Chapter 13. Binary Compatibility

Table of Contents

13.1. The Form of a Binary
13.2. What Binary Compatibility Is and Is Not
13.3. Evolution of Packages
13.4. Evolution of Classes
13.4.1. abstract Classes
13.4.2. final Classes
13.4.3. public Classes
13.4.4. Superclasses and Superinterfaces
13.4.5. Class Type Parameters
13.4.6. Class Body and Member Declarations
13.4.7. Access to Members and Constructors
13.4.8. Field Declarations
13.4.9. final Fields and static Constant Variables
13.4.10. static Fields
13.4.11. transient Fields
13.4.12. Method and Constructor Declarations
13.4.13. Method and Constructor Type Parameters
13.4.14. Method and Constructor Formal Parameters
13.4.15. Method Result Type
13.4.16. abstract Methods
13.4.17. final Methods
13.4.18. native Methods
13.4.19. static Methods
13.4.20. synchronized Methods
13.4.21. Method and Constructor Throws
13.4.22. Method and Constructor Body
13.4.23. Method and Constructor Overloading
13.4.24. Method Overriding
13.4.25. Static Initializers
13.4.26. Evolution of Enums
13.5. Evolution of Interfaces
13.5.1. public Interfaces
13.5.2. Superinterfaces
13.5.3. Interface Members
13.5.4. Interface Type Parameters
13.5.5. Field Declarations
13.5.6. Interface Method Declarations
13.5.7. Evolution of Annotation Types

Development tools for the Java programming language should support automatic recompilation as necessary whenever source code is available. Particular implementations may also store the source and binary of types in a versioning database and implement a ClassLoader that uses integrity mechanisms of the database to prevent linkage errors by providing binary-compatible versions of types to clients.

Developers of packages and classes that are to be widely distributed face a different set of problems. In the Internet, which is our favorite example of a widely distributed system, it is often impractical or impossible to automatically recompile the pre-existing binaries that directly or indirectly depend on a type that is to be changed. Instead, this specification defines a set of changes that developers are permitted to make to a package or to a class or interface type while preserving (not breaking) compatibility with pre-existing binaries.

Within the framework of Release-to-Release Binary Compatibility in SOM (Forman, Conner, Danforth, and Raper, Proceedings of OOPSLA '95), Java programming language binaries are binary compatible under all relevant transformations that the authors identify (with some caveats with respect to the addition of instance variables). Using their scheme, here is a list of some important binary compatible changes that the Java programming language supports:

This chapter specifies minimum standards for binary compatibility guaranteed by all implementations. The Java programming language guarantees compatibility when binaries of classes and interfaces are mixed that are not known to be from compatible sources, but whose sources have been modified in the compatible ways described here. Note that we are discussing compatibility between releases of an application. A discussion of compatibility among releases of the Java SE platform is beyond the scope of this chapter.

We encourage development systems to provide facilities that alert developers to the impact of changes on pre-existing binaries that cannot be recompiled.

This chapter first specifies some properties that any binary format for the Java programming language must have (§13.1). It next defines binary compatibility, explaining what it is and what it is not (§13.2). It finally enumerates a large set of possible changes to packages (§13.3), classes (§13.4), and interfaces (§13.5), specifying which of these changes are guaranteed to preserve binary compatibility and which are not.

13.1. The Form of a Binary

Programs must be compiled either into the class file format specified by The Java Virtual Machine Specification, Java SE 8 Edition, or into a representation that can be mapped into that format by a class loader written in the Java programming language.

Furthermore, the resulting class file must have certain properties. A number of these properties are specifically chosen to support source code transformations that preserve binary compatibility. The required properties are:

  1. The class or interface must be named by its binary name, which must meet the following constraints:

    • The binary name of a top level type (§7.6) is its canonical name (§6.7).

    • The binary name of a member type (§8.5, §9.5) consists of the binary name of its immediately enclosing type, followed by $, followed by the simple name of the member.

    • The binary name of a local class (§14.3) consists of the binary name of its immediately enclosing type, followed by $, followed by a non-empty sequence of digits, followed by the simple name of the local class.

    • The binary name of an anonymous class (§15.9.5) consists of the binary name of its immediately enclosing type, followed by $, followed by a non-empty sequence of digits.

    • The binary name of a type variable declared by a generic class or interface (§8.1.2, §9.1.2) is the binary name of its immediately enclosing type, followed by $, followed by the simple name of the type variable.

    • The binary name of a type variable declared by a generic method (§8.4.4) is the binary name of the type declaring the method, followed by $, followed by the descriptor of the method as defined in The Java Virtual Machine Specification, Java SE 8 Edition (JVMS §4.3.3), followed by $, followed by the simple name of the type variable.

    • The binary name of a type variable declared by a generic constructor (§8.8.4) is the binary name of the type declaring the constructor, followed by $, followed by the descriptor of the constructor as defined in The Java Virtual Machine Specification, Java SE 8 Edition (JVMS §4.3.3), followed by $, followed by the simple name of the type variable.

  2. A reference to another class or interface type must be symbolic, using the binary name of the type.

  3. A reference to a field that is a constant variable (§4.12.4) must be resolved at compile time to the value V denoted by the constant variable's initializer.

    If such a field is static, then no reference to the field should be present in the code in a binary file, including the class or interface which declared the field. Such a field must always appear to have been initialized (§12.4.2); the default initial value for the field (if different than V) must never be observed.

    If such a field is non-static, then no reference to the field should be present in the code in a binary file, except in the class containing the field. (It will be a class rather than an interface, since an interface has only static fields.) The class should have code to set the field's value to V during instance creation (§12.5).

  4. Given a legal expression denoting a field access in a class C, referencing a field named f that is not a constant variable and is declared in a (possibly distinct) class or interface D, we define the qualifying type of the field reference as follows:

    • If the expression is referenced by a simple name, then if f is a member of the current class or interface, C, then let T be C. Otherwise, let T be the innermost lexically enclosing type declaration of which f is a member. In either case, T is the qualifying type of the reference.

    • If the reference is of the form TypeName.f, where TypeName denotes a class or interface, then the class or interface denoted by TypeName is the qualifying type of the reference.

    • If the expression is of the form ExpressionName.f or Primary.f, then:

      • If the compile-time type of ExpressionName or Primary is an intersection type V1 & ... & Vn (§4.9), then the qualifying type of the reference is V1.

      • Otherwise, the compile-time type of ExpressionName or Primary is the qualifying type of the reference.

    • If the expression is of the form super.f, then the superclass of C is the qualifying type of the reference.

    • If the expression is of the form TypeName.super.f, then the superclass of the class denoted by TypeName is the qualifying type of the reference.

    The reference to f must be compiled into a symbolic reference to the erasure (§4.6) of the qualifying type of the reference, plus the simple name of the field, f. The reference must also include a symbolic reference to the erasure of the declared type of the field so that the verifier can check that the type is as expected.

  5. Given a method invocation expression or a method reference expression in a class or interface C, referencing a method named m declared (or implicitly declared (§9.2)) in a (possibly distinct) class or interface D, we define the qualifying type of the method invocation as follows:

    • If D is Object then the qualifying type of the expression is Object.

    • Otherwise:

      • If the method is referenced by a simple name, then if m is a member of the current class or interface C, let T be C; otherwise, let T be the innermost lexically enclosing type declaration of which m is a member. In either case, T is the qualifying type of the method invocation.

      • If the expression is of the form TypeName.m or ReferenceType::m, then the type denoted by TypeName or ReferenceType is the qualifying type of the method invocation.

      • If the expression is of the form ExpressionName.m or Primary.m or ExpressionName::m or Primary::m, then:

        • If the compile-time type of ExpressionName or Primary is an intersection type V1 & ... & Vn (§4.9), then the qualifying type of the method invocation is V1.

        • Otherwise, the compile-time type of ExpressionName or Primary is the qualifying type of the method invocation.

      • If the expression is of the form super.m or super::m, then the superclass of C is the qualifying type of the method invocation.

      • If the expression is of the form TypeName.super.m or TypeName.super::m, then if TypeName denotes a class X, the superclass of X is the qualifying type of the method invocation; if TypeName denotes an interface X, X is the qualifying type of the method invocation.

    A reference to a method must be resolved at compile time to a symbolic reference to the erasure (§4.6) of the qualifying type of the invocation, plus the erasure of the signature (§8.4.2) of the method. The signature of a method must include all of the following as determined by §15.12.3:

    • The simple name of the method

    • The number of parameters to the method

    • A symbolic reference to the type of each parameter

    A reference to a method must also include either a symbolic reference to the erasure of the return type of the denoted method or an indication that the denoted method is declared void and does not return a value.

  6. Given a class instance creation expression (§15.9) or an explicit constructor invocation statement (§8.8.7.1) or a method reference expression of the form ClassType :: new (§15.13) in a class or interface C referencing a constructor m declared in a (possibly distinct) class or interface D, we define the qualifying type of the constructor invocation as follows:

    • If the expression is of the form new D(...) or ExpressionName.new D(...) or Primary.new D(...) or D :: new, then the qualifying type of the invocation is D.

    • If the expression is of the form new D(...){...} or ExpressionName.new D(...){...} or Primary.new D(...){...}, then the qualifying type of the expression is the compile-time type of the expression.

    • If the expression is of the form super(...) or ExpressionName.super(...) or Primary.super(...), then the qualifying type of the expression is the direct superclass of C.

    • If the expression is of the form this(...), then the qualifying type of the expression is C.

    A reference to a constructor must be resolved at compile time to a symbolic reference to the erasure (§4.6) of the qualifying type of the invocation, plus the signature of the constructor (§8.8.2). The signature of a constructor must include both:

    • The number of parameters of the constructor

    • A symbolic reference to the type of each formal parameter

A binary representation for a class or interface must also contain all of the following:

  1. If it is a class and is not Object, then a symbolic reference to the erasure of the direct superclass of this class.

  2. A symbolic reference to the erasure of each direct superinterface, if any.

  3. A specification of each field declared in the class or interface, given as the simple name of the field and a symbolic reference to the erasure of the type of the field.

  4. If it is a class, then the erased signature of each constructor, as described above.

  5. For each method declared in the class or interface (excluding, for an interface, its implicitly declared methods (§9.2)), its erased signature and return type, as described above.

  6. The code needed to implement the class or interface:

    • For an interface, code for the field initializers and the implementation of each default method.

    • For a class, code for the field initializers, the instance and static initializers, and the implementation of each method or constructor.

  7. Every type must contain sufficient information to recover its canonical name (§6.7).

  8. Every member type must have sufficient information to recover its source level access modifier.

  9. Every nested class and nested interface must have a symbolic reference to its immediately enclosing class (§8.1.3).

  10. Every class must contain symbolic references to all of its member types (§8.5), and to all local and anonymous classes that appear in its methods, constructors, static initializers, instance initializers, and field initializers.

    Every interface must contain symbolic references to all of its member types (§9.5), and to all local and anonymous classes that appear in its default methods and field initializers.

  11. A construct emitted by a Java compiler must be marked as synthetic if it does not correspond to a construct declared explicitly or implicitly in source code, unless the emitted construct is a class initialization method (JVMS §2.9).

  12. A construct emitted by a Java compiler must be marked as mandated if it corresponds to a formal parameter declared implicitly in source code (§8.8.1, §8.8.9, §8.9.3, §15.9.5.1).

The following formal parameters are declared implicitly in source code:

  • The first formal parameter of a constructor of a non-private inner member class (§8.8.1, §8.8.9).

  • The first formal parameter of an anonymous constructor of an anonymous class whose superclass is inner or local (not in a static context) (§15.9.5.1).

  • The formal parameter name of the valueOf method which is implicitly declared in an enum type (§8.9.3).

For reference, the following constructs are declared implicitly in source code, but are not marked as mandated because only formal parameters can be so marked in a class file (JVMS §4.7.22):

  • Default constructors of classes and enum types (§8.8.9, §8.9.2)

  • Anonymous constructors (§15.9.5.1)

  • The values and valueOf methods of enum types (§8.9.3)

  • Certain public fields of enum types (§8.9.3)

  • Certain public methods of interfaces (§9.2)

  • Container annotations (§9.7.5)

The following sections discuss changes that may be made to class and interface type declarations without breaking compatibility with pre-existing binaries. Under the translation requirements given above, the Java Virtual Machine and its class file format support these changes. Any other valid binary format, such as a compressed or encrypted representation that is mapped back into class files by a class loader under the above requirements, will necessarily support these changes as well.

13.2. What Binary Compatibility Is and Is Not

A change to a type is binary compatible with (equivalently, does not break binary compatibility with) pre-existing binaries if pre-existing binaries that previously linked without error will continue to link without error.

Binaries are compiled to rely on the accessible members and constructors of other classes and interfaces. To preserve binary compatibility, a class or interface should treat its accessible members and constructors, their existence and behavior, as a contract with its users.

The Java programming language is designed to prevent additions to contracts and accidental name collisions from breaking binary compatibility. Specifically, addition of more methods overloading a particular method name does not break compatibility with pre-existing binaries. The method signature that the pre-existing binary will use for method lookup is chosen by the overload resolution algorithm at compile time (§15.12.2).

If the Java programming language had been designed so that the particular method to be executed was chosen at run time, then such an ambiguity might be detected at run time. Such a rule would imply that adding an additional overloaded method so as to make ambiguity possible at a call site could break compatibility with an unknown number of pre-existing binaries. See §13.4.23 for more discussion.

Binary compatibility is not the same as source compatibility. In particular, the example in §13.4.6 shows that a set of compatible binaries can be produced from sources that will not compile all together. This example is typical: a new declaration is added, changing the meaning of a name in an unchanged part of the source code, while the pre-existing binary for that unchanged part of the source code retains the fully-qualified, previous meaning of the name. Producing a consistent set of source code requires providing a qualified name or field access expression corresponding to the previous meaning.

13.3. Evolution of Packages

A new top level class or interface type may be added to a package without breaking compatibility with pre-existing binaries, provided the new type does not reuse a name previously given to an unrelated type.

If a new type reuses a name previously given to an unrelated type, then a conflict may result, since binaries for both types could not be loaded by the same class loader.

Changes in top level class and interface types that are not public and that are not a superclass or superinterface, respectively, of a public type, affect only types within the package in which they are declared. Such types may be deleted or otherwise changed, even if incompatibilities are otherwise described here, provided that the affected binaries of that package are updated together.

13.4. Evolution of Classes

This section describes the effects of changes to the declaration of a class and its members and constructors on pre-existing binaries.

13.4.1. abstract Classes

If a class that was not declared abstract is changed to be declared abstract, then pre-existing binaries that attempt to create new instances of that class will throw either an InstantiationError at link time, or (if a reflective method is used) an InstantiationException at run time; such a change is therefore not recommended for widely distributed classes.

Changing a class that is declared abstract to no longer be declared abstract does not break compatibility with pre-existing binaries.

13.4.2. final Classes

If a class that was not declared final is changed to be declared final, then a VerifyError is thrown if a binary of a pre-existing subclass of this class is loaded, because final classes can have no subclasses; such a change is not recommended for widely distributed classes.

Changing a class that is declared final to no longer be declared final does not break compatibility with pre-existing binaries.

13.4.3. public Classes

Changing a class that is not declared public to be declared public does not break compatibility with pre-existing binaries.

If a class that was declared public is changed to not be declared public, then an IllegalAccessError is thrown if a pre-existing binary is linked that needs but no longer has access to the class type; such a change is not recommended for widely distributed classes.

13.4.4. Superclasses and Superinterfaces

A ClassCircularityError is thrown at load time if a class would be a superclass of itself. Changes to the class hierarchy that could result in such a circularity when newly compiled binaries are loaded with pre-existing binaries are not recommended for widely distributed classes.

Changing the direct superclass or the set of direct superinterfaces of a class type will not break compatibility with pre-existing binaries, provided that the total set of superclasses or superinterfaces, respectively, of the class type loses no members.

If a change to the direct superclass or the set of direct superinterfaces results in any class or interface no longer being a superclass or superinterface, respectively, then linkage errors may result if pre-existing binaries are loaded with the binary of the modified class. Such changes are not recommended for widely distributed classes.

Example 13.4.4-1. Changing A Superclass

Suppose that the following test program:

class Hyper { char h = 'h'; } 
class Super extends Hyper { char s = 's'; }
class Test extends Super {
    public static void printH(Hyper h) {
        System.out.println(h.h);
    }
    public static void main(String[] args) {
        printH(new Super());
    }
}

is compiled and executed, producing the output:

h

Suppose that a new version of class Super is then compiled:

class Super { char s = 's'; }

This version of class Super is not a subclass of Hyper. If we then run the existing binaries of Hyper and Test with the new version of Super, then a VerifyError is thrown at link time. The verifier objects because the result of new Super() cannot be passed as an argument in place of a formal parameter of type Hyper, because Super is not a subclass of Hyper.

It is instructive to consider what might happen without the verification step: the program might run and print:

s

This demonstrates that without the verifier, the Java type system could be defeated by linking inconsistent binary files, even though each was produced by a correct Java compiler.

The lesson is that an implementation that lacks a verifier or fails to use it will not maintain type safety and is, therefore, not a valid implementation.


The requirement that alternatives in a multi-catch clause (§14.20) not be subclasses or superclasses of each other is only a source restriction. Assuming the following client code is legal:

try {
    throwAorB();
} catch(ExceptionA | ExceptionB e) {
    ...
}

where ExceptionA and ExceptionB do not have a subclass/superclass relationship when the client is compiled, it is binary compatible with respect to the client for ExceptionA and ExceptionB to have such a relationship when the client is executed.

This is analogous to other situations where a class transformation that is binary compatible for a client might not be source compatible for the same client.

13.4.5. Class Type Parameters

Adding or removing a type parameter of a class does not, in itself, have any implications for binary compatibility.

If such a type parameter is used in the type of a field or method, that may have the normal implications of changing the aforementioned type.

Renaming a type parameter of a class has no effect with respect to pre-existing binaries.

Changing the first bound of a type parameter of a class may change the erasure (§4.6) of any member that uses that type parameter in its own type, and this may affect binary compatibility. The change of such a bound is analogous to the change of the first bound of a type parameter of a method or constructor (§13.4.13).

Changing any other bound has no effect on binary compatibility.

13.4.6. Class Body and Member Declarations

No incompatibility with pre-existing binaries is caused by adding an instance (respectively static) member that has the same name and accessibility (for fields), or same name and accessibility and signature and return type (for methods), as an instance (respectively static) member of a superclass or subclass. No error occurs even if the set of classes being linked would encounter a compile-time error.

Deleting a class member or constructor that is not declared private may cause a linkage error if the member or constructor is used by a pre-existing binary.

Example 13.4.6-1. Changing A Class Body

class Hyper {
    void hello() { System.out.println("hello from Hyper"); }
}
class Super extends Hyper {
    void hello() { System.out.println("hello from Super"); }
}
class Test {
    public static void main(String[] args) {
        new Super().hello();
    }
}

This program produces the output:

hello from Super

Suppose that a new version of class Super is produced:

class Super extends Hyper {}

Then, recompiling Super and executing this new binary with the original binaries for Test and Hyper produces the output:

hello from Hyper

as expected.


The super keyword can be used to access a method declared in a superclass, bypassing any methods declared in the current class. The expression super.Identifier is resolved, at compile time, to a method m in the superclass S. If the method m is an instance method, then the method which is invoked at run time is the method with the same signature as m that is a member of the direct superclass of the class containing the expression involving super.

Example 13.4.6-2. Changing A Superclass

class Hyper {
    void hello() { System.out.println("hello from Hyper"); }
}
class Super extends Hyper { }
class Test extends Super {
    public static void main(String[] args) {
        new Test().hello();
    }
    void hello() {
        super.hello();
    }
}

This program produces the output:

hello from Hyper

Suppose that a new version of class Super is produced:

class Super extends Hyper {
    void hello() { System.out.println("hello from Super"); }
}

Then, if Super and Hyper are recompiled but not Test, then running the new binaries with the existing binary of Test produces the output:

hello from Super

as you might expect.


13.4.7. Access to Members and Constructors

Changing the declared access of a member or constructor to permit less access may break compatibility with pre-existing binaries, causing a linkage error to be thrown when these binaries are resolved. Less access is permitted if the access modifier is changed from package access to private access; from protected access to package or private access; or from public access to protected, package, or private access. Changing a member or constructor to permit less access is therefore not recommended for widely distributed classes.

Perhaps surprisingly, the binary format is defined so that changing a member or constructor to be more accessible does not cause a linkage error when a subclass (already) defines a method to have less access.

Example 13.4.7-1. Changing Accessibility

If the package points defines the class Point:

package points;
public class Point {
    public int x, y;
    protected void print() {
        System.out.println("(" + x + "," + y + ")");
    }
}

used by the program:

class Test extends points.Point {
    public static void main(String[] args) {
        Test t = new Test();
        t.print();
    }
    protected void print() { 
        System.out.println("Test"); 
    }
}

then these classes compile and Test executes to produce the output:

Test

If the method print in class Point is changed to be public, and then only the Point class is recompiled, and then executed with the previously existing binary for Test, then no linkage error occurs. This happens even though it is improper, at compile time, for a public method to be overridden by a protected method (as shown by the fact that the class Test could not be recompiled using this new Point class unless print in Test were changed to be public.)


Allowing superclasses to change protected methods to be public without breaking binaries of pre-existing subclasses helps make binaries less fragile. The alternative, where such a change would cause a linkage error, would create additional binary incompatibilities.

13.4.8. Field Declarations

Widely distributed programs should not expose any fields to their clients. Apart from the binary compatibility issues discussed below, this is generally good software engineering practice. Adding a field to a class may break compatibility with pre-existing binaries that are not recompiled.

Assume a reference to a field f with qualifying type T. Assume further that f is in fact an instance (respectively static) field declared in a superclass of T, S, and that the type of f is X.

If a new field of type X with the same name as f is added to a subclass of S that is a superclass of T or T itself, then a linkage error may occur. Such a linkage error will occur only if, in addition to the above, either one of the following is true:

  • The new field is less accessible than the old one.

  • The new field is a static (respectively instance) field.

In particular, no linkage error will occur in the case where a class could no longer be recompiled because a field access previously referenced a field of a superclass with an incompatible type. The previously compiled class with such a reference will continue to reference the field declared in a superclass.

Example 13.4.8-1. Adding A Field Declaration

class Hyper { String h = "hyper"; }
class Super extends Hyper { String s = "super"; }
class Test {
    public static void main(String[] args) {
        System.out.println(new Super().h);
    }
}

This program produces the output:

hyper

Suppose a new version of class Super is produced:

class Super extends Hyper {
    String s = "super";
    int h = 0;
}

Then, recompiling Hyper and Super, and executing the resulting new binaries with the old binary of Test produces the output:

hyper

The field h of Hyper is output by the original binary of Test. While this may seem surprising at first, it serves to reduce the number of incompatibilities that occur at run time. (In an ideal world, all source files that needed recompilation would be recompiled whenever any one of them changed, eliminating such surprises. But such a mass recompilation is often impractical or impossible, especially in the Internet. And, as was previously noted, such recompilation would sometimes require further changes to the source code.)

As another example, if the program:

class Hyper { String h = "Hyper"; }
class Super extends Hyper { }
class Test extends Super {
    public static void main(String[] args) {
        String s = new Test().h;
        System.out.println(s);
    }
}

is compiled and executed, it produces the output:

Hyper

Suppose that a new version of class Super is then compiled:

class Super extends Hyper { char h = 'h'; }

If the resulting binary is used with the existing binaries for Hyper and Test, then the output is still:

Hyper

even though compiling the source for these binaries:

class Hyper { String h = "Hyper"; }
class Super extends Hyper { char h = 'h'; }
class Test extends Super {
    public static void main(String[] args) {
        String s = new Test().h;
        System.out.println(s);
    }
}

would result in a compile-time error, because the h in the source code for main would now be construed as referring to the char field declared in Super, and a char value can't be assigned to a String.


Deleting a field from a class will break compatibility with any pre-existing binaries that reference this field, and a NoSuchFieldError will be thrown when such a reference from a pre-existing binary is linked. Only private fields may be safely deleted from a widely distributed class.

For purposes of binary compatibility, adding or removing a field f whose type involves type variables (§4.4) or parameterized types (§4.5) is equivalent to the addition (respectively, removal) of a field of the same name whose type is the erasure (§4.6) of the type of f.

13.4.9. final Fields and static Constant Variables

If a field that was not declared final is changed to be declared final, then it can break compatibility with pre-existing binaries that attempt to assign new values to the field.

Example 13.4.9-1. Changing A Variable To Be final

class Super { char s; }
class Test extends Super {
    public static void main(String[] args) {
        Super x = new Super();
        x.s = 'a';
        System.out.println(x.s);
    }
}

This program produces the output:

a

Suppose that a new version of class Super is produced:

class Super { final char s = 'b'; }

If Super is recompiled but not Test, then running the new binary with the existing binary of Test results in a IllegalAccessError.


Deleting the keyword final or changing the value to which a field is initialized does not break compatibility with existing binaries.

If a field is a constant variable (§4.12.4), and moreover is static, then deleting the keyword final or changing its value will not break compatibility with pre-existing binaries by causing them not to run, but they will not see any new value for a usage of the field unless they are recompiled. This result is a side-effect of the decision to support conditional compilation (§14.21). (One might suppose that the new value is not seen if the usage occurs in a constant expression (§15.28) but is seen otherwise. This is not so; pre-existing binaries do not see the new value at all.)

Another reason for requiring inlining of values of static constant variables is because of switch statements. They are the only kind of statement that relies on constant expressions, namely that each case label of a switch statement must be a constant expression whose value is different than every other case label. case labels are often references to static constant variables so it may not be immediately obvious that all the labels have different values. If it is proven that there are no duplicate labels at compile time, then inlining the values into the class file ensures there are no duplicate labels at run time either - a very desirable property.

Example 13.4.9-2. Conditional Compilation

If the example:

class Flags { static final boolean debug = true; }
class Test {
    public static void main(String[] args) {
        if (Flags.debug)
            System.out.println("debug is true");
    }
}

is compiled and executed, it produces the output:

debug is true

Suppose that a new version of class Flags is produced:

class Flags { static final boolean debug = false; }

If Flags is recompiled but not Test, then running the new binary with the existing binary of Test produces the output:

debug is true

because the value of debug was a constant expression, and could have been used in compiling Test without making a reference to the class Flags.

This behavior would not change if Flags were changed to be an interface, as in the modified example:

interface Flags { boolean debug = true; }
class Test {
    public static void main(String[] args) {
        if (Flags.debug)
            System.out.println("debug is true");
    }
}

Conditional compilation is discussed further at the end of §14.21.


The best way to avoid problems with "inconstant constants" in widely-distributed code is to use static constant variables only for values which truly are unlikely ever to change. Other than for true mathematical constants, we recommend that source code make very sparing use of static constant variables.

If the read-only nature of final is required, a better choice is to declare a private static variable and a suitable accessor method to get its value. Thus we recommend:


private static int N;
public static int getN() { return N; }

rather than:


public static final int N = ...;

There is no problem with:


public static int N = ...;

if N need not be read-only.

We recommend, as a general rule, that only constant expressions be assigned to fields of interfaces.

We note, but do not recommend, that if a field of primitive type of an interface may change, its value may be expressed idiomatically as in:


interface Flags {
    boolean debug = new Boolean(true).booleanValue();
}

ensuring that this value is not a constant. Similar idioms exist for the other primitive types.

One other thing to note is that static constant variables must never appear to have the default initial value for their type (§4.12.5). This means that all such fields appear to be initialized first during class initialization (§8.3.2, §9.3.1, §12.4.2).

13.4.10. static Fields

If a field that is not declared private was not declared static and is changed to be declared static, or vice versa, then a linkage error, specifically an IncompatibleClassChangeError, will result if the field is used by a pre-existing binary which expected a field of the other kind. Such changes are not recommended in code that has been widely distributed.

13.4.11. transient Fields

Adding or deleting a transient modifier of a field does not break compatibility with pre-existing binaries.

13.4.12. Method and Constructor Declarations

Adding a method or constructor declaration to a class will not break compatibility with any pre-existing binaries, even in the case where a type could no longer be recompiled because an invocation previously referenced a method or constructor of a superclass with an incompatible type. The previously compiled class with such a reference will continue to reference the method or constructor declared in a superclass.

Assume a reference to a method m with qualifying type T. Assume further that m is in fact an instance (respectively static) method declared in a superclass of T, S.

If a new method of type X with the same signature and return type as m is added to a subclass of S that is a superclass of T or T itself, then a linkage error may occur. Such a linkage error will occur only if, in addition to the above, either one of the following is true:

  • The new method is less accessible than the old one.

  • The new method is a static (respectively instance) method.

Deleting a method or constructor from a class may break compatibility with any pre-existing binary that referenced this method or constructor; a NoSuchMethodError may be thrown when such a reference from a pre-existing binary is linked. Such an error will occur only if no method with a matching signature and return type is declared in a superclass.

If the source code for a non-inner class contains no declared constructors, then a default constructor with no parameters is implicitly declared (§8.8.9). Adding one or more constructor declarations to the source code of such a class will prevent this default constructor from being implicitly declared, effectively deleting a constructor, unless one of the new constructors also has no parameters, thus replacing the default constructor. The default constructor with no parameters is given the same access modifier as the class of its declaration, so any replacement should have as much or more access if compatibility with pre-existing binaries is to be preserved.

13.4.13. Method and Constructor Type Parameters

Adding or removing a type parameter of a method or constructor does not, in itself, have any implications for binary compatibility.

If such a type parameter is used in the type of the method or constructor, that may have the normal implications of changing the aforementioned type.

Renaming a type parameter of a method or constructor has no effect with respect to pre-existing binaries.

Changing the first bound of a type parameter of a method or constructor may change the erasure (§4.6) of any member that uses that type parameter in its own type, and this may affect binary compatibility. Specifically:

  • If the type parameter is used as the type of a field, the effect is as if the field was removed and a field with the same name, whose type is the new erasure of the type variable, was added.

  • If the type parameter is used as the type of any formal parameter of a method, but not as the return type, the effect is as if that method were removed, and replaced with a new method that is identical except for the types of the aforementioned formal parameters, which now have the new erasure of the type parameter as their type.

  • If the type parameter is used as a return type of a method, but not as the type of any formal parameter of the method, the effect is as if that method were removed, and replaced with a new method that is identical except for the return type, which is now the new erasure of the type parameter.

  • If the type parameter is used as a return type of a method and as the type of one or more formal parameters of the method, the effect is as if that method were removed, and replaced with a new method that is identical except for the return type, which is now the new erasure of the type parameter, and except for the types of the aforementioned formal parameters, which now have the new erasure of the type parameter as their types.

Changing any other bound has no effect on binary compatibility.

13.4.14. Method and Constructor Formal Parameters

Changing the name of a formal parameter of a method or constructor does not impact pre-existing binaries.

Changing the name of a method, or the type of a formal parameter to a method or constructor, or adding a parameter to or deleting a parameter from a method or constructor declaration creates a method or constructor with a new signature, and has the combined effect of deleting the method or constructor with the old signature and adding a method or constructor with the new signature (§13.4.12).

Changing the type of the last formal parameter of a method from T[] to a variable arity parameter (§8.4.1) of type T (i.e. to T...), and vice versa, does not impact pre-existing binaries.

For purposes of binary compatibility, adding or removing a method or constructor m whose signature involves type variables (§4.4) or parameterized types (§4.5) is equivalent to the addition (respectively, removal) of an otherwise equivalent method whose signature is the erasure (§4.6) of the signature of m.

13.4.15. Method Result Type

Changing the result type of a method, or replacing a result type with void, or replacing void with a result type, has the combined effect of deleting the old method and adding a new method with the new result type or newly void result (see §13.4.12).

For purposes of binary compatibility, adding or removing a method or constructor m whose return type involves type variables (§4.4) or parameterized types (§4.5) is equivalent to the addition (respectively, removal) of the an otherwise equivalent method whose return type is the erasure (§4.6) of the return type of m.

13.4.16. abstract Methods

Changing a method that is declared abstract to no longer be declared abstract does not break compatibility with pre-existing binaries.

Changing a method that is not declared abstract to be declared abstract will break compatibility with pre-existing binaries that previously invoked the method, causing an AbstractMethodError.

Example 13.4.16-1. Changing A Method To Be abstract

class Super { void out() { System.out.println("Out"); } }
class Test extends Super {
    public static void main(String[] args) {
        Test t = new Test();
        System.out.println("Way ");
        t.out();
    }
}

This program produces the output:

Way
Out

Suppose that a new version of class Super is produced:

abstract class Super {
    abstract void out();
}

If Super is recompiled but not Test, then running the new binary with the existing binary of Test results in an AbstractMethodError, because class Test has no implementation of the method out, and is therefore is (or should be) abstract.


13.4.17. final Methods

Changing a method that is declared final to no longer be declared final does not break compatibility with pre-existing binaries.

Changing an instance method that is not declared final to be declared final may break compatibility with existing binaries that depend on the ability to override the method.

Example 13.4.17-1. Changing A Method To Be final

class Super { void out() { System.out.println("out"); } }
class Test extends Super {
    public static void main(String[] args) {
        Test t = new Test();
        t.out();
    }
    void out() { super.out(); }
}

This program produces the output:

out

Suppose that a new version of class Super is produced:

class Super { final void out() { System.out.println("!"); } }

If Super is recompiled but not Test, then running the new binary with the existing binary of Test results in a VerifyError because the class Test improperly tries to override the instance method out.


Changing a class (static) method that is not declared final to be declared final does not break compatibility with existing binaries, because the method could not have been overridden.

13.4.18. native Methods

Adding or deleting a native modifier of a method does not break compatibility with pre-existing binaries.

The impact of changes to types on pre-existing native methods that are not recompiled is beyond the scope of this specification and should be provided with the description of an implementation. Implementations are encouraged, but not required, to implement native methods in a way that limits such impact.

13.4.19. static Methods

If a method that is not declared private is also declared static (that is, a class method) and is changed to not be declared static (that is, to an instance method), or vice versa, then compatibility with pre-existing binaries may be broken, resulting in a linkage time error, namely an IncompatibleClassChangeError, if these methods are used by the pre-existing binaries. Such changes are not recommended in code that has been widely distributed.

13.4.20. synchronized Methods

Adding or deleting a synchronized modifier of a method does not break compatibility with pre-existing binaries.

13.4.21. Method and Constructor Throws

Changes to the throws clause of methods or constructors do not break compatibility with pre-existing binaries; these clauses are checked only at compile time.

13.4.22. Method and Constructor Body

Changes to the body of a method or constructor do not break compatibility with pre-existing binaries.

The keyword final on a method does not mean that the method can be safely inlined; it means only that the method cannot be overridden. It is still possible that a new version of that method will be provided at link-time. Furthermore, the structure of the original program must be preserved for purposes of reflection.

Therefore, we note that a Java compiler cannot expand a method inline at compile time. In general we suggest that implementations use late-bound (run-time) code generation and optimization.

13.4.23. Method and Constructor Overloading

Adding new methods or constructors that overload existing methods or constructors does not break compatibility with pre-existing binaries. The signature to be used for each invocation was determined when these existing binaries were compiled; therefore newly added methods or constructors will not be used, even if their signatures are both applicable and more specific than the signature originally chosen.

While adding a new overloaded method or constructor may cause a compile-time error the next time a class or interface is compiled because there is no method or constructor that is most specific (§15.12.2.5), no such error occurs when a program is executed, because no overload resolution is done at execution time.

Example 13.4.23-1. Adding An Overloaded Method

class Super {
    static void out(float f) {
        System.out.println("float");
    }
}
class Test {
    public static void main(String[] args) {
        Super.out(2);
    }
}

This program produces the output:

float

Suppose that a new version of class Super is produced:

class Super {
    static void out(float f) { System.out.println("float"); }
    static void out(int i)   { System.out.println("int");   }
}

If Super is recompiled but not Test, then running the new binary with the existing binary of Test still produces the output:

float

However, if Test is then recompiled, using this new Super, the output is then:

int

as might have been naively expected in the previous case.


13.4.24. Method Overriding

If an instance method is added to a subclass and it overrides a method in a superclass, then the subclass method will be found by method invocations in pre-existing binaries, and these binaries are not impacted.

If a class method is added to a class, then this method will not be found unless the qualifying type of the reference is the subclass type.

13.4.25. Static Initializers

Adding, deleting, or changing a static initializer (§8.7) of a class does not impact pre-existing binaries.

13.4.26. Evolution of Enums

Adding or reordering constants in an enum will not break compatibility with pre-existing binaries.

If a pre-existing binary attempts to access an enum constant that no longer exists, the client will fail at run time with a NoSuchFieldError. Therefore such a change is not recommended for widely distributed enums.

In all other respects, the binary compatibility rules for enums are identical to those for classes.

13.5. Evolution of Interfaces

This section describes the impact of changes to the declaration of an interface and its members on pre-existing binaries.

13.5.1. public Interfaces

Changing an interface that is not declared public to be declared public does not break compatibility with pre-existing binaries.

If an interface that is declared public is changed to not be declared public, then an IllegalAccessError is thrown if a pre-existing binary is linked that needs but no longer has access to the interface type, so such a change is not recommended for widely distributed interfaces.

13.5.2. Superinterfaces

Changes to the interface hierarchy cause errors in the same way that changes to the class hierarchy do, as described in §13.4.4. In particular, changes that result in any previous superinterface of a class no longer being a superinterface can break compatibility with pre-existing binaries, resulting in a VerifyError.

13.5.3. Interface Members

Adding an abstract method to an interface does not break compatibility with pre-existing binaries.

A field added to a superinterface of C may hide a field inherited from a superclass of C. If the original reference was to an instance field, an IncompatibleClassChangeError will result. If the original reference was an assignment, an IllegalAccessError will result.

Deleting a member from an interface may cause linkage errors in pre-existing binaries.

Example 13.5.3-1. Deleting An Interface Member

interface I { void hello(); }
class Test implements I {
    public static void main(String[] args) {
        I anI = new Test();
        anI.hello();
    }
    public void hello() { System.out.println("hello"); }
}

This program produces the output:

hello

Suppose that a new version of interface I is compiled:

interface I {}

If I is recompiled but not Test, then running the new binary with the existing binary for Test will result in a NoSuchMethodError.


13.5.4. Interface Type Parameters

The effects of changes to the type parameters of an interface are the same as those of analogous changes to the type parameters of a class.

13.5.5. Field Declarations

The considerations for changing field declarations in interfaces are the same as those for static final fields in classes, as described in §13.4.8 and §13.4.9.

13.5.6. Interface Method Declarations

The considerations for changing abstract method declarations in interfaces include those for abstract methods in classes, as described in §13.4.14, §13.4.15, §13.4.19, §13.4.21, and §13.4.23.

Adding a default method, or changing a method from abstract to default, does not break compatibility with pre-existing binaries, but may cause an IncompatibleClassChangeError if a pre-existing binary attempts to invoke the method. This error occurs if the qualifying type, T, is a subtype of two interfaces, I and J, where both I and J declare a default method with the same signature and result, and neither I nor J is a subinterface of the other.

In other words, adding a default method is a binary-compatible change because it does not introduce errors at link time, even if it introduces errors at compile time or invocation time. In practice, the risk of accidental clashes occurring by introducing a default method are similar to those associated with adding a new method to a non-final class. In the event of a clash, adding a method to a class is unlikely to trigger a LinkageError, but an accidental override of the method in a child can lead to unpredictable method behavior. Both changes can cause errors at compile time.

Example 13.5.6-1. Adding A Default Method

interface Painter {
    default void draw() {
        System.out.println("Here's a picture...");
    }
}

interface Cowboy {}

public class CowboyArtist implements Cowboy, Painter {
    public static void main(String... args) {
        new CowboyArtist().draw();
   }
}

This program produces the output:

Here's a picture...

Suppose that a default method is added to Cowboy:

interface Cowboy {
    default void draw() {
        System.out.println("Bang!");
    }
}

If Cowboy is recompiled but not CowboyArtist, then running the new binary with the existing binary for CowboyArtist will link without error but cause an IncompatibleClassChangeError when main attempts to invoke draw().


13.5.7. Evolution of Annotation Types

Annotation types behave exactly like any other interface. Adding or removing an element from an annotation type is analogous to adding or removing a method. There are important considerations governing other changes to annotation types, such as making an annotation type repeatable (§9.6.3), but these have no effect on the linkage of binaries by the Java Virtual Machine. Rather, such changes affect the behavior of reflective APIs that manipulate annotations. The documentation of these APIs specifies their behavior when various changes are made to the underlying annotation types.

Adding or removing annotations has no effect on the correct linkage of the binary representations of programs in the Java programming language.