains the method reference is the type to search.

    • For the two other forms (involving :: new), the referenced method is notional and there is no type to search.

  • Second, given a targeted function type with n parameters, a set of potentially applicable methods is identified:

    • If the method reference expression has the form ReferenceType :: [TypeArguments] Identifier, the potentially applicable methods are the member methods of the type to search that have an appropriate name (given by Identifier), accessibility, arity (n or n-1), and type argument arity (derived from [TypeArguments]), as specified in §15.12.2.1.

      Two different arities, n and n-1, are considered, to account for the possibility that this form refers to either a static method or an instance method.

    • If the method reference expression has the form ClassType :: [TypeArguments] new, the potentially applicable methods are a set of notional methods corresponding to the constructors of ClassType.

      If ClassType is a raw type, but is not a non-static member type of a raw type, the candidate notional member methods are those specified in §15.9.3 for a class instance creation expression that uses <> to elide the type arguments to a class.

      Otherwise, the candidate notional member methods are the constructors of ClassType, treated as if they were methods with return type ClassType. Among these candidates, the methods with appropriate accessibility, arity (n), and type argument arity (derived from [TypeArguments]) are selected, as specified in §15.12.2.1.

    • If the method reference expression has the form ArrayType :: new, a single notional method is considered. The method has a single parameter of type int, returns the ArrayType, and has no throws clause. If n = 1, this is the only potentially applicable method; otherwise, there are no potentially applicable methods.

    • For all other forms, the potentially applicable methods are the member methods of the type to search that have an appropriate name (given by Identifier), accessibility, arity (n), and type argument arity (derived from [TypeArguments]), as specified in §15.12.2.1.

  • Finally, if there are no potentially applicable methods, then there is no compile-time declaration.

    Otherwise, given a targeted function type with parameter types P1, ..., Pn and a set of potentially applicable methods, the compile-time declaration is selected as follows:

    • If the method reference expression has the form ReferenceType :: [TypeArguments] Identifier, then two searches for a most specific applicable method are performed. Each search is as specified in §15.12.2.2 through §15.12.2.5, with the clarifications below. Each search may produce a method or, in the case of an error as specified in §15.12.2.2 through §15.12.2.5, no result.

      In the first search, the method reference is treated as if it were an invocation with argument expressions of types P1, ..., Pn; the type arguments, if any, are given by the method reference expression.

      In the second search, if P1, ..., Pn is not empty and P1 is a subtype of ReferenceType, then the method reference expression is treated as if it were a method invocation expression with argument expressions of types P2, ..., Pn. If ReferenceType is a raw type, and there exists a parameterization of this type, G<...>, that is a supertype of P1, the type to search is the result of capture conversion (§5.1.10) applied to G<...>; otherwise, the type to search is the same as the type of the first search. Again, the type arguments, if any, are given by the method reference expression.

      If the first search produces a static method, and no non-static method is applicable by §15.12.2.2, §15.12.2.3, or §15.12.2.4 during the second search, then the compile-time declaration is the result of the first search.

      Otherwise, if no static method is applicable by §15.12.2.2, §15.12.2.3, or §15.12.2.4 during the first search, and the second search produces a non-static method, then the compile-time declaration is the result of the second search.

      Otherwise, there is no compile-time declaration.

    • For all other forms of method reference expression, one search for a most specific applicable method is performed. The search is as specified in §15.12.2.2 through §15.12.2.5, with the clarifications below.

      The method reference is treated as if it were an invocation with argument expressions of types P1, ..., Pn; the type arguments, if any, are given by the method reference expression.

      If the search results in an error as specified in §15.12.2.2 through §15.12.2.5, or if the most specific applicable method is static, there is no compile-time declaration.

      Otherwise, the compile-time declaration is the most specific applicable method.

It is a compile-time error if a method reference expression has the form ReferenceType :: [TypeArguments] Identifier, and the compile-time declaration is static, and ReferenceType is not a simple or qualified name (§6.2).

It is a compile-time error if the method reference expression has the form super :: [TypeArguments] Identifier or TypeName . super :: [TypeArguments] Identifier, and the compile-time declaration is abstract.

It is a compile-time error if the method reference expression has the form TypeName . super :: [TypeArguments] Identifier, and TypeName denotes an interface, and there exists a method, distinct from the compile-time declaration, that overrides (§8.4.8, §9.4.1) the compile-time declaration from a direct superclass or direct superinterface of the type whose declaration immediately encloses the method reference expression.

It is a compile-time error if the method reference expression is of the form ClassType :: [TypeArguments] new and a compile-time error would occur when determining an enclosing instance for ClassType as specified in §15.9.2 (treating the method reference expression as if it were a class instance creation expression).

A method reference expression of the form ReferenceType :: [TypeArguments] Identifier can be interpreted in different ways. If Identifier refers to an instance method, then the implicit lambda expression has an extra parameter compared to if Identifier refers to a static method. It is possible for ReferenceType to have both kinds of applicable methods, so the search algorithm described above identifies them separately, since there are different parameter types for each case.

An example of ambiguity is:

interface Fun<T,R> { R apply(T arg); }

class C {
    int size() { return 0; }
    static int size(Object arg) { return 0; }

    void test() {
        Fun<C, Integer> f1 = C::size;
          // Error: instance method size()
          // or static method size(Object)?
    }
}

This ambiguity cannot be resolved by providing an applicable instance method which is more specific than an applicable static method:

interface Fun<T,R> { R apply(T arg); }

class C {
    int size() { return 0; }
    static int size(Object arg) { return 0; }
    int size(C arg) { return 0; }

    void test() {
        Fun<C, Integer> f1 = C::size;
          // Error: instance method size()
          // or static method size(Object)?
    }
}

The search is smart enough to ignore ambiguities in which all the applicable methods (from both searches) are instance methods:

interface Fun<T,R> { R apply(T arg); }

class C {
    int size() { return 0; }
    int size(Object arg) { return 0; }
    int size(C arg) { return 0; }

    void test() {
        Fun<C, Integer> f1 = C::size;
          // OK: reference is to instance method size()
    }
}

For convenience, when the name of a generic type is used to refer to an instance method (where the receiver becomes the first parameter), the target type is used to determine the type arguments. This facilitates usage like Pair::first in place of Pair<String,Integer>::first. Similarly, a method reference like Pair::new is treated like a "diamond" instance creation (new Pair<>()). Because the "diamond" is implicit, this form does not instantiate a raw type; in fact, there is no way to express a reference to the constructor of a raw type.

For some method reference expressions, there is only one possible compile-time declaration with only one possible invocation type (§15.12.2.6), regardless of the targeted function type. Such method reference expressions are said to be exact. A method reference expression that is not exact is said to be inexact.

A method reference expression ending with Identifier is exact if it satisfies all of the following:

  • If the method reference expression has the form ReferenceType :: [TypeArguments] Identifier, then ReferenceType does not denote a raw type.

  • The type to search has exactly one member method with the name Identifier that is accessible to the class or interface in which the method reference expression appears.

  • This method is not variable arity (§8.4.1).

  • If this method is generic (§8.4.4), then the method reference expression provides TypeArguments.

A method reference expression of the form ClassType :: [TypeArguments] new is exact if it satisfies all of the following:

  • The type denoted by ClassType is not raw, or is a non-static member type of a raw type.

  • The type denoted by ClassType has exactly one constructor that is accessible to the class or interface in which the method reference expression appears.

  • This constructor is not variable arity.

  • If this constructor is generic, then the method reference expression provides TypeArguments.

A method reference expression of the form ArrayType :: new is always exact.

15.13.2. Type of a Method Reference

A method reference expression is compatible in an assignment context, invocation context, or casting context with a target type T if T is a functional interface type (§9.8) and the expression is congruent with the function type of the ground target type derived from T.

The ground target type is derived from T as follows:

  • If T is a wildcard-parameterized functional interface type, then the ground target type is the non-wildcard parameterization (§9.9) of T.

  • Otherwise, the ground target type is T.

A method reference expression is congruent with a function type if both of the following are true:

  • The function type identifies a single compile-time declaration corresponding to the reference.

  • One of the following is true:

    • The result of the function type is void.

    • The result of the function type is R, and the result of applying capture conversion (§5.1.10) to the return type of the invocation type (§15.12.2.6) of the chosen compile-time declaration is R' (where R is the target type that may be used to infer R'), and neither R nor R' is void, and R' is compatible with R in an assignment context.

A compile-time unchecked warning occurs if unchecked conversion was necessary for the compile-time declaration to be applicable, and this conversion would cause an unchecked warning in an invocation context.

A compile-time unchecked warning occurs if unchecked conversion was necessary for the return type R', described above, to be compatible with the function type's return type, R, and this conversion would cause an unchecked warning in an assignment context.

If a method reference expression is compatible with a target type T, then the type of the expression, U, is the ground target type derived from T.

It is a compile-time error if any class or interface mentioned by either U or the function type of U is not accessible from the class or interface in which the method reference expression appears.

For each non-static member method m of U, if the function type of U has a subsignature of the signature of m, then a notional method whose method type is the function type of U is said to override m, and any compile-time error or unchecked warning specified in §8.4.8.3 may occur.

For each checked exception type X listed in the throws clause of the invocation type of the compile-time declaration, X or a superclass of X must be mentioned in the throws clause of the function type of U, or a compile-time error occurs.

The key idea driving the compatibility definition is that a method reference is compatible if and only if the equivalent lambda expression (x, y, z) -> exp.<T1, T2>method(x, y, z) is compatible. (This is informal, and there are issues that make it difficult or impossible to formally define the semantics in terms of such a rewrite.)

These compatibility rules provide a convenient facility for converting from one functional interface to another:

Task t = () -> System.out.println("hi");
Runnable r = t::invoke;

The implementation may be optimized so that when a lambda-derived object is passed around and converted to various types, this does not result in many levels of adaptation logic around the core lambda body.

Unlike a lambda expression, a method reference can be congruent with a generic function type (that is, a function type that has type parameters). This is because the lambda expression would need to be able to declare type parameters, and no syntax supports this; while for a method reference, no such declaration is necessary. For example, the following program is legal:


interface ListFactory {
    <T> List<T> make();
}

ListFactory lf  = ArrayList::new;
List<String> ls = lf.make();
List<Number> ln = lf.make();

15.13.3. Run-time Evaluation of Method References

At run time, evaluation of a method reference expression is similar to evaluation of a class instance creation expression, insofar as normal completion produces a reference to an object. Evaluation of a method reference expression is distinct from invocation of the method itself.

First, if the method reference expression begins with an ExpressionName or a Primary, this subexpression is evaluated. If the subexpression evaluates to null, a NullPointerException is raised, and the method reference expression completes abruptly. If the subexpression completes abruptly, the method reference expression completes abruptly for the same reason.

Next, either a new instance of a class with the properties below is allocated and initialized, or an existing instance of a class with the properties below is referenced. If a new instance is to be created, but there is insufficient space to allocate the object, evaluation of the method reference expression completes abruptly by throwing an OutOfMemoryError.

The value of a method reference expression is a reference to an instance of a class with the following properties:

  • The class implements the targeted functional interface type and, if the target type is an intersection type, every other interface type mentioned in the intersection.

  • Where the method reference expression has type U, for each non-static member method m of U:

    If the function type of U has a subsignature of the signature of m, then the class declares an invocation method that overrides m. The invocation method's body invokes the referenced method, creates a class instance, or creates an array, as described below. If the invocation method's result is not void, then the body returns the result of the method invocation or object creation, after any necessary assignment conversions (§5.2).

    If the erasure of the type of a method being overridden differs in its signature from the erasure of the function type of U, then before the method invocation or object creation, an invocation method's body checks that each argument value is an instance of a subclass or subinterface of the erasure of the corresponding parameter type in the function type of U; if not, a ClassCastException is thrown.

  • The class overrides no other methods of the functional interface type or other interface types mentioned above, although it may override methods of the Object class.

The body of an invocation method depends on the form of the method reference expression, as follows:

  • If the form is ExpressionName :: [TypeArguments] Identifier or Primary :: [TypeArguments] Identifier, then the body of the invocation method has the effect of a method invocation expression for a compile-time declaration which is the compile-time declaration of the method reference expression. Run-time evaluation of the method invocation expression is as specified in §15.12.4.3, §15.12.4.4, and §15.12.4.5, where:

    • The invocation mode is derived from the compile-time declaration as specified in §15.12.3.

    • The target reference is the value of ExpressionName or Primary, as determined when the method reference expression was evaluated.

    • The arguments to the method invocation expression are the formal parameters of the invocation method.

  • If the form is ReferenceType :: [TypeArguments] Identifier, the body of the invocation method similarly has the effect of a method invocation expression for a compile-time declaration which is the compile-time declaration of the method reference expression. Run-time evaluation of the method invocation expression is as specified in §15.12.4.3, §15.12.4.4, and §15.12.4.5, where:

    • The invocation mode is derived from the compile-time declaration as specified in §15.12.3.

    • If the compile-time declaration is an instance method, then the target reference is the first formal parameter of the invocation method. Otherwise, there is no target reference.

    • If the compile-time declaration is an instance method, then the arguments to the method invocation expression (if any) are the second and subsequent formal parameters of the invocation method. Otherwise, the arguments to the method invocation expression are the formal parameters of the invocation method.

  • If the form is super :: [TypeArguments] Identifier or TypeName . super :: [TypeArguments] Identifier, the body of the invocation method has the effect of a method invocation expression for a compile-time declaration which is the compile-time declaration of the method reference expression. Run-time evaluation of the method invocation expression is as specified in §15.12.4.3, §15.12.4.4, and §15.12.4.5, where:

    • The invocation mode is super.

    • If the method reference expression begins with a TypeName that names a class, the target reference is the value of TypeName . this at the point at which the method reference is evaluated. Otherwise, the target reference is the value of this at the point at which the method reference is evaluated.

    • The arguments to the method invocation expression are the formal parameters of the invocation method.

  • If the form is ClassType :: [TypeArguments] new, the body of the invocation method has the effect of a class instance creation expression of the form new [TypeArguments] ClassType(A1, ..., An), where the arguments A1, ..., An are the formal parameters of the invocation method, and where:

    • The enclosing instance for the new object, if any, is derived from the site of the method reference expression, as specified in §15.9.2.

    • The constructor to invoke is the constructor that corresponds to the compile-time declaration of the method reference (§15.13.1).

  • If the form is Type[]k :: new (k 1), then the body of the invocation method has the same effect as an array creation expression of the form new Type [ size ] []k-1, where size is the invocation method's single parameter. (The notation []k indicates a sequence of k bracket pairs.)

If the body of the invocation method has the effect of a method invocation expression, then the compile-time parameter types and the compile-time result of the method invocation are determined as specified in §15.12.3. For the purpose of determining the compile-time result, the method invocation expression is an expression statement if the invocation method's result is void, and the Expression of a return statement if the invocation method's result is non-void.

The effect of this determination when the compile-time declaration of the method reference is signature polymorphic is that:

  • The types of the parameters for the method invocation are the types of the corresponding arguments.

  • The method invocation is either void or has a return type of Object, depending on whether the invocation method which encloses the method invocation is void or has a return type.

The timing of method reference expression evaluation is more complex than that of lambda expressions (§15.27.4). When a method reference expression has an expression (rather than a type) preceding the :: separator, that subexpression is evaluated immediately. The result of evaluation is stored until the method of the corresponding functional interface type is invoked; at that point, the result is used as the target reference for the invocation. This means the expression preceding the :: separator is evaluated only when the program encounters the method reference expression, and is not re-evaluated on subsequent invocations on the functional interface type.

It is interesting to contrast the treatment of null here with its treatment during method invocation. When a method invocation expression is evaluated, it is possible for the Primary that qualifies the invocation to evaluate to null but for no NullPointerException to be raised. This occurs when the invoked method is static (despite the syntax of the invocation suggesting an instance method). Since the applicable method for a method reference expression qualified by a Primary is prohibited from being static (§15.13.1), the evaluation of the method reference expression is simpler - a null Primary always raises a NullPointerException.

15.14. Postfix Expressions

Postfix expressions include uses of the postfix ++ and -- operators. Names are not considered to be primary expressions (§15.8), but are handled separately in the grammar to avoid certain ambiguities. They become interchangeable only here, at the level of precedence of postfix expressions.

15.14.1. Expression Names

The rules for evaluating expression names are given in §6.5.6.

15.14.2. Postfix Increment Operator ++

A postfix expression followed by a ++ operator is a postfix increment expression.

PostIncrementExpression:

The result of the postfix expression must be a variable of a type that is convertible (§5.1.8) to a numeric type, or a compile-time error occurs.

The type of the postfix increment expression is the type of the variable. The result of the postfix increment expression is not a variable, but a value.

At run time, if evaluation of the operand expression completes abruptly, then the postfix increment expression completes abruptly for the same reason and no incrementation occurs. Otherwise, the value 1 is added to the value of the variable and the sum is stored back into the variable. Before the addition, binary numeric promotion (§5.6.2) is performed on the value 1 and the value of the variable. If necessary, the sum is narrowed by a narrowing primitive conversion (§5.1.3) and/or subjected to boxing conversion (§5.1.7) to the type of the variable before it is stored. The value of the postfix increment expression is the value of the variable before the new value is stored.

Note that the binary numeric promotion mentioned above may include unboxing conversion (§5.1.8) and value set conversion (§5.1.13). If necessary, value set conversion is applied to the sum prior to its being stored in the variable.

A variable that is declared final cannot be incremented because when an access of such a final variable is used as an expression, the result is a value, not a variable. Thus, it cannot be used as the operand of a postfix increment operator.

15.14.3. Postfix Decrement Operator --

A postfix expression followed by a -- operator is a postfix decrement expression.

PostDecrementExpression:

The result of the postfix expression must be a variable of a type that is convertible (§5.1.8) to a numeric type, or a compile-time error occurs.

The type of the postfix decrement expression is the type of the variable. The result of the postfix decrement expression is not a variable, but a value.

At run time, if evaluation of the operand expression completes abruptly, then the postfix decrement expression completes abruptly for the same reason and no decrementation occurs. Otherwise, the value 1 is subtracted from the value of the variable and the difference is stored back into the variable. Before the subtraction, binary numeric promotion (§5.6.2) is performed on the value 1 and the value of the variable. If necessary, the difference is narrowed by a narrowing primitive conversion (§5.1.3) and/or subjected to boxing conversion (§5.1.7) to the type of the variable before it is stored. The value of the postfix decrement expression is the value of the variable before the new value is stored.

Note that the binary numeric promotion mentioned above may include unboxing conversion (§5.1.8) and value set conversion (§5.1.13). If necessary, value set conversion is applied to the difference prior to its being stored in the variable.

A variable that is declared final cannot be decremented because when an access of such a final variable is used as an expression, the result is a value, not a variable. Thus, it cannot be used as the operand of a postfix decrement operator.

15.15. Unary Operators

The operators +, -, ++, --, ~, !, and the cast operator (§15.16) are called the unary operators.

The following production from §15.16 is shown here for convenience:

Expressions with unary operators group right-to-left, so that -~x means the same as -(~x).

This portion of the grammar contains some tricks to avoid two potential syntactic ambiguities.

The first potential ambiguity would arise in expressions such as (p)+q, which looks, to a C or C++ programmer, as though it could be either a cast to type p of a unary + operating on q, or a binary addition of two quantities p and q. In C and C++, the parser handles this problem by performing a limited amount of semantic analysis as it parses, so that it knows whether p is the name of a type or the name of a variable.

Java takes a different approach. The result of the + operator must be numeric, and all type names involved in casts on numeric values are known keywords. Thus, if p is a keyword naming a primitive type, then (p)+q can make sense only as a cast of a unary expression. However, if p is not a keyword naming a primitive type, then (p)+q can make sense only as a binary arithmetic operation. Similar remarks apply to the - operator. The grammar shown above splits CastExpression into two cases to make this distinction. The nonterminal UnaryExpression includes all unary operators, but the nonterminal UnaryExpressionNotPlusMinus excludes uses of all unary operators that could also be binary operators, which in Java are + and -.

The second potential ambiguity is that the expression (p)++ could, to a C or C++ programmer, appear to be either a postfix increment of a parenthesized expression or the beginning of a cast, for example, in (p)++q. As before, parsers for C and C++ know whether p is the name of a type or the name of a variable. But a parser using only one-token lookahead and no semantic analysis during the parse would not be able to tell, when ++ is the lookahead token, whether (p) should be considered a Primary expression or left alone for later consideration as part of a CastExpression.

In Java, the result of the ++ operator must be numeric, and all type names involved in casts on numeric values are known keywords. Thus, if p is a keyword naming a primitive type, then (p)++ can make sense only as a cast of a prefix increment expression, and there had better be an operand such as q following the ++. However, if p is not a keyword naming a primitive type, then (p)++ can make sense only as a postfix increment of p. Similar remarks apply to the -- operator. The nonterminal UnaryExpressionNotPlusMinus therefore also excludes uses of the prefix operators ++ and --.

15.15.1. Prefix Increment Operator ++

A unary expression preceded by a ++ operator is a prefix increment expression.

The result of the unary expression must be a variable of a type that is convertible (§5.1.8) to a numeric type, or a compile-time error occurs.

The type of the prefix increment expression is the type of the variable. The result of the prefix increment expression is not a variable, but a value.

At run time, if evaluation of the operand expression completes abruptly, then the prefix increment expression completes abruptly for the same reason and no incrementation occurs. Otherwise, the value 1 is added to the value of the variable and the sum is stored back into the variable. Before the addition, binary numeric promotion (§5.6.2) is performed on the value 1 and the value of the variable. If necessary, the sum is narrowed by a narrowing primitive conversion (§5.1.3) and/or subjected to boxing conversion (§5.1.7) to the type of the variable before it is stored. The value of the prefix increment expression is the value of the variable after the new value is stored.

Note that the binary numeric promotion mentioned above may include unboxing conversion (§5.1.8) and value set conversion (§5.1.13). If necessary, value set conversion is applied to the sum prior to its being stored in the variable.

A variable that is declared final cannot be incremented because when an access of such a final variable is used as an expression, the result is a value, not a variable. Thus, it cannot be used as the operand of a prefix increment operator.

15.15.2. Prefix Decrement Operator --

A unary expression preceded by a -- operator is a prefix decrement expression.

The result of the unary expression must be a variable of a type that is convertible (§5.1.8) to a numeric type, or a compile-time error occurs.

The type of the prefix decrement expression is the type of the variable. The result of the prefix decrement expression is not a variable, but a value.

At run time, if evaluation of the operand expression completes abruptly, then the prefix decrement expression completes abruptly for the same reason and no decrementation occurs. Otherwise, the value 1 is subtracted from the value of the variable and the difference is stored back into the variable. Before the subtraction, binary numeric promotion (§5.6.2) is performed on the value 1 and the value of the variable. If necessary, the difference is narrowed by a narrowing primitive conversion (§5.1.3) and/or subjected to boxing conversion (§5.1.7) to the type of the variable before it is stored. The value of the prefix decrement expression is the value of the variable after the new value is stored.

Note that the binary numeric promotion mentioned above may include unboxing conversion (§5.1.8) and value set conversion (§5.1.13). If necessary, format conversion is applied to the difference prior to its being stored in the variable.

A variable that is declared final cannot be decremented because when an access of such a final variable is used as an expression, the result is a value, not a variable. Thus, it cannot be used as the operand of a prefix decrement operator.

15.15.3. Unary Plus Operator +

The type of the operand expression of the unary + operator must be a type that is convertible (§5.1.8) to a primitive numeric type, or a compile-time error occurs.

Unary numeric promotion (§5.6.1) is performed on the operand. The type of the unary plus expression is the promoted type of the operand. The result of the unary plus expression is not a variable, but a value, even if the result of the operand expression is a variable.

At run time, the value of the unary plus expression is the promoted value of the operand.

15.15.4. Unary Minus Operator -

The type of the operand expression of the unary - operator must be a type that is convertible (§5.1.8) to a primitive numeric type, or a compile-time error occurs.

Unary numeric promotion (§5.6.1) is performed on the operand.

The type of the unary minus expression is the promoted type of the operand.

Note that unary numeric promotion performs value set conversion (§5.1.13). Whatever value set the promoted operand value is drawn from, the unary negation operation is carried out and the result is drawn from that same value set. That result is then subject to further value set conversion.

At run time, the value of the unary minus expression is the arithmetic negation of the promoted value of the operand.

For integer values, negation is the same as subtraction from zero. The Java programming language uses two's-complement representation for integers, and the range of two's-complement values is not symmetric, so negation of the maximum negative int or long results in that same maximum negative number. Overflow occurs in this case, but no exception is thrown. For all integer values x, -x equals (~x)+1.

For floating-point values, negation is not the same as subtraction from zero, because if x is +0.0, then 0.0-x is +0.0, but -x is -0.0. Unary minus merely inverts the sign of a floating-point number. Special cases of interest:

  • If the operand is NaN, the result is NaN. (Recall that NaN has no sign (§4.2.3).)

  • If the operand is an infinity, the result is the infinity of opposite sign.

  • If the operand is a zero, the result is the zero of opposite sign.

15.15.5. Bitwise Complement Operator ~

The type of the operand expression of the unary ~ operator must be a type that is convertible (§5.1.8) to a primitive integral type, or a compile-time error occurs.

Unary numeric promotion (§5.6.1) is performed on the operand. The type of the unary bitwise complement expression is the promoted type of the operand.

At run time, the value of the unary bitwise complement expression is the bitwise complement of the promoted value of the operand. In all cases, ~x equals (-x)-1.

15.15.6. Logical Complement Operator !

The type of the operand expression of the unary ! operator must be boolean or Boolean, or a compile-time error occurs.

The type of the unary logical complement expression is boolean.

At run time, the operand is subject to unboxing conversion (§5.1.8) if necessary. The value of the unary logical complement expression is true if the (possibly converted) operand value is false, and false if the (possibly converted) operand value is true.

15.16. Cast Expressions

A cast expression converts, at run time, a value of one numeric type to a similar value of another numeric type; or confirms, at compile time, that the type of an expression is boolean; or checks, at run time, that a reference value refers to an object whose class is compatible with a specified reference type or list of reference types.

The parentheses and the type or list of types they contain are sometimes called the cast operator.

The following production from §4.4 is shown here for convenience:

AdditionalBound:

If the cast operator contains a list of types - that is, a ReferenceType followed by one or more AdditionalBound terms - then all of the following must be true, or a compile-time error occurs:

  • ReferenceType must denote a class or interface type.

  • The erasures (§4.6) of all the listed types must be pairwise different.

  • No two listed types may be subtypes of different parameterizations of the same generic interface.

The target type for the casting context (§5.5) introduced by the cast expression is either the PrimitiveType or the ReferenceType (if not followed by AdditionalBound terms) appearing in the cast operator, or the intersection type denoted by the ReferenceType and AdditionalBound terms appearing in the cast operator.

The type of a cast expression is the result of applying capture conversion (§5.1.10) to this target type.

Casts can be used to explicitly "tag" a lambda expression or a method reference expression with a particular target type. To provide an appropriate degree of flexibility, the target type may be a list of types denoting an intersection type, provided the intersection induces a functional interface (§9.8).

The result of a cast expression is not a variable, but a value, even if the result of the operand expression is a variable.

A cast operator has no effect on the choice of value set (§4.2.3) for a value of type float or type double. Consequently, a cast to type float within an expression that is not FP-strict (§15.4) does not necessarily cause its value to be converted to an element of the float value set, and a cast to type double within an expression that is not FP-strict does not necessarily cause its value to be converted to an element of the double value set.

It is a compile-time error if the compile-time type of the operand may never be cast to the type specified by the cast operator according to the rules of casting conversion (§5.5).

Otherwise, at run time, the operand value is converted (if necessary) by casting conversion to the type specified by the cast operator.

A ClassCastException is thrown if a cast is found at run time to be impermissible.

Some casts result in an error at compile time. Some casts can be proven, at compile time, always to be correct at run time. For example, it is always correct to convert a value of a class type to the type of its superclass; such a cast should require no special action at run time. Finally, some casts cannot be proven to be either always correct or always incorrect at compile time. Such casts require a test at run time. See §5.5 for details.

15.17. Multiplicative Operators

The operators *, /, and % are called the multiplicative operators.

The multiplicative operators have the same precedence and are syntactically left-associative (they group left-to-right).

The type of each of the operands of a multiplicative operator must be a type that is convertible (§5.1.8) to a primitive numeric type, or a compile-time error occurs.

Binary numeric promotion is performed on the operands (§5.6.2).

Note that binary numeric promotion performs value set conversion (§5.1.13) and may perform unboxing conversion (§5.1.8).

The type of a multiplicative expression is the promoted type of its operands.

If the promoted type is int or long, then integer arithmetic is performed.

If the promoted type is float or double, then floating-point arithmetic is performed.

15.17.1. Multiplication Operator *

The binary * operator performs multiplication, producing the product of its operands.

Multiplication is a commutative operation if the operand expressions have no side effects.

Integer multiplication is associative when the operands are all of the same type.

Floating-point multiplication is not associative.

If an integer multiplication overflows, then the result is the low-order bits of the mathematical product as represented in some sufficiently large two's-complement format. As a result, if overflow occurs, then the sign of the result may not be the same as the sign of the mathematical product of the two operand values.

The result of a floating-point multiplication is determined by the rules of IEEE 754 arithmetic:

  • If either operand is NaN, the result is NaN.

  • If the result is not NaN, the sign of the result is positive if both operands have the same sign, and negative if the operands have different signs.

  • Multiplication of an infinity by a zero results in NaN.

  • Multiplication of an infinity by a finite value results in a signed infinity. The sign is determined by the rule stated above.

  • In the remaining cases, where neither an infinity nor NaN is involved, the exact mathematical product is computed. A floating-point value set is then chosen:

    • If the multiplication expression is FP-strict (§15.4):

      • If the type of the multiplication expression is float, then the float value set must be chosen.

      • If the type of the multiplication expression is double, then the double value set must be chosen.

    • If the multiplication expression is not FP-strict:

      • If the type of the multiplication expression is float, then either the float value set or the float-extended-exponent value set may be chosen, at the whim of the implementation.

      • If the type of the multiplication expression is double, then either the double value set or the double-extended-exponent value set may be chosen, at the whim of the implementation.

    Next, a value must be chosen from the chosen value set to represent the product.

    If the magnitude of the product is too large to represent, we say the operation overflows; the result is then an infinity of appropriate sign.

    Otherwise, the product is rounded to the nearest value in the chosen value set using IEEE 754 round-to-nearest mode. The Java programming language requires support of gradual underflow as defined by IEEE 754 (§4.2.4).

Despite the fact that overflow, underflow, or loss of information may occur, evaluation of a multiplication operator * never throws a run-time exception.

15.17.2. Division Operator /

The binary / operator performs division, producing the quotient of its operands. The left-hand operand is the dividend and the right-hand operand is the divisor.

Integer division rounds toward 0. That is, the quotient produced for operands n and d that are integers after binary numeric promotion (§5.6.2) is an integer value q whose magnitude is as large as possible while satisfying |d q| |n|. Moreover, q is positive when |n| |d| and n and d have the same sign, but q is negative when |n| |d| and n and d have opposite signs.

There is one special case that does not satisfy this rule: if the dividend is the negative integer of largest possible magnitude for its type, and the divisor is -1, then integer overflow occurs and the result is equal to the dividend. Despite the overflow, no exception is thrown in this case. On the other hand, if the value of the divisor in an integer division is 0, then an ArithmeticException is thrown.

The result of a floating-point division is determined by the rules of IEEE 754 arithmetic:

  • If either operand is NaN, the result is NaN.

  • If the result is not NaN, the sign of the result is positive if both operands have the same sign, and negative if the operands have different signs.

  • Division of an infinity by an infinity results in NaN.

  • Division of an infinity by a finite value results in a signed infinity. The sign is determined by the rule stated above.

  • Division of a finite value by an infinity results in a signed zero. The sign is determined by the rule stated above.

  • Division of a zero by a zero results in NaN; division of zero by any other finite value results in a signed zero. The sign is determined by the rule stated above.

  • Division of a nonzero finite value by a zero results in a signed infinity. The sign is determined by the rule stated above.

  • In the remaining cases, where neither an infinity nor NaN is involved, the exact mathematical quotient is computed. A floating-point value set is then chosen:

    • If the division expression is FP-strict (§15.4):

      • If the type of the division expression is float, then the float value set must be chosen.

      • If the type of the division expression is double, then the double value set must be chosen.

    • If the division expression is not FP-strict:

      • If the type of the division expression is float, then either the float value set or the float-extended-exponent value set may be chosen, at the whim of the implementation.

      • If the type of the division expression is double, then either the double value set or the double-extended-exponent value set may be chosen, at the whim of the implementation.

    Next, a value must be chosen from the chosen value set to represent the quotient.

    If the magnitude of the quotient is too large to represent, we say the operation overflows; the result is then an infinity of appropriate sign.

    Otherwise, the quotient is rounded to the nearest value in the chosen value set using IEEE 754 round-to-nearest mode. The Java programming language requires support of gradual underflow as defined by IEEE 754 (§4.2.4).

Despite the fact that overflow, underflow, division by zero, or loss of information may occur, evaluation of a floating-point division operator / never throws a run-time exception.

15.17.3. Remainder Operator %

The binary % operator is said to yield the remainder of its operands from an implied division; the left-hand operand is the dividend and the right-hand operand is the divisor.

In C and C++, the remainder operator accepts only integral operands, but in the Java programming language, it also accepts floating-point operands.

The remainder operation for operands that are integers after binary numeric promotion (§5.6.2) produces a result value such that (a/b)*b+(a%b) is equal to a.

This identity holds even in the special case that the dividend is the negative integer of largest possible magnitude for its type and the divisor is -1 (the remainder is 0).

It follows from this rule that the result of the remainder operation can be negative only if the dividend is negative, and can be positive only if the dividend is positive. Moreover, the magnitude of the result is always less than the magnitude of the divisor.

If the value of the divisor for an integer remainder operator is 0, then an ArithmeticException is thrown.

Example 15.17.3-1. Integer Remainder Operator

class Test1 {
    public static void main(String[] args) {
        int a = 5%3;  // 2
        int b = 5/3;  // 1
        System.out.println("5%3 produces " + a +
                           " (note that 5/3 produces " + b + ")");

        int c = 5%(-3);  // 2
        int d = 5/(-3);  // -1
        System.out.println("5%(-3) produces " + c +
                           " (note that 5/(-3) produces " + d + ")");

        int e = (-5)%3;  // -2
        int f = (-5)/3;  // -1
        System.out.println("(-5)%3 produces " + e +
                           " (note that (-5)/3 produces " + f + ")");

        int g = (-5)%(-3);  // -2
        int h = (-5)/(-3);  // 1
        System.out.println("(-5)%(-3) produces " + g +
                           " (note that (-5)/(-3) produces " + h + ")");
    }
}

This program produces the output:

5%3 produces 2 (note that 5/3 produces 1)
5%(-3) produces 2 (note that 5/(-3) produces -1)
(-5)%3 produces -2 (note that (-5)/3 produces -1)
(-5)%(-3) produces -2 (note that (-5)/(-3) produces 1)

The result of a floating-point remainder operation as computed by the % operator is not the same as that produced by the remainder operation defined by IEEE 754. The IEEE 754 remainder operation computes the remainder from a rounding division, not a truncating division, and so its behavior is not analogous to that of the usual integer remainder operator. Instead, the Java programming language defines % on floating-point operations to behave in a manner analogous to that of the integer remainder operator; this may be compared with the C library function fmod. The IEEE 754 remainder operation may be computed by the library routine Math.IEEEremainder.

The result of a floating-point remainder operation is determined by the rules of IEEE 754 arithmetic:

  • If either operand is NaN, the result is NaN.

  • If the result is not NaN, the sign of the result equals the sign of the dividend.

  • If the dividend is an infinity, or the divisor is a zero, or both, the result is NaN.

  • If the dividend is finite and the divisor is an infinity, the result equals the dividend.

  • If the dividend is a zero and the divisor is finite, the result equals the dividend.

  • In the remaining cases, where neither an infinity, nor a zero, nor NaN is involved, the floating-point remainder r from the division of a dividend n by a divisor d is defined by the mathematical relation r = n - (d q) where q is an integer that is negative only if n/d is negative and positive only if n/d is positive, and whose magnitude is as large as possible without exceeding the magnitude of the true mathematical quotient of n and d.

Evaluation of a floating-point remainder operator % never throws a run-time exception, even if the right-hand operand is zero. Overflow, underflow, or loss of precision cannot occur.

Example 15.17.3-2. Floating-Point Remainder Operator

class Test2 {
    public static void main(String[] args) {
        double a = 5.0%3.0;  // 2.0
        System.out.println("5.0%3.0 produces " + a);

        double b = 5.0%(-3.0);  // 2.0
        System.out.println("5.0%(-3.0) produces " + b);

        double c = (-5.0)%3.0;  // -2.0
        System.out.println("(-5.0)%3.0 produces " + c);

        double d = (-5.0)%(-3.0);  // -2.0
        System.out.println("(-5.0)%(-3.0) produces " + d);
    }
}

This program produces the output:

5.0%3.0 produces 2.0
5.0%(-3.0) produces 2.0
(-5.0)%3.0 produces -2.0
(-5.0)%(-3.0) produces -2.0

15.18. Additive Operators

The operators + and - are called the additive operators.

The additive operators have the same precedence and are syntactically left-associative (they group left-to-right).

If the type of either operand of a + operator is String, then the operation is string concatenation.

Otherwise, the type of each of the operands of the + operator must be a type that is convertible (§5.1.8) to a primitive numeric type, or a compile-time error occurs.

In every case, the type of each of the operands of the binary - operator must be a type that is convertible (§5.1.8) to a primitive numeric type, or a compile-time error occurs.

15.18.1. String Concatenation Operator +

If only one operand expression is of type String, then string conversion (§5.1.11) is performed on the other operand to produce a string at run time.

The result of string concatenation is a reference to a String object that is the concatenation of the two operand strings. The characters of the left-hand operand precede the characters of the right-hand operand in the newly created string.

The String object is newly created (§12.5) unless the expression is a constant expression (§15.28).

An implementation may choose to perform conversion and concatenation in one step to avoid creating and then discarding an intermediate String object. To increase the performance of repeated string concatenation, a Java compiler may use the StringBuffer class or a similar technique to reduce the number of intermediate String objects that are created by evaluation of an expression.

For primitive types, an implementation may also optimize away the creation of a wrapper object by converting directly from a primitive type to a string.

Example 15.18.1-1. String Concatenation

The example expression:

"The square root of 2 is " + Math.sqrt(2)

produces the result:

"The square root of 2 is 1.4142135623730952"

The + operator is syntactically left-associative, no matter whether it is determined by type analysis to represent string concatenation or numeric addition. In some cases care is required to get the desired result. For example, the expression:

a + b + c

is always regarded as meaning:

(a + b) + c

Therefore the result of the expression:

1 + 2 + " fiddlers"

is:

"3 fiddlers"

but the result of:

"fiddlers " + 1 + 2

is:

"fiddlers 12"

Example 15.18.1-2. String Concatenation and Conditionals

In this jocular little example:

class Bottles {
    static void printSong(Object stuff, int n) {
        String plural = (n == 1) ? "" : "s";
  loop: while (true) {
            System.out.println(n + " bottle" + plural
                    + " of " + stuff + " on the wall,");
            System.out.println(n + " bottle" + plural
                    + " of " + stuff + ";");
            System.out.println("You take one down "
                    + "and pass it around:");
            --n;
            plural = (n == 1) ? "" : "s";
            if (n == 0)
                break loop;
            System.out.println(n + " bottle" + plural
                    + " of " + stuff + " on the wall!");
            System.out.println();
        }
        System.out.println("No bottles of " +
                    stuff + " on the wall!");
    }

    public static void main(String[] args) {
        printSong("slime", 3);
    }
}

the method printSong will print a version of a children's song. Popular values for stuff include "pop" and "beer"; the most popular value for n is 100. Here is the output that results from running the program:

3 bottles of slime on the wall,
3 bottles of slime;
You take one down and pass it around:
2 bottles of slime on the wall!

2 bottles of slime on the wall,
2 bottles of slime;
You take one down and pass it around:
1 bottle of slime on the wall!

1 bottle of slime on the wall,
1 bottle of slime;
You take one down and pass it around:
No bottles of slime on the wall!

In the code, note the careful conditional generation of the singular "bottle" when appropriate rather than the plural "bottles"; note also how the string concatenation operator was used to break the long constant string:

"You take one down and pass it around:"

into two pieces to avoid an inconveniently long line in the source code.


15.18.2. Additive Operators (+ and -) for Numeric Types

The binary + operator performs addition when applied to two operands of numeric type, producing the sum of the operands.

The binary - operator performs subtraction, producing the difference of two numeric operands.

Binary numeric promotion is performed on the operands (§5.6.2).

Note that binary numeric promotion performs value set conversion (§5.1.13) and may perform unboxing conversion (§5.1.8).

The type of an additive expression on numeric operands is the promoted type of its operands.

If this promoted type is int or long, then integer arithmetic is performed.

If this promoted type is float or double, then floating-point arithmetic is performed.

Addition is a commutative operation if the operand expressions have no side effects.

Integer addition is associative when the operands are all of the same type.

Floating-point addition is not associative.

If an integer addition overflows, then the result is the low-order bits of the mathematical sum as represented in some sufficiently large two's-complement format. If overflow occurs, then the sign of the result is not the same as the sign of the mathematical sum of the two operand values.

The result of a floating-point addition is determined using the following rules of IEEE 754 arithmetic:

  • If either operand is NaN, the result is NaN.

  • The sum of two infinities of opposite sign is NaN.

  • The sum of two infinities of the same sign is the infinity of that sign.

  • The sum of an infinity and a finite value is equal to the infinite operand.

  • The sum of two zeros of opposite sign is positive zero.

  • The sum of two zeros of the same sign is the zero of that sign.

  • The sum of a zero and a nonzero finite value is equal to the nonzero operand.

  • The sum of two nonzero finite values of the same magnitude and opposite sign is positive zero.

  • In the remaining cases, where neither an infinity, nor a zero, nor NaN is involved, and the operands have the same sign or have different magnitudes, the exact mathematical sum is computed. A floating-point value set is then chosen:

    • If the addition expression is FP-strict (§15.4):

      • If the type of the addition expression is float, then the float value set must be chosen.

      • If the type of the addition expression is double, then the double value set must be chosen.

    • If the addition expression is not FP-strict:

      • If the type of the addition expression is float, then either the float value set or the float-extended-exponent value set may be chosen, at the whim of the implementation.

      • If the type of the addition expression is double, then either the double value set or the double-extended-exponent value set may be chosen, at the whim of the implementation.

    Next, a value must be chosen from the chosen value set to represent the sum.

    If the magnitude of the sum is too large to represent, we say the operation overflows; the result is then an infinity of appropriate sign.

    Otherwise, the sum is rounded to the nearest value in the chosen value set using IEEE 754 round-to-nearest mode. The Java programming language requires support of gradual underflow as defined by IEEE 754 (§4.2.4).

The binary - operator performs subtraction when applied to two operands of numeric type, producing the difference of its operands; the left-hand operand is the minuend and the right-hand operand is the subtrahend.

For both integer and floating-point subtraction, it is always the case that a-b produces the same result as a+(-b).

Note that, for integer values, subtraction from zero is the same as negation. However, for floating-point operands, subtraction from zero is not the same as negation, because if x is +0.0, then 0.0-x is +0.0, but -x is -0.0.

Despite the fact that overflow, underflow, or loss of information may occur, evaluation of a numeric additive operator never throws a run-time exception.

15.19. Shift Operators

The operators << (left shift), >> (signed right shift), and >>> (unsigned right shift) are called the shift operators. The left-hand operand of a shift operator is the value to be shifted; the right-hand operand specifies the shift distance.

The shift operators are syntactically left-associative (they group left-to-right).

Unary numeric promotion (§5.6.1) is performed on each operand separately. (Binary numeric promotion (§5.6.2) is not performed on the operands.)

It is a compile-time error if the type of each of the operands of a shift operator, after unary numeric promotion, is not a primitive integral type.

The type of the shift expression is the promoted type of the left-hand operand.

If the promoted type of the left-hand operand is int, only the five lowest-order bits of the right-hand operand are used as the shift distance. It is as if the right-hand operand were subjected to a bitwise logical AND operator & (§15.22.1) with the mask value 0x1f (0b11111). The shift distance actually used is therefore always in the range 0 to 31, inclusive.

If the promoted type of the left-hand operand is long, then only the six lowest-order bits of the right-hand operand are used as the shift distance. It is as if the right-hand operand were subjected to a bitwise logical AND operator & (§15.22.1) with the mask value 0x3f (0b111111). The shift distance actually used is therefore always in the range 0 to 63, inclusive.

At run time, shift operations are performed on the two's-complement integer representation of the value of the left operand.

The value of n << s is n left-shifted s bit positions; this is equivalent (even if overflow occurs) to multiplication by two to the power s.

The value of n >> s is n right-shifted s bit positions with sign-extension. The resulting value is n / 2s . For non-negative values of n, this is equivalent to truncating integer division, as computed by the integer division operator /, by two to the power s.

The value of n >>> s is n right-shifted s bit positions with zero-extension, where:

  • If n is positive, then the result is the same as that of n >> s.

  • If n is negative and the type of the left-hand operand is int, then the result is equal to that of the expression (n >> s) + (2 << ~s).

  • If n is negative and the type of the left-hand operand is long, then the result is equal to that of the expression (n >> s) + (2L << ~s).

The added term (2 << ~s) or (2L << ~s) cancels out the propagated sign bit.

Note that, because of the implicit masking of the right-hand operand of a shift operator, ~s as a shift distance is equivalent to 31-s when shifting an int value and to 63-s when shifting a long value.

15.20. Relational Operators

The numerical comparison operators <, >, <=, and >=, and the instanceof operator, are called the relational operators.

The relational operators are syntactically left-associative (they group left-to-right).

However, this fact is not useful. For example, a<b<c parses as (a<b)<c, which is always a compile-time error, because the type of a<b is always boolean and < is not an operator on boolean values.

The type of a relational expression is always boolean.

15.20.1. Numerical Comparison Operators <, <=, >, and >=

The type of each of the operands of a numerical comparison operator must be a type that is convertible (§5.1.8) to a primitive numeric type, or a compile-time error occurs.

Binary numeric promotion is performed on the operands (§5.6.2).

Note that binary numeric promotion performs value set conversion (§5.1.13) and may perform unboxing conversion (§5.1.8).

If the promoted type of the operands is int or long, then signed integer comparison is performed.

If the promoted type is float or double, then floating-point comparison is performed.

Comparison is carried out accurately on floating-point values, no matter what value sets their representing values were drawn from.

The result of a floating-point comparison, as determined by the specification of the IEEE 754 standard, is:

  • If either operand is NaN, then the result is false.

  • All values other than NaN are ordered, with negative infinity less than all finite values, and positive infinity greater than all finite values.

  • Positive zero and negative zero are considered equal.

    For example, -0.0<0.0 is false, but -0.0<=0.0 is true.

    Note, however, that the methods Math.min and Math.max treat negative zero as being strictly smaller than positive zero.

Subject to these considerations for floating-point numbers, the following rules then hold for integer operands or for floating-point operands other than NaN:

  • The value produced by the < operator is true if the value of the left-hand operand is less than the value of the right-hand operand, and otherwise is false.

  • The value produced by the <= operator is true if the value of the left-hand operand is less than or equal to the value of the right-hand operand, and otherwise is false.

  • The value produced by the > operator is true if the value of the left-hand operand is greater than the value of the right-hand operand, and otherwise is false.

  • The value produced by the >= operator is true if the value of the left-hand operand is greater than or equal to the value of the right-hand operand, and otherwise is false.

15.20.2. Type Comparison Operator instanceof

The type of the RelationalExpression operand of the instanceof operator must be a reference type or the null type; otherwise, a compile-time error occurs.

It is a compile-time error if the ReferenceType mentioned after the instanceof operator does not denote a reference type that is reifiable (§4.7).

If a cast (§15.16) of the RelationalExpression to the ReferenceType would be rejected as a compile-time error, then the instanceof relational expression likewise produces a compile-time error. In such a situation, the result of the instanceof expression could never be true.

At run time, the result of the instanceof operator is true if the value of the RelationalExpression is not null and the reference could be cast to the ReferenceType without raising a ClassCastException. Otherwise the result is false.

Example 15.20.2-1. The instanceof Operator

class Point   { int x, y; }
class Element { int atomicNumber; }
class Test {
    public static void main(String[] args) {
        Point   p = new Point();
        Element e = new Element();
        if (e instanceof Point) {  // compile-time error
            System.out.println("I get your point!");
            p = (Point)e;  // compile-time error
        }
    }
}

This program results in two compile-time errors. The cast (Point)e is incorrect because no instance of Element or any of its possible subclasses (none are shown here) could possibly be an instance of any subclass of Point. The instanceof expression is incorrect for exactly the same reason. If, on the other hand, the class Point were a subclass of Element (an admittedly strange notion in this example):


class Point extends Element { int x, y; }

then the cast would be possible, though it would require a run-time check, and the instanceof expression would then be sensible and valid. The cast (Point)e would never raise an exception because it would not be executed if the value of e could not correctly be cast to type Point.


15.21. Equality Operators

The operators == (equal to) and != (not equal to) are called the equality operators.

The equality operators are syntactically left-associative (they group left-to-right).

However, this fact is essentially never useful. For example, a==b==c parses as (a==b)==c. The result type of a==b is always boolean, and c must therefore be of type boolean or a compile-time error occurs. Thus, a==b==c does not test to see whether a, b, and c are all equal.

The equality operators are commutative if the operand expressions have no side effects.

The equality operators are analogous to the relational operators except for their lower precedence. Thus, a<b==c<d is true whenever a<b and c<d have the same truth value.

The equality operators may be used to compare two operands that are convertible (§5.1.8) to numeric type, or two operands of type boolean or Boolean, or two operands that are each of either reference type or the null type. All other cases result in a compile-time error.

The type of an equality expression is always boolean.

In all cases, a!=b produces the same result as !(a==b).

15.21.1. Numerical Equality Operators == and !=

If the operands of an equality operator are both of numeric type, or one is of numeric type and the other is convertible (§5.1.8) to numeric type, binary numeric promotion is performed on the operands (§5.6.2).

Note that binary numeric promotion performs value set conversion (§5.1.13) and may perform unboxing conversion (§5.1.8).

If the promoted type of the operands is int or long, then an integer equality test is performed.

If the promoted type is float or double, then a floating-point equality test is performed.

Comparison is carried out accurately on floating-point values, no matter what value sets their representing values were drawn from.

Floating-point equality testing is performed in accordance with the rules of the IEEE 754 standard:

  • If either operand is NaN, then the result of == is false but the result of != is true.

    Indeed, the test x!=x is true if and only if the value of x is NaN.

    The methods Float.isNaN and Double.isNaN may also be used to test whether a value is NaN.

  • Positive zero and negative zero are considered equal.

    For example, -0.0==0.0 is true.

  • Otherwise, two distinct floating-point values are considered unequal by the equality operators.

    In particular, there is one value representing positive infinity and one value representing negative infinity; each compares equal only to itself, and each compares unequal to all other values.

Subject to these considerations for floating-point numbers, the following rules then hold for integer operands or for floating-point operands other than NaN:

  • The value produced by the == operator is true if the value of the left-hand operand is equal to the value of the right-hand operand; otherwise, the result is false.

  • The value produced by the != operator is true if the value of the left-hand operand is not equal to the value of the right-hand operand; otherwise, the result is false.

15.21.2. Boolean Equality Operators == and !=

If the operands of an equality operator are both of type boolean, or if one operand is of type boolean and the other is of type Boolean, then the operation is boolean equality.

The boolean equality operators are associative.

If one of the operands is of type Boolean, it is subjected to unboxing conversion (§5.1.8).

The result of == is true if the operands (after any required unboxing conversion) are both true or both false; otherwise, the result is false.

The result of != is false if the operands are both true or both false; otherwise, the result is true.

Thus != behaves the same as ^ (§15.22.2) when applied to boolean operands.

15.21.3. Reference Equality Operators == and !=

If the operands of an equality operator are both of either reference type or the null type, then the operation is object equality.

It is a compile-time error if it is impossible to convert the type of either operand to the type of the other by a casting conversion (§5.5). The run-time values of the two operands would necessarily be unequal.

At run time, the result of == is true if the operand values are both null or both refer to the same object or array; otherwise, the result is false.

The result of != is false if the operand values are both null or both refer to the same object or array; otherwise, the result is true.

While == may be used to compare references of type String, such an equality test determines whether or not the two operands refer to the same String object. The result is false if the operands are distinct String objects, even if they contain the same sequence of characters (§3.10.5). The contents of two strings s and t can be tested for equality by the method invocation s.equals(t).

15.22. Bitwise and Logical Operators

The bitwise operators and logical operators include the AND operator &, exclusive OR operator ^, and inclusive OR operator |.

These operators have different precedence, with & having the highest precedence and | the lowest precedence.

Each of these operators is syntactically left-associative (each groups left-to-right).

Each operator is commutative if the operand expressions have no side effects.

Each operator is associative.

The bitwise and logical operators may be used to compare two operands of numeric type or two operands of type boolean. All other cases result in a compile-time error.

15.22.1. Integer Bitwise Operators &, ^, and |

When both operands of an operator &, ^, or | are of a type that is convertible (§5.1.8) to a primitive integral type, binary numeric promotion is first performed on the operands (§5.6.2).

The type of the bitwise operator expression is the promoted type of the operands.

For &, the result value is the bitwise AND of the operand values.

For ^, the result value is the bitwise exclusive OR of the operand values.

For |, the result value is the bitwise inclusive OR of the operand values.

For example, the result of the expression:

0xff00 & 0xf0f0

is:

0xf000

The result of the expression:

0xff00 ^ 0xf0f0

is:

0x0ff0

The result of the expression:

0xff00 | 0xf0f0

is:

0xfff0

15.22.2. Boolean Logical Operators &, ^, and |

When both operands of a &, ^, or | operator are of type boolean or Boolean, then the type of the bitwise operator expression is boolean. In all cases, the operands are subject to unboxing conversion (§5.1.8) as necessary.

For &, the result value is true if both operand values are true; otherwise, the result is false.

For ^, the result value is true if the operand values are different; otherwise, the result is false.

For |, the result value is false if both operand values are false; otherwise, the result is true.

15.23. Conditional-And Operator &&

The conditional-and operator && is like & (§15.22.2), but evaluates its right-hand operand only if the value of its left-hand operand is true.

The conditional-and operator is syntactically left-associative (it groups left-to-right).

The conditional-and operator is fully associative with respect to both side effects and result value. That is, for any expressions a, b, and c, evaluation of the expression ((a) && (b)) && (c) produces the same result, with the same side effects occurring in the same order, as evaluation of the expression (a) && ((b) && (c)).

Each operand of the conditional-and operator must be of type boolean or Boolean, or a compile-time error occurs.

The type of a conditional-and expression is always boolean.

At run time, the left-hand operand expression is evaluated first; if the result has type Boolean, it is subjected to unboxing conversion (§5.1.8).

If the resulting value is false, the value of the conditional-and expression is false and the right-hand operand expression is not evaluated.

If the value of the left-hand operand is true, then the right-hand expression is evaluated; if the result has type Boolean, it is subjected to unboxing conversion (§5.1.8). The resulting value becomes the value of the conditional-and expression.

Thus, && computes the same result as & on boolean operands. It differs only in that the right-hand operand expression is evaluated conditionally rather than always.

15.24. Conditional-Or Operator ||

The conditional-or operator || operator is like | (§15.22.2), but evaluates its right-hand operand only if the value of its left-hand operand is false.

ConditionalOrExpression:

The conditional-or operator is syntactically left-associative (it groups left-to-right).

The conditional-or operator is fully associative with respect to both side effects and result value. That is, for any expressions a, b, and c, evaluation of the expression ((a) || (b)) || (c) produces the same result, with the same side effects occurring in the same order, as evaluation of the expression (a) || ((b) || (c)).

Each operand of the conditional-or operator must be of type boolean or Boolean, or a compile-time error occurs.

The type of a conditional-or expression is always boolean.

At run time, the left-hand operand expression is evaluated first; if the result has type Boolean, it is subjected to unboxing conversion (§5.1.8).

If the resulting value is true, the value of the conditional-or expression is true and the right-hand operand expression is not evaluated.

If the value of the left-hand operand is false, then the right-hand expression is evaluated; if the result has type Boolean, it is subjected to unboxing conversion (§5.1.8). The resulting value becomes the value of the conditional-or expression.

Thus, || computes the same result as | on boolean or Boolean operands. It differs only in that the right-hand operand expression is evaluated conditionally rather than always.

15.25. Conditional Operator ? :

The conditional operator ? : uses the boolean value of one expression to decide which of two other expressions should be evaluated.

The conditional operator is syntactically right-associative (it groups right-to-left). Thus, a?b:c?d:e?f:g means the same as a?b:(c?d:(e?f:g)).

The conditional operator has three operand expressions. ? appears between the first and second expressions, and : appears between the second and third expressions.

The first expression must be of type boolean or Boolean, or a compile-time error occurs.

It is a compile-time error for either the second or the third operand expression to be an invocation of a void method.

In fact, by the grammar of expression statements (§14.8), it is not permitted for a conditional expression to appear in any context where an invocation of a void method could appear.

There are three kinds of conditional expressions, classified according to the second and third operand expressions: boolean conditional expressions, numeric conditional expressions, and reference conditional expressions. The classification rules are as follows:

  • If both the second and the third operand expressions are boolean expressions, the conditional expression is a boolean conditional expression.

    For the purpose of classifying a conditional, the following expressions are boolean expressions:

    • An expression of a standalone form (§15.2) that has type boolean or Boolean.

    • A parenthesized boolean expression (§15.8.5).

    • A class instance creation expression (§15.9) for class Boolean.

    • A method invocation expression (§15.12) for which the chosen most specific method (§15.12.2.5) has return type boolean or Boolean.

      Note that, for a generic method, this is the type before instantiating the method's type arguments.

    • A boolean conditional expression.

  • If both the second and the third operand expressions are numeric expressions, the conditional expression is a numeric conditional expression.

    For the purpose of classifying a conditional, the following expressions are numeric expressions:

    • An expression of a standalone form (§15.2) with a type that is convertible to a numeric type (§4.2, §5.1.8).

    • A parenthesized numeric expression (§15.8.5).

    • A class instance creation expression (§15.9) for a class that is convertible to a numeric type.

    • A method invocation expression (§15.12) for which the chosen most specific method (§15.12.2.5) has a return type that is convertible to a numeric type.

    • A numeric conditional expression.

  • Otherwise, the conditional expression is a reference conditional expression.

The process for determining the type of a conditional expression depends on the kind of conditional expression, as outlined in the following sections.

The following tables summarize the rules above by giving the type of a conditional expression for all possible types of its second and third operands. bnp(..) means to apply binary numeric promotion. The form "T | bnp(..)" is used where one operand is a constant expression of type int and may be representable in type T, where binary numeric promotion is used if the operand is not representable in type T. The operand type Object means any reference type other than the null type and the eight wrapper classes Boolean, Byte, Short, Character, Integer, Long, Float, Double.

Table 15.1. Conditional expression type (Primitive 3rd operand, Part I)

3rd byte short char int
2nd        
byte byte short bnp(byte,char) byte | bnp(byte,int)
Byte byte short bnp(Byte,char) byte | bnp(Byte,int)
short short short bnp(short,char) short | bnp(short,int)
Short short short bnp(Short,char) short | bnp(Short,int)
char bnp(char,byte) bnp(char,short) char char | bnp(char,int)
Character bnp(Character,byte) bnp(Character,short) char char | bnp(Character,int)
int byte | bnp(int,byte) short | bnp(int,short) char | bnp(int,char) int
Integer bnp(Integer,byte) bnp(Integer,short) bnp(Integer,char) int
long bnp(long,byte) bnp(long,short) bnp(long,char) bnp(long,int)
Long bnp(Long,byte) bnp(Long,short) bnp(Long,char) bnp(Long,int)
float bnp(float,byte) bnp(float,short) bnp(float,char) bnp(float,int)
Float bnp(Float,byte) bnp(Float,short) bnp(Float,char) bnp(Float,int)
double bnp(double,byte) bnp(double,short) bnp(double,char) bnp(double,int)
Double bnp(Double,byte) bnp(Double,short) bnp(Double,char) bnp(Double,int)
boolean lub(Boolean,Byte) lub(Boolean,Short) lub(Boolean,Character) lub(Boolean,Integer)
Boolean lub(Boolean,Byte) lub(Boolean,Short) lub(Boolean,Character) lub(Boolean,Integer)
null lub(null,Byte) lub(null,Short) lub(null,Character) lub(null,Integer)
Object lub(Object,Byte) lub(Object,Short) lub(Object,Character) lub(Object,Integer)

Table 15.2. Conditional expression type (Primitive 3rd operand, Part II)

3rd long float double boolean
2nd        
byte bnp(byte,long) bnp(byte,float) bnp(byte,double) lub(Byte,Boolean)
Byte bnp(Byte,long) bnp(Byte,float) bnp(Byte,double) lub(Byte,Boolean)
short bnp(short,long) bnp(short,float) bnp(short,double) lub(Short,Boolean)
Short bnp(Short,long) bnp(Short,float) bnp(Short,double) lub(Short,Boolean)
char bnp(char,long) bnp(char,float) bnp(char,double) lub(Character,Boolean)
Character bnp(Character,long) bnp(Character,float) bnp(Character,double) lub(Character,Boolean)
int bnp(int,long) bnp(int,float) bnp(int,double) lub(Integer,Boolean)
Integer bnp(Integer,long) bnp(Integer,float) bnp(Integer,double) lub(Integer,Boolean)
long long bnp(long,float) bnp(long,double) lub(Long,Boolean)
Long long bnp(Long,float) bnp(Long,double) lub(Long,Boolean)
float bnp(float,long) float bnp(float,double) lub(Float,Boolean)
Float bnp(Float,long) float bnp(Float,double) lub(Float,Boolean)
double bnp(double,long) bnp(double,float) double lub(Double,Boolean)
Double bnp(Double,long) bnp(Double,float) double lub(Double,Boolean)
boolean lub(Boolean,Long) lub(Boolean,Float) lub(Boolean,Double) boolean
Boolean lub(Boolean,Long) lub(Boolean,Float) lub(Boolean,Double) boolean
null lub(null,Long) lub(null,Float) lub(null,Double) lub(null,Boolean)
Object lub(Object,Long) lub(Object,Float) lub(Object,Double) lub(Object,Boolean)

Table 15.3. Conditional expression type (Reference 3rd operand, Part I)

3rd Byte Short Character Integer
2nd        
byte byte short bnp(byte,Character) bnp(byte,Integer)
Byte Byte short bnp(Byte,Character) bnp(Byte,Integer)
short short short bnp(short,Character) bnp(short,Integer)
Short short Short bnp(Short,Character) bnp(Short,Integer)
char bnp(char,Byte) bnp(char,Short) char bnp(char,Integer)
Character bnp(Character,Byte) bnp(Character,Short) Character bnp(Character,Integer)
int byte | bnp(int,Byte) short | bnp(int,Short) char | bnp(int,Character) int
Integer bnp(Integer,Byte) bnp(Integer,Short) bnp(Integer,Character) Integer
long bnp(long,Byte) bnp(long,Short) bnp(long,Character) bnp(long,Integer)
Long bnp(Long,Byte) bnp(Long,Short) bnp(Long,Character) bnp(Long,Integer)
float bnp(float,Byte) bnp(float,Short) bnp(float,Character) bnp(float,Integer)
Float bnp(Float,Byte) bnp(Float,Short) bnp(Float,Character) bnp(Float,Integer)
double bnp(double,Byte) bnp(double,Short) bnp(double,Character) bnp(double,Integer)
Double bnp(Double,Byte) bnp(Double,Short) bnp(Double,Character) bnp(Double,Integer)
boolean lub(Boolean,Byte) lub(Boolean,Short) lub(Boolean,Character) lub(Boolean,Integer)
Boolean lub(Boolean,Byte) lub(Boolean,Short) lub(Boolean,Character) lub(Boolean,Integer)
null Byte Short Character Integer
Object lub(Object,Byte) lub(Object,Short) lub(Object,Character) lub(Object,Integer)

Table 15.4. Conditional expression type (Reference 3rd operand, Part II)

3rd Long Float Double Boolean
2nd        
byte bnp(byte,Long) bnp(byte,Float) bnp(byte,Double) lub(Byte,Boolean)
Byte bnp(Byte,Long) bnp(Byte,Float) bnp(Byte,Double) lub(Byte,Boolean)
short bnp(short,Long) bnp(short,Float) bnp(short,Double) lub(Short,Boolean)
Short bnp(Short,Long) bnp(Short,Float) bnp(Short,Double) lub(Short,Boolean)
char bnp(char,Long) bnp(char,Float) bnp(char,Double) lub(Character,Boolean)
Character bnp(Character,Long) bnp(Character,Float) bnp(Character,Double) lub(Character,Boolean)
int bnp(int,Long) bnp(int,Float) bnp(int,Double) lub(Integer,Boolean)
Integer bnp(Integer,Long) bnp(Integer,Float) bnp(Integer,Double) lub(Integer,Boolean)
long long bnp(long,Float) bnp(long,Double) lub(Long,Boolean)
Long Long bnp(Long,Float) bnp(Long,Double) lub(Long,Boolean)
float bnp(float,Long) float bnp(float,Double) lub(Float,Boolean)
Float bnp(Float,Long) Float bnp(Float,Double) lub(Float,Boolean)
double bnp(double,Long) bnp(double,Float) double lub(Double,Boolean)
Double bnp(Double,Long) bnp(Double,Float) Double lub(Double,Boolean)
boolean lub(Boolean,Long) lub(Boolean,Float) lub(Boolean,Double) boolean
Boolean lub(Boolean,Long) lub(Boolean,Float) lub(Boolean,Double) Boolean
null Long Float Double Boolean
Object lub(Object,Long) lub(Object,Float) lub(Object,Double) lub(Object,Boolean)

Table 15.5. Conditional expression type (Reference 3rd operand, Part III)

3rd null Object
2nd    
byte lub(Byte,null) lub(Byte,Object)
Byte Byte lub(Byte,Object)
short lub(Short,null) lub(Short,Object)
Short Short lub(Short,Object)
char lub(Character,null) lub(Character,Object)
Character Character lub(Character,Object)
int lub(Integer,null) lub(Integer,Object)
Integer Integer lub(Integer,Object)
long lub(Long,null) lub(Long,Object)
Long Long lub(Long,Object)
float lub(Float,null) lub(Float,Object)
Float Float lub(Float,Object)
double lub(Double,null) lub(Double,Object)
Double Double lub(Double,Object)
boolean lub(Boolean,null) lub(Boolean,Object)
Boolean Boolean lub(Boolean,Object)
null null lub(null,Object)
Object Object Object

At run time, the first operand expression of the conditional expression is evaluated first. If necessary, unboxing conversion is performed on the result.

The resulting boolean value is then used to choose either the second or the third operand expression:

  • If the value of the first operand is true, then the second operand expression is chosen.

  • If the value of the first operand is false, then the third operand expression is chosen.

The chosen operand expression is then evaluated and the resulting value is converted to the type of the conditional expression as determined by the rules stated below.

This conversion may include boxing or unboxing conversion (§5.1.7, §5.1.8).

The operand expression not chosen is not evaluated for that particular evaluation of the conditional expression.

15.25.1. Boolean Conditional Expressions

Boolean conditional expressions are standalone expressions (§15.2).

The type of a boolean conditional expression is determined as follows:

  • If the second and third operands are both of type Boolean, the conditional expression has type Boolean.

  • Otherwise, the conditional expression has type boolean.

15.25.2. Numeric Conditional Expressions

Numeric conditional expressions are standalone expressions (§15.2).

The type of a numeric conditional expression is determined as follows:

  • If the second and third operands have the same type, then that is the type of the conditional expression.

  • If one of the second and third operands is of primitive type T, and the type of the other is the result of applying boxing conversion (§5.1.7) to T, then the type of the conditional expression is T.

  • If one of the operands is of type byte or Byte and the other is of type short or Short, then the type of the conditional expression is short.

  • If one of the operands is of type T where T is byte, short, or char, and the other operand is a constant expression (§15.28) of type int whose value is representable in type T, then the type of the conditional expression is T.

  • If one of the operands is of type T, where T is Byte, Short, or Character, and the other operand is a constant expression of type int whose value is representable in the type U which is the result of applying unboxing conversion to T, then the type of the conditional expression is U.

  • Otherwise, binary numeric promotion (§5.6.2) is applied to the operand types, and the type of the conditional expression is the promoted type of the second and third operands.

    Note that binary numeric promotion performs value set conversion (§5.1.13) and may perform unboxing conversion (§5.1.8).

15.25.3. Reference Conditional Expressions

A reference conditional expression is a poly expression if it appears in an assignment context or an invocation context (§5.2. §5.3). Otherwise, it is a standalone expression.

Where a poly reference conditional expression appears in a context of a particular kind with target type T, its second and third operand expressions similarly appear in a context of the same kind with target type T.

The type of a poly reference conditional expression is the same as its target type.

The type of a standalone reference conditional expression is determined as follows:

  • If the second and third operands have the same type (which may be the null type), then that is the type of the conditional expression.

  • If the type of one of the second and third operands is the null type, and the type of the other operand is a reference type, then the type of the conditional expression is that reference type.

  • Otherwise, the second and third operands are of types S1 and S2 respectively. Let T1 be the type that results from applying boxing conversion to S1, and let T2 be the type that results from applying boxing conversion to S2. The type of the conditional expression is the result of applying capture conversion (§5.1.10) to lub(T1, T2).

Because reference conditional expressions can be poly expressions, they can "pass down" context to their operands. This allows lambda expressions and method reference expressions to appear as operands:

return ... ? (x -> x) : (x -> -x);

It also allows use of extra information to improve type checking of generic method invocations. Prior to Java SE 8, this assignment was well-typed:

List<String> ls = Arrays.asList();

but this was not:

List<String> ls = ... ? Arrays.asList() : Arrays.asList("a","b");

The rules above allow both assignments to be considered well-typed.

Note that a reference conditional expression does not have to contain a poly expression as an operand in order to be a poly expression. It is a poly expression simply by virtue of the context in which it appears. For example, in the following code, the conditional expression is a poly expression, and each operand is considered to be in an assignment context targeting Class<? super Integer>:


Class<? super Integer> choose(boolean b,
                              Class<Integer> c1,
                              Class<Number> c2) {
    return b ? c1 : c2;
}

If the conditional expression was not a poly expression, then a compile-time error would occur, as its type would be lub(Class<Integer>, Class<Number>) = Class<? extends Number> which is incompatible with the return type of choose.

15.26. Assignment Operators

There are 12 assignment operators; all are syntactically right-associative (they group right-to-left). Thus, a=b=c means a=(b=c), which assigns the value of c to b and then assigns the value of b to a.

AssignmentExpression:
AssignmentOperator:
=  *=  /=  %=  +=  -=  <<=  >>=  >>>=  &=  ^=  |=

The result of the first operand of an assignment operator must be a variable, or a compile-time error occurs.

This operand may be a named variable, such as a local variable or a field of the current object or class, or it may be a computed variable, as can result from a field access (§15.11) or an array access (§15.10.3).

The type of the assignment expression is the type of the variable after capture conversion (§5.1.10).

At run time, the result of the assignment expression is the value of the variable after the assignment has occurred. The result of an assignment expression is not itself a variable.

A variable that is declared final cannot be assigned to (unless it is definitely unassigned (§16 (Definite Assignment))), because when an access of such a final variable is used as an expression, the result is a value, not a variable, and so it cannot be used as the first operand of an assignment operator.

15.26.1. Simple Assignment Operator =

A compile-time error occurs if the type of the right-hand operand cannot be converted to the type of the variable by assignment conversion (§5.2).

At run time, the expression is evaluated in one of three ways.

If the left-hand operand expression is a field access expression e.f (§15.11), possibly enclosed in one or more pairs of parentheses, then:

  • First, the expression e is evaluated. If evaluation of e completes abruptly, the assignment expression completes abruptly for the same reason.

  • Next, the right hand operand is evaluated. If evaluation of the right hand expression completes abruptly, the assignment expression completes abruptly for the same reason.

  • Then, if the field denoted by e.f is not static and the result of the evaluation of e above is null, then a NullPointerException is thrown.

  • Otherwise, the variable denoted by e.f is assigned the value of the right hand operand as computed above.

If the left-hand operand is an array access expression (§15.10.3), possibly enclosed in one or more pairs of parentheses, then:

  • First, the array reference subexpression of the left-hand operand array access expression is evaluated. If this evaluation completes abruptly, then the assignment expression completes abruptly for the same reason; the index subexpression (of the left-hand operand array access expression) and the right-hand operand are not evaluated and no assignment occurs.

  • Otherwise, the index subexpression of the left-hand operand array access expression is evaluated. If this evaluation completes abruptly, then the assignment expression completes abruptly for the same reason and the right-hand operand is not evaluated and no assignment occurs.

  • Otherwise, the right-hand operand is evaluated. If this evaluation completes abruptly, then the assignment expression completes abruptly for the same reason and no assignment occurs.

  • Otherwise, if the value of the array reference subexpression is null, then no assignment occurs and a NullPointerException is thrown.

  • Otherwise, the value of the array reference subexpression indeed refers to an array. If the value of the index subexpression is less than zero, or greater than or equal to the length of the array, then no assignment occurs and an ArrayIndexOutOfBoundsException is thrown.

  • Otherwise, the value of the index subexpression is used to select a component of the array referred to by the value of the array reference subexpression.

    This component is a variable; call its type SC. Also, let TC be the type of the left-hand operand of the assignment operator as determined at compile time. Then there are two possibilities:

    • If TC is a primitive type, then SC is necessarily the same as TC.

      The value of the right-hand operand is converted to the type of the selected array component, is subjected to value set conversion (§5.1.13) to the appropriate standard value set (not an extended-exponent value set), and the result of the conversion is stored into the array component.

    • If TC is a reference type, then SC may not be the same as TC, but rather a type that extends or implements TC.

      Let RC be the class of the object referred to by the value of the right-hand operand at run time.

      A Java compiler may be able to prove at compile time that the array component will be of type TC exactly (for example, TC might be final). But if a Java compiler cannot prove at compile time that the array component will be of type TC exactly, then a check must be performed at run time to ensure that the class RC is assignment compatible (§5.2) with the actual type SC of the array component.

      This check is similar to a narrowing cast (§5.5, §15.16), except that if the check fails, an ArrayStoreException is thrown rather than a ClassCastException.

      If class RC is not assignable to type SC, then no assignment occurs and an ArrayStoreException is thrown.

      Otherwise, the reference value of the right-hand operand is stored into the selected array component.

Otherwise, three steps are required:

  • First, the left-hand operand is evaluated to produce a variable. If this evaluation completes abruptly, then the assignment expression completes abruptly for the same reason; the right-hand operand is not evaluated and no assignment occurs.

  • Otherwise, the right-hand operand is evaluated. If this evaluation completes abruptly, then the assignment expression completes abruptly for the same reason and no assignment occurs.

  • Otherwise, the value of the right-hand operand is converted to the type of the left-hand variable, is subjected to value set conversion (§5.1.13) to the appropriate standard value set (not an extended-exponent value set), and the result of the conversion is stored into the variable.

Example 15.26.1-1. Simple Assignment To An Array Component

class ArrayReferenceThrow extends RuntimeException { }
class IndexThrow          extends RuntimeException { }
class RightHandSideThrow  extends RuntimeException { }

class IllustrateSimpleArrayAssignment {
    static Object[] objects = { new Object(), new Object() };
    static Thread[] threads = { new Thread(), new Thread() };

    static Object[] arrayThrow() {
        throw new ArrayReferenceThrow();
    }
    static int indexThrow() {
        throw new IndexThrow();
    }
    static Thread rightThrow() {
        throw new RightHandSideThrow();
    }
    static String name(Object q) {
        String sq = q.getClass().getName();
        int k = sq.lastIndexOf('.');
        return (k < 0) ? sq : sq.substring(k+1);
    }

    static void testFour(Object[] x, int j, Object y) {
        String sx = x == null ? "null" : name(x[0]) + "s";
        String sy = name(y);
        System.out.println();
        try {
            System.out.print(sx + "[throw]=throw => ");
            x[indexThrow()] = rightThrow();
            System.out.println("Okay!");
        } catch (Throwable e) { System.out.println(name(e)); }
        try {
            System.out.print(sx + "[throw]=" + sy + " => ");
            x[indexThrow()] = y;
            System.out.println("Okay!");
        } catch (Throwable e) { System.out.println(name(e)); }
        try {
            System.out.print(sx + "[" + j + "]=throw => ");
            x[j] = rightThrow();
            System.out.println("Okay!");
        } catch (Throwable e) { System.out.println(name(e)); }
        try {
            System.out.print(sx + "[" + j + "]=" + sy + " => ");
            x[j] = y;
            System.out.println("Okay!");
        } catch (Throwable e) { System.out.println(name(e)); }
    }

    public static void main(String[] args) {
        try {
            System.out.print("throw[throw]=throw => ");
            arrayThrow()[indexThrow()] = rightThrow();
            System.out.println("Okay!");
        } catch (Throwable e) { System.out.println(name(e)); }
        try {
            System.out.print("throw[throw]=Thread => ");
            arrayThrow()[indexThrow()] = new Thread();
            System.out.println("Okay!");
        } catch (Throwable e) { System.out.println(name(e)); }
        try {
            System.out.print("throw[1]=throw => ");
            arrayThrow()[1] = rightThrow();
            System.out.println("Okay!");
        } catch (Throwable e) { System.out.println(name(e)); }
        try {
            System.out.print("throw[1]=Thread => ");
            arrayThrow()[1] = new Thread();
            System.out.println("Okay!");
        } catch (Throwable e) { System.out.println(name(e)); }

        testFour(null, 1, new StringBuffer());
        testFour(null, 9, new Thread());
        testFour(objects, 1, new StringBuffer());
        testFour(objects, 1, new Thread());
        testFour(objects, 9, new StringBuffer());
        testFour(objects, 9, new Thread());
        testFour(threads, 1, new StringBuffer());
        testFour(threads, 1, new Thread());
        testFour(threads, 9, new StringBuffer());
        testFour(threads, 9, new Thread());
    }
}

This program produces the output:

throw[throw]=throw => ArrayReferenceThrow
throw[throw]=Thread => ArrayReferenceThrow
throw[1]=throw => ArrayReferenceThrow
throw[1]=Thread => ArrayReferenceThrow

null[throw]=throw => IndexThrow
null[throw]=StringBuffer => IndexThrow
null[1]=throw => RightHandSideThrow
null[1]=StringBuffer => NullPointerException

null[throw]=throw => IndexThrow
null[throw]=Thread => IndexThrow
null[9]=throw => RightHandSideThrow
null[9]=Thread => NullPointerException

Objects[throw]=throw => IndexThrow
Objects[throw]=StringBuffer => IndexThrow
Objects[1]=throw => RightHandSideThrow
Objects[1]=StringBuffer => Okay!

Objects[throw]=throw => IndexThrow
Objects[throw]=Thread => IndexThrow
Objects[1]=throw => RightHandSideThrow
Objects[1]=Thread => Okay!

Objects[throw]=throw => IndexThrow
Objects[throw]=StringBuffer => IndexThrow
Objects[9]=throw => RightHandSideThrow
Objects[9]=StringBuffer => ArrayIndexOutOfBoundsException

Objects[throw]=throw => IndexThrow
Objects[throw]=Thread => IndexThrow
Objects[9]=throw => RightHandSideThrow
Objects[9]=Thread => ArrayIndexOutOfBoundsException

Threads[throw]=throw => IndexThrow
Threads[throw]=StringBuffer => IndexThrow
Threads[1]=throw => RightHandSideThrow
Threads[1]=StringBuffer => ArrayStoreException

Threads[throw]=throw => IndexThrow
Threads[throw]=Thread => IndexThrow
Threads[1]=throw => RightHandSideThrow
Threads[1]=Thread => Okay!

Threads[throw]=throw => IndexThrow
Threads[throw]=StringBuffer => IndexThrow
Threads[9]=throw => RightHandSideThrow
Threads[9]=StringBuffer => ArrayIndexOutOfBoundsException

Threads[throw]=throw => IndexThrow
Threads[throw]=Thread => IndexThrow
Threads[9]=throw => RightHandSideThrow
Threads[9]=Thread => ArrayIndexOutOfBoundsException

The most interesting case of the lot is thirteenth from the end:

Threads[1]=StringBuffer => ArrayStoreException

which indicates that the attempt to store a reference to a StringBuffer into an array whose components are of type Thread throws an ArrayStoreException. The code is type-correct at compile time: the assignment has a left-hand side of type Object[] and a right-hand side of type Object. At run time, the first actual argument to method testFour is a reference to an instance of "array of Thread" and the third actual argument is a reference to an instance of class StringBuffer.


15.26.2. Compound Assignment Operators

A compound assignment expression of the form E1 op= E2 is equivalent to E1 = (T) ((E1) op (E2)), where T is the type of E1, except that E1 is evaluated only once.

For example, the following code is correct:

short x = 3;
x += 4.6;

and results in x having the value 7 because it is equivalent to:

short x = 3;
x = (short)(x + 4.6);

At run time, the expression is evaluated in one of two ways.

If the left-hand operand expression is not an array access expression, then:

  • First, the left-hand operand is evaluated to produce a variable. If this evaluation completes abruptly, then the assignment expression completes abruptly for the same reason; the right-hand operand is not evaluated and no assignment occurs.

  • Otherwise, the value of the left-hand operand is saved and then the right-hand operand is evaluated. If this evaluation completes abruptly, then the assignment expression completes abruptly for the same reason and no assignment occurs.

  • Otherwise, the saved value of the left-hand variable and the value of the right-hand operand are used to perform the binary operation indicated by the compound assignment operator. If this operation completes abruptly, then the assignment expression completes abruptly for the same reason and no assignment occurs.

  • Otherwise, the result of the binary operation is converted to the type of the left-hand variable, subjected to value set conversion (§5.1.13) to the appropriate standard value set (not an extended-exponent value set), and the result of the conversion is stored into the variable.

If the left-hand operand expression is an array access expression (§15.10.3), then:

  • First, the array reference subexpression of the left-hand operand array access expression is evaluated. If this evaluation completes abruptly, then the assignment expression completes abruptly for the same reason; the index subexpression (of the left-hand operand array access expression) and the right-hand operand are not evaluated and no assignment occurs.

  • Otherwise, the index subexpression of the left-hand operand array access expression is evaluated. If this evaluation completes abruptly, then the assignment expression completes abruptly for the same reason and the right-hand operand is not evaluated and no assignment occurs.

  • Otherwise, if the value of the array reference subexpression is null, then no assignment occurs and a NullPointerException is thrown.

  • Otherwise, the value of the array reference subexpression indeed refers to an array. If the value of the index subexpression is less than zero, or greater than or equal to the length of the array, then no assignment occurs and an ArrayIndexOutOfBoundsException is thrown.

  • Otherwise, the value of the index subexpression is used to select a component of the array referred to by the value of the array reference subexpression. The value of this component is saved and then the right-hand operand is evaluated. If this evaluation completes abruptly, then the assignment expression completes abruptly for the same reason and no assignment occurs.

    For a simple assignment operator, the evaluation of the right-hand operand occurs before the checks of the array reference subexpression and the index subexpression, but for a compound assignment operator, the evaluation of the right-hand operand occurs after these checks.

  • Otherwise, consider the array component selected in the previous step, whose value was saved. This component is a variable; call its type S. Also, let T be the type of the left-hand operand of the assignment operator as determined at compile time.

    • If T is a primitive type, then S is necessarily the same as T.

      The saved value of the array component and the value of the right-hand operand are used to perform the binary operation indicated by the compound assignment operator.

      If this operation completes abruptly (the only possibility is an integer division by zero - see §15.17.2), then the assignment expression completes abruptly for the same reason and no assignment occurs.

      Otherwise, the result of the binary operation is converted to the type of the selected array component, subjected to value set conversion (§5.1.13) to the appropriate standard value set (not an extended-exponent value set), and the result of the conversion is stored into the array component.

    • If T is a reference type, then it must be String. Because class String is a final class, S must also be String.

      Therefore the run-time check that is sometimes required for the simple assignment operator is never required for a compound assignment operator.

      The saved value of the array component and the value of the right-hand operand are used to perform the binary operation (string concatenation) indicated by the compound assignment operator (which is necessarily +=). If this operation completes abruptly, then the assignment expression completes abruptly for the same reason and no assignment occurs.

      Otherwise, the String result of the binary operation is stored into the array component.

Example 15.26.2-1. Compound Assignment To An Array Component

class ArrayReferenceThrow extends RuntimeException { }
class IndexThrow          extends RuntimeException { }
class RightHandSideThrow  extends RuntimeException { }

class IllustrateCompoundArrayAssignment {
    static String[] strings = { "Simon", "Garfunkel" };
    static double[] doubles = { Math.E, Math.PI };

    static String[] stringsThrow() {
        throw new ArrayReferenceThrow();
    }
    static double[] doublesThrow() {
        throw new ArrayReferenceThrow();
    }
    static int indexThrow() {
        throw new IndexThrow();
    }
    static String stringThrow() {
        throw new RightHandSideThrow();
    }
    static double doubleThrow() {
        throw new RightHandSideThrow();
    }
    static String name(Object q) {
        String sq = q.getClass().getName();
        int k = sq.lastIndexOf('.');
        return (k < 0) ? sq : sq.substring(k+1);
    }

    static void testEight(String[] x, double[] z, int j) {
        String sx = (x == null) ? "null" : "Strings";
        String sz = (z == null) ? "null" : "doubles";
        System.out.println();
        try {
            System.out.print(sx + "[throw]+=throw => ");
            x[indexThrow()] += stringThrow();
            System.out.println("Okay!");
        } catch (Throwable e) { System.out.println(name(e)); }
        try {
            System.out.print(sz + "[throw]+=throw => ");
            z[indexThrow()] += doubleThrow();
            System.out.println("Okay!");
        } catch (Throwable e) { System.out.println(name(e)); }
        try {
            System.out.print(sx + "[throw]+=\"heh\" => ");
            x[indexThrow()] += "heh";
            System.out.println("Okay!");
        } catch (Throwable e) { System.out.println(name(e)); }
        try {
            System.out.print(sz + "[throw]+=12345 => ");
            z[indexThrow()] += 12345;
            System.out.println("Okay!");
        } catch (Throwable e) { System.out.println(name(e)); }
        try {
            System.out.print(sx + "[" + j + "]+=throw => ");
            x[j] += stringThrow();
            System.out.println("Okay!");
        } catch (Throwable e) { System.out.println(name(e)); }
        try {
            System.out.print(sz + "[" + j + "]+=throw => ");
            z[j] += doubleThrow();
            System.out.println("Okay!");
        } catch (Throwable e) { System.out.println(name(e)); }
        try {
            System.out.print(sx + "[" + j + "]+=\"heh\" => ");
            x[j] += "heh";
            System.out.println("Okay!");
        } catch (Throwable e) { System.out.println(name(e)); }
        try {
            System.out.print(sz + "[" + j + "]+=12345 => ");
            z[j] += 12345;
            System.out.println("Okay!");
        } catch (Throwable e) { System.out.println(name(e)); }
    }

    public static void main(String[] args) {
        try {
            System.out.print("throw[throw]+=throw => ");
            stringsThrow()[indexThrow()] += stringThrow();
            System.out.println("Okay!");
        } catch (Throwable e) { System.out.println(name(e)); }
        try {
            System.out.print("throw[throw]+=throw => ");
            doublesThrow()[indexThrow()] += doubleThrow();
            System.out.println("Okay!");
        } catch (Throwable e) { System.out.println(name(e)); }
        try {
            System.out.print("throw[throw]+=\"heh\" => ");
            stringsThrow()[indexThrow()] += "heh";
            System.out.println("Okay!");
        } catch (Throwable e) { System.out.println(name(e)); }
        try {
            System.out.print("throw[throw]+=12345 => ");
            doublesThrow()[indexThrow()] += 12345;
            System.out.println("Okay!");
        } catch (Throwable e) { System.out.println(name(e)); }
        try {
            System.out.print("throw[1]+=throw => ");
            stringsThrow()[1] += stringThrow();
            System.out.println("Okay!");
        } catch (Throwable e) { System.out.println(name(e)); }
        try {
            System.out.print("throw[1]+=throw => ");
            doublesThrow()[1] += doubleThrow();
            System.out.println("Okay!");
        } catch (Throwable e) { System.out.println(name(e)); }
        try {
            System.out.print("throw[1]+=\"heh\" => ");
            stringsThrow()[1] += "heh";
            System.out.println("Okay!");
        } catch (Throwable e) { System.out.println(name(e)); }
        try {
            System.out.print("throw[1]+=12345 => ");
            doublesThrow()[1] += 12345;
            System.out.println("Okay!");
        } catch (Throwable e) { System.out.println(name(e)); }
        testEight(null, null, 1);
        testEight(null, null, 9);
        testEight(strings, doubles, 1);
        testEight(strings, doubles, 9);
    }
}

This program produces the output:

throw[throw]+=throw => ArrayReferenceThrow
throw[throw]+=throw => ArrayReferenceThrow
throw[throw]+="heh" => ArrayReferenceThrow
throw[throw]+=12345 => ArrayReferenceThrow
throw[1]+=throw => ArrayReferenceThrow
throw[1]+=throw => ArrayReferenceThrow
throw[1]+="heh" => ArrayReferenceThrow
throw[1]+=12345 => ArrayReferenceThrow

null[throw]+=throw => IndexThrow
null[throw]+=throw => IndexThrow
null[throw]+="heh" => IndexThrow
null[throw]+=12345 => IndexThrow
null[1]+=throw => NullPointerException
null[1]+=throw => NullPointerException
null[1]+="heh" => NullPointerException
null[1]+=12345 => NullPointerException

null[throw]+=throw => IndexThrow
null[throw]+=throw => IndexThrow
null[throw]+="heh" => IndexThrow
null[throw]+=12345 => IndexThrow
null[9]+=throw => NullPointerException
null[9]+=throw => NullPointerException
null[9]+="heh" => NullPointerException
null[9]+=12345 => NullPointerException

Strings[throw]+=throw => IndexThrow
doubles[throw]+=throw => IndexThrow
Strings[throw]+="heh" => IndexThrow
doubles[throw]+=12345 => IndexThrow
Strings[1]+=throw => RightHandSideThrow
doubles[1]+=throw => RightHandSideThrow
Strings[1]+="heh" => Okay!
doubles[1]+=12345 => Okay!

Strings[throw]+=throw => IndexThrow
doubles[throw]+=throw => IndexThrow
Strings[throw]+="heh" => IndexThrow
doubles[throw]+=12345 => IndexThrow
Strings[9]+=throw => ArrayIndexOutOfBoundsException
doubles[9]+=throw => ArrayIndexOutOfBoundsException
Strings[9]+="heh" => ArrayIndexOutOfBoundsException
doubles[9]+=12345 => ArrayIndexOutOfBoundsException

The most interesting cases of the lot are eleventh and twelfth from the end:

Strings[1]+=throw => RightHandSideThrow
doubles[1]+=throw => RightHandSideThrow

They are the cases where a right-hand side that throws an exception actually gets to throw the exception; moreover, they are the only such cases in the lot. This demonstrates that the evaluation of the right-hand operand indeed occurs after the checks for a null array reference value and an out-of-bounds index value.


Example 15.26.2-2. Value Of Left-Hand Side Of Compound Assignment Is Saved Before Evaluation Of Right-Hand Side

class Test {
    public static void main(String[] args) {
        int k = 1;
        int[] a = { 1 };
        k += (k = 4) * (k + 2);
        a[0] += (a[0] = 4) * (a[0] + 2);
        System.out.println("k==" + k + " and a[0]==" + a[0]);
    }
}

This program produces the output:

k==25 and a[0]==25

The value 1 of k is saved by the compound assignment operator += before its right-hand operand (k = 4) * (k + 2) is evaluated. Evaluation of this right-hand operand then assigns 4 to k, calculates the value 6 for k + 2, and then multiplies 4 by 6 to get 24. This is added to the saved value 1 to get 25, which is then stored into k by the += operator. An identical analysis applies to the case that uses a[0].

In short, the statements:

k += (k = 4) * (k + 2);
a[0] += (a[0] = 4) * (a[0] + 2);

behave in exactly the same manner as the statements:

k = k + (k = 4) * (k + 2);
a[0] = a[0] + (a[0] = 4) * (a[0] + 2);

15.27. Lambda Expressions

A lambda expression is like a method: it provides a list of formal parameters and a body - an expression or block - expressed in terms of those parameters.

LambdaExpression:

Lambda expressions are always poly expressions (§15.2).

It is a compile-time error if a lambda expression occurs in a program in someplace other than an assignment context (§5.2), an invocation context (§5.3), or a casting context (§5.5).

Evaluation of a lambda expression produces an instance of a functional interface (§9.8). Lambda expression evaluation does not cause the execution of the expression's body; instead, this may occur at a later time when an appropriate method of the functional interface is invoked.

Here are some examples of lambda expressions:


() -> {}                // No parameters; result is void
() -> 42                // No parameters, expression body
() -> null              // No parameters, expression body
() -> { return 42; }    // No parameters, block body with return
() -> { System.gc(); }  // No parameters, void block body

() -> {                 // Complex block body with returns
  if (true) return 12;
  else {
    int result = 15;
    for (int i = 1; i < 10; i++)
      result *= i;
    return result;
  }
}                          

(int x) -> x+1              // Single declared-type parameter
(int x) -> { return x+1; }  // Single declared-type parameter
(x) -> x+1                  // Single inferred-type parameter
x -> x+1                    // Parentheses optional for
                            // single inferred-type parameter

(String s) -> s.length()      // Single declared-type parameter
(Thread t) -> { t.start(); }  // Single declared-type parameter
s -> s.length()               // Single inferred-type parameter
t -> { t.start(); }           // Single inferred-type parameter

(int x, int y) -> x+y  // Multiple declared-type parameters
(x, y) -> x+y          // Multiple inferred-type parameters
(x, int y) -> x+y    // Illegal: can't mix inferred and declared types
(x, final y) -> x+y  // Illegal: no modifiers with inferred types

This syntax has the advantage of minimizing bracket noise around simple lambda expressions, which is especially beneficial when a lambda expression is an argument to a method, or when the body is another lambda expression. It also clearly distinguishes between its expression and statement forms, which avoids ambiguities or over-reliance on ';' tokens. When some extra bracketing is needed to visually distinguish either the full lambda expression or its body expression, parentheses are naturally supported (just as in other cases in which operator precedence is unclear).

The syntax has some parsing challenges. The Java programming language has always had an ambiguity between types and expressions after a '(' token: what follows may be a cast or a parenthesized expression. This was made worse when generics reused the binary operators '<' and '>' in types. Lambda expressions introduce a new possibility: the tokens following '(' may describe a type, an expression, or a lambda parameter list. Some tokens (annotations, final) are unique to parameter lists, while in other cases there are certain patterns that must be interpreted as parameter lists (two names in a row, a ',' not nested inside of '<' and '>'). And sometimes, the ambiguity cannot be resolved until a '->' is encountered, after a ')'. The simplest way to think of how this might be efficiently parsed is with a state machine: each state represents a subset of possible interpretations (type, expression, or parameters), and when the machine transitions to a state in which the set is a singleton, the parser knows which case it is. This does not map very elegantly to a fixed-lookahead grammar, however.

There is no special nullary form: a lambda expression with zero arguments is expressed as () -> .... The obvious special-case syntax, -> ..., does not work because it introduces an ambiguity between argument lists and casts: (x) -> ....

Lambda expressions cannot declare type parameters. While it would make sense semantically to do so, the natural syntax (preceding the parameter list with a type parameter list) introduces messy ambiguities. For example, consider:

foo( (x) < y , z > (w) -> v )

This could be an invocation of foo with one argument (a generic lambda cast to type x), or it could be an invocation of foo with two arguments, both the results of comparisons, the second comparing z with a lambda expression. (Strictly speaking, a lambda expression is meaningless as an operand to the relational operator >, but that is a tenuous assumption on which to build the grammar.)

There is a precedent for ambiguity resolution involving casts, which essentially prohibits the use of - and + following a non-primitive cast (§15.15), but to extend that approach to generic lambdas would involve invasive changes to the grammar.

15.27.1. Lambda Parameters

The formal parameters of a lambda expression may have either declared types or inferred types. These styles cannot be mixed: it is not possible for a lambda expression to declare the types of some of its parameters but leave others to be inferred. Only parameters with declared types can have modifiers.

InferredFormalParameterList:

The following productions from §4.3, §8.3, and §8.4.1 are shown here for convenience:

VariableModifier:
Annotation final
VariableDeclaratorId:
Dims:
{Annotation} [ ] {{Annotation} [ ]}

Receiver parameters are not permitted in the FormalParameters of a lambda expression, as specified in §8.4.1.

A lambda expression whose formal parameters have declared types is said to be explicitly typed, while a lambda expression whose formal parameters have inferred types is said to be implicitly typed. A lambda expression with zero parameters is explicitly typed.

If the formal parameters have inferred types, then these types are derived (§15.27.3) from the functional interface type targeted by the lambda expression.

The syntax for formal parameters with declared types is the same as the syntax for the parameters of a method declaration (§8.4.1).

The declared type of a formal parameter is denoted by the UnannType that appears in its parameter specifier, followed by any bracket pairs that follow the Identifier in the declarator, except for a variable arity parameter, whose declared type is an array type whose component type is the UnannType that appears in its parameter specifier.

No distinction is made between the following lambda parameter lists:

(int... x) -> ..
(int[] x) -> ..

Consistent with the rules for overriding, either can be used, whether the functional interface's abstract method is fixed arity or variable arity. Since lambda expressions are never directly invoked, introducing int... where the functional interface uses int[] can have no impact on the surrounding program. In a lambda body, a variable arity parameter is treated just like an array-typed parameter.

The rules for annotation modifiers on a formal parameter declaration are specified in §9.7.4 and §9.7.5.

It is a compile-time error if final appears more than once as a modifier for a formal parameter declaration.

It is a compile-time error to use mixed array notation (§10.2) for a variable arity parameter.

The scope and shadowing of a formal parameter declaration is specified in §6.3 and §6.4.

It is a compile-time error for a lambda expression to declare two formal parameters with the same name. (That is, their declarations mention the same Identifier.)

It is a compile-time error if a lambda parameter has the name _ (that is, a single underscore character).

The use of the variable name _ in any context is discouraged. Future versions of the Java programming language may reserve this name as a keyword and/or give it special semantics.

It is a compile-time error if a receiver parameter (§8.4.1) appears in the FormalParameters of a lambda expression.

It is a compile-time error if a formal parameter that is declared final is assigned to within the body of the lambda expression.

When the lambda expression is invoked (via a method invocation expression (§15.12)), the values of the actual argument expressions initialize newly created parameter variables, each of the declared or inferred type, before execution of the lambda body. The Identifier that appears in the VariableDeclaratorId or the InferredFormalParameterList may be used as a simple name in the lambda body to refer to the formal parameter.

A lambda parameter of type float always contains an element of the float value set (§4.2.3); similarly, a lambda parameter of type double always contains an element of the double value set. It is not permitted for a lambda parameter of type float to contain an element of the float-extended-exponent value set that is not also an element of the float value set, nor for a lambda parameter of type double to contain an element of the double-extended-exponent value set that is not also an element of the double value set.

When the parameter types of a lambda expression are inferred, the same lambda body can be interpreted in different ways, depending on the context in which it appears. Specifically, the types of expressions in the body, the checked exceptions thrown by the body, and the type correctness of code in the body all depend on the parameters' inferred types. This implies that inference of parameter types must occur "before" attempting to type-check the body of the lambda expression.

15.27.2. Lambda Body

A lambda body is either a single expression or a block (§14.2). Like a method body, a lambda body describes code that will be executed whenever an invocation occurs.

LambdaBody:

Unlike code appearing in anonymous class declarations, the meaning of names and the this and super keywords appearing in a lambda body, along with the accessibility of referenced declarations, are the same as in the surrounding context (except that lambda parameters introduce new names).

The transparency of this (both explicit and implicit) in the body of a lambda expression - that is, treating it the same as in the surrounding context - allows more flexibility for implementations, and prevents the meaning of unqualified names in the body from being dependent on overload resolution.

Practically speaking, it is unusual for a lambda expression to need to talk about itself (either to call itself recursively or to invoke its other methods), while it is more common to want to use names to refer to things in the enclosing class that would otherwise be shadowed (this, toString()). If it is necessary for a lambda expression to refer to itself (as if via this), a method reference or an anonymous inner class should be used instead.

A block lambda body is void-compatible if every return statement in the block has the form return;.

A block lambda body is value-compatible if it cannot complete normally (§14.21) and every return statement in the block has the form return Expression;.

It is a compile-time error if a block lambda body is neither void-compatible nor value-compatible.

In a value-compatible block lambda body, the result expressions are any expressions that may produce an invocation's value. Specifically, for each statement of the form return Expression ; contained by the body, the Expression is a result expression.

The following lambda bodies are void-compatible:

() -> {}
() -> { System.out.println("done"); }

These are value-compatible:

() -> { return "done"; }
() -> { if (...) return 1; else return 0; }

These are both:

() -> { throw new RuntimeException(); }
() -> { while (true); }

This is neither:

() -> { if (...) return "done"; System.out.println("done"); }

The handling of void/value-compatible and the meaning of names in the body jointly serve to minimize the dependency on a particular target type in the given context, which is useful both for implementations and for programmer comprehension. While expressions can be assigned different types during overload resolution depending on the target type, the meaning of unqualified names and the basic structure of the lambda body do not change.

Note that the void/value-compatible definition is not a strictly structural property: "can complete normally" depends on the values of constant expressions, and these may include names that reference constant variables.

Any local variable, formal parameter, or exception parameter used but not declared in a lambda expression must either be declared final or be effectively final (§4.12.4), or a compile-time error occurs where the use is attempted.

Any local variable used but not declared in a lambda body must be definitely assigned (§16 (Definite Assignment)) before the lambda body, or a compile-time error occurs.

Similar rules on variable use apply in the body of an inner class (§8.1.3). The restriction to effectively final variables prohibits access to dynamically-changing local variables, whose capture would likely introduce concurrency problems. Compared to the final restriction, it reduces the clerical burden on programmers.

The restriction to effectively final variables includes standard loop variables, but not enhanced-for loop variables, which are treated as distinct for each iteration of the loop (§14.14.2).

The following lambda bodies demonstrate use of effectively final variables.


void m1(int x) {
    int y = 1;
    foo(() -> x+y);
    // Legal: x and y are both effectively final.
}

void m2(int x) {
    int y;
    y = 1;
    foo(() -> x+y);
    // Legal: x and y are both effectively final.
}

void m3(int x) {
    int y;
    if (...) y = 1;
    foo(() -> x+y);
    // Illegal: y is effectively final, but not definitely assigned.
}

void m4(int x) {
    int y;
    if (...) y = 1; else y = 2;
    foo(() ->; x+y);
    // Legal: x and y are both effectively final.
}


void m5(int x) {
    int y;
    if (...) y = 1;
    y = 2;
    foo(() -> x+y);
    // Illegal: y is not effectively final.
}

void m6(int x) {
    foo(() -> x+1);
    x++;
    // Illegal: x is not effectively final.
}

void m7(int x) {
    foo(() -> x=1);
    // Illegal: x is not effectively final.
}

void m8() {
    int y;
    foo(() -> y=1);
    // Illegal: y is not definitely assigned before the lambda.
}

void m9(String[] arr) {
    for (String s : arr) {
        foo(() -> s);
        // Legal: s is effectively final
        // (it is a new variable on each iteration)
    }
}

void m10(String[] arr) {
    for (int i = 0; i < arr.length; i++) {
        foo(() -> arr[i]);
        // Illegal: i is not effectively final
        // (it is not final, and is incremented)
    }
}

15.27.3. Type of a Lambda Expression

A lambda expression is compatible in an assignment context, invocation context, or casting context with a target type T if T is a functional interface type (§9.8) and the expression is congruent with the function type of the ground target type derived from T.

The ground target type is derived from T as follows:

  • If T is a wildcard-parameterized functional interface type and the lambda expression is explicitly typed, then the ground target type is inferred as described in §18.5.3.

  • If T is a wildcard-parameterized functional interface type and the lambda expression is implicitly typed, then the ground target type is the non-wildcard parameterization (§9.9) of T.

  • Otherwise, the ground target type is T.

A lambda expression is congruent with a function type if all of the following are true:

  • The function type has no type parameters.

  • The number of lambda parameters is the same as the number of parameter types of the function type.

  • If the lambda expression is explicitly typed, its formal parameter types are the same as the parameter types of the function type.

  • If the lambda parameters are assumed to have the same types as the function type's parameter types, then:

    • If the function type's result is void, the lambda body is either a statement expression or a void-compatible block.

    • If the function type's result is a (non-void) type R, then either i) the lambda body is an expression that is compatible with R in an assignment context, or ii) the lambda body is a value-compatible block, and each result expression (§15.27.2) is compatible with R in an assignment context.

If a lambda expression is compatible with a target type T, then the type of the expression, U, is the ground target type derived from T.

It is a compile-time error if any class or interface mentioned by either U or the function type of U is not accessible from the class or interface in which the lambda expression appears.

For each non-static member method m of U, if the function type of U has a subsignature of the signature of m, then a notional method whose method type is the function type of U is deemed to override m, and any compile-time error or unchecked warning specified in §8.4.8.3 may occur.

A checked exception that can be thrown in the body of the lambda expression may cause a compile-time error, as specified in §11.2.3.

The parameter types of explicitly typed lambdas are required to exactly match those of the function type. While it would be possible to be more flexible - allow boxing or contravariance, for example - this kind of generality seems unnecessary, and is inconsistent with the way overriding works in class declarations. A programmer ought to know exactly what function type is being targeted when writing a lambda expression, so he should thus know exactly what signature must be overridden. (In contrast, this is not the case for method references, and so more flexibility is allowed when they are used.) In addition, more flexibility with parameter types would add to the complexity of type inference and overload resolution.

Note that while boxing is not allowed in a strict invocation context, boxing of lambda result expressions is always allowed - that is, the result expression appears in an assignment context, regardless of the context enclosing the lambda expression. However, if an explicitly typed lambda expression is an argument to an overloaded method, a method signature that avoids boxing or unboxing the lambda result is preferred by the most specific check (§15.12.2.5).

If the body of a lambda is a statement expression (that is, an expression that would be allowed to stand alone as a statement), it is compatible with a void-producing function type; any result is simply discarded. So, for example, both of the following are legal:

// Predicate has a boolean result
java.util.function.Predicate<String> p = s -> list.add(s);
// Consumer has a void result
java.util.function.Consumer<String> c = s -> list.add(s);

Generally speaking, a lambda of the form () -> expr, where expr is a statement expression, is interpreted as either () -> { return expr; } or () -> { expr; }, depending on the target type.

15.27.4. Run-time Evaluation of Lambda Expressions

At run time, evaluation of a lambda expression is similar to evaluation of a class instance creation expression, insofar as normal completion produces a reference to an object. Evaluation of a lambda expression is distinct from execution of the lambda body.

Either a new instance of a class with the properties below is allocated and initialized, or an existing instance of a class with the properties below is referenced. If a new instance is to be created, but there is insufficient space to allocate the object, evaluation of the lambda expression completes abruptly by throwing an OutOfMemoryError.

The value of a lambda expression is a reference to an instance of a class with the following properties:

  • The class implements the targeted functional interface type and, if the target type is an intersection type, every other interface type mentioned in the intersection.

  • Where the lambda expression has type U, for each non-static member method m of U:

    If the function type of U has a subsignature of the signature of m, then the class declares a method that overrides m. The method's body has the effect of evaluating the lambda body, if it is an expression, or of executing the lambda body, if it is a block; if a result is expected, it is returned from the method.

    If the erasure of the type of a method being overridden differs in its signature from the erasure of the function type of U, then before evaluating or executing the lambda body, the method's body checks that each argument value is an instance of a subclass or subinterface of the erasure of the corresponding parameter type in the function type of U; if not, a ClassCastException is thrown.

  • The class overrides no other methods of the targeted functional interface type or other interface types mentioned above, although it may override methods of the Object class.

These rules are meant to offer flexibility to implementations of the Java programming language, in that:

  • A new object need not be allocated on every evaluation.

  • Objects produced by different lambda expressions need not belong to different classes (if the bodies are identical, for example).

  • Every object produced by evaluation need not belong to the same class (captured local variables might be inlined, for example).

  • If an "existing instance" is available, it need not have been created at a previous lambda evaluation (it might have been allocated during the enclosing class's initialization, for example).

If the targeted functional interface type is a subtype of java.io.Serializable, the resulting object will automatically be an instance of a serializable class. Making an object derived from a lambda expression serializable can have extra run time overhead and security implications, so lambda-derived objects are not required to be serializable "by default".

15.28. Constant Expressions

ConstantExpression:

A constant expression is an expression denoting a value of primitive type or a String that does not complete abruptly and is composed using only the following:

  • Literals of primitive type and literals of type String (§3.10.1, §3.10.2, §3.10.3, §3.10.4, §3.10.5)

  • Casts to primitive types and casts to type String (§15.16)

  • The unary operators +, -, ~, and ! (but not ++ or --) (§15.15.3, §15.15.4, §15.15.5, §15.15.6)

  • The multiplicative operators *, /, and % (§15.17)

  • The additive operators + and - (§15.18)

  • The shift operators <<, >>, and >>> (§15.19)

  • The relational operators <, <=, >, and >= (but not instanceof) (§15.20)

  • The equality operators == and != (§15.21)

  • The bitwise and logical operators &, ^, and | (§15.22)

  • The conditional-and operator && and the conditional-or operator || (§15.23, §15.24)

  • The ternary conditional operator ? : (§15.25)

  • Parenthesized expressions (§15.8.5) whose contained expression is a constant expression.

  • Simple names (§6.5.6.1) that refer to constant variables (§4.12.4).

  • Qualified names (§6.5.6.2) of the form TypeName . Identifier that refer to constant variables (§4.12.4).

Constant expressions of type String are always "interned" so as to share unique instances, using the method String.intern.

A constant expression is always treated as FP-strict (§15.4), even if it occurs in a context where a non-constant expression would not be considered to be FP-strict.

Constant expressions are used as case labels in switch statements (§14.11) and have a special significance for assignment conversion (§5.2) and initialization of a class or interface (§12.4.2). They may also govern the ability of a while, do, or for statement to complete normally (§14.21), and the type of a conditional operator ? : with numeric operands.

Example 15.28-1. Constant Expressions

true
(short)(1*2*3*4*5*6)
Integer.MAX_VALUE / 2
2.0 * Math.PI
"The integer " + Long.MAX_VALUE + " is mighty big."