Chapter 8. Classes

A class declaration may include class modifiers.

The rules for annotation modifiers on a class declaration are specified in §9.7.4 and §9.7.5.

The access modifier public (§6.6) pertains only to top level classes (§7.6) and member classes (§8.5), not to local classes (§14.3) or anonymous classes (§15.9.5).

The access modifiers protected and private pertain only to member classes within a directly enclosing class declaration (§8.5).

The modifier static pertains only to member classes (§8.5.1), not to top level or local or anonymous classes.

It is a compile-time error if the same keyword appears more than once as a modifier for a class declaration, or if a class declaration has more than one of the access modifiers public, protected, and private (§6.6).

If two or more (distinct) class modifiers appear in a class declaration, then it is customary, though not required, that they appear in the order consistent with that shown above in the production for ClassModifier.

abstract class Point {
    int x = 1, y = 1;
    void move(int dx, int dy) {
        x += dx;
        y += dy;
        alert();
    }
    abstract void alert();
}
abstract class ColoredPoint extends Point {
    int color;
}
class SimplePoint extends Point {
    void alert() { }
}

Point p = new Point();

Point p = new SimplePoint();

interface Colorable {
    void setColor(int color);
}
abstract class Colored implements Colorable {
    public abstract int setColor(int color);
}

public final class Math {
    private Math() { }  // never instantiate this class
    . . . declarations of class variables and methods . . .
}

A class is generic if it declares one or more type variables (§4.4).

These type variables are known as the type parameters of the class. The type parameter section follows the class name and is delimited by angle brackets.

The following productions from §4.4 are shown here for convenience:

The rules for annotation modifiers on a type parameter declaration are specified in §9.7.4 and §9.7.5.

In a class's type parameter section, a type variable T directly depends on a type variable S if S is the bound of T, while T depends on S if either T directly depends on S or T directly depends on a type variable U that depends on S (using this definition recursively). It is a compile-time error if a type variable in a class's type parameter section depends on itself.

The scope and shadowing of a class's type parameter is specified in §6.3 and §6.4.

A generic class declaration defines a set of parameterized types (§4.5), one for each possible parameterization of the type parameter section by type arguments. All of these parameterized types share the same class at run time.

Vector<String>  x = new Vector<String>();
Vector<Integer> y = new Vector<Integer>();
boolean b = x.getClass() == y.getClass();

It is a compile-time error if a generic class is a direct or indirect subclass of Throwable (§11.1.1).

This restriction is needed since the catch mechanism of the Java Virtual Machine works only with non-generic classes.

It is a compile-time error to refer to a type parameter of a generic class C in any of the following:

interface ConvertibleTo<T> {
    T convert();
}
class ReprChange<T extends ConvertibleTo<S>,
                 S extends ConvertibleTo<T>> { 
    T t; 
    void set(S s) { t = s.convert();    } 
    S get()       { return t.convert(); } 
}

class Seq<T> { 
    T      head;
    Seq<T> tail;

    Seq() { this(null, null); } 
    Seq(T head, Seq<T> tail) {
        this.head = head;
        this.tail = tail;
    }
    boolean isEmpty() { return tail == null; }

    class Zipper<S> { 
        Seq<Pair<T,S>> zip(Seq<S> that) { 
            if (isEmpty() || that.isEmpty()) {
                return new Seq<Pair<T,S>>(); 
            } else {
                Seq<T>.Zipper<S> tailZipper =
                    tail.new Zipper<S>();
                return new Seq<Pair<T,S>>( 
                    new Pair<T,S>(head, that.head),
                    tailZipper.zip(that.tail));
            }
        }
    }
}
class Pair<T, S> {
    T fst; S snd;
    Pair(T f, S s) { fst = f; snd = s; }
}
class Test {
    public static void main(String[] args) {
        Seq<String> strs =
            new Seq<String>(
                "a",
                new Seq<String>("b",
                                new Seq<String>()));
        Seq<Number> nums =
            new Seq<Number>(
                new Integer(1),
                new Seq<Number>(new Double(1.5),
                                new Seq<Number>()));

        Seq<String>.Zipper<Number> zipper =
            strs.new Zipper<Number>();

        Seq<Pair<String,Number>> combined =
            zipper.zip(nums);
    }
}

An inner class is a nested class that is not explicitly or implicitly declared static.

An inner class may be a non-static member class (§8.5), a local class (§14.3), or an anonymous class (§15.9.5). A member class of an interface is implicitly static (§9.5) so is never considered to be an inner class.

It is a compile-time error if an inner class declares a static initializer (§8.7).

It is a compile-time error if an inner class declares a member that is explicitly or implicitly static, unless the member is a constant variable (§4.12.4).

An inner class may inherit static members that are not constant variables even though it cannot declare them.

A nested class that is not an inner class may declare static members freely, in accordance with the usual rules of the Java programming language.

class HasStatic {
    static int j = 100;
}
class Outer {
    class Inner extends HasStatic {
        static final int x = 3;  // OK: constant variable
        static int y = 4;  // Compile-time error: an inner class
    }
    static class NestedButNotInner{
        static int z = 5;    // OK: not an inner class
    }
    interface NeverInner {}  // Interfaces are never inner
}

A statement or expression occurs in a static context if and only if the innermost method, constructor, instance initializer, static initializer, field initializer, or explicit constructor invocation statement enclosing the statement or expression is a static method, a static initializer, the variable initializer of a static variable, or an explicit constructor invocation statement (§8.8.7.1).

An inner class C is a direct inner class of a class or interface O if O is the immediately enclosing type declaration of C and the declaration of C does not occur in a static context.

A class C is an inner class of class or interface O if it is either a direct inner class of O or an inner class of an inner class of O.

It is unusual, but possible, for the immediately enclosing type declaration of an inner class to be an interface. This only occurs if the class is declared in a default method body (§9.4). Specifically, it occurs if an anonymous or local class is declared in a default method body, or a member class is declared in the body of an anonymous class that is declared in a default method body.

A class or interface O is the zeroth lexically enclosing type declaration of itself.

A class O is the n'th lexically enclosing type declaration of a class C if it is the immediately enclosing type declaration of the n-1'th lexically enclosing type declaration of C.

An instance i of a direct inner class C of a class or interface O is associated with an instance of O, known as the immediately enclosing instance of i. The immediately enclosing instance of an object, if any, is determined when the object is created (§15.9.2).

An object o is the zeroth lexically enclosing instance of itself.

An object o is the n'th lexically enclosing instance of an instance i if it is the immediately enclosing instance of the n-1'th lexically enclosing instance of i.

An instance of an inner class I whose declaration occurs in a static context has no lexically enclosing instances. However, if I is immediately declared within a static method or static initializer then I does have an enclosing block, which is the innermost block statement lexically enclosing the declaration of I.

For every superclass S of C which is itself a direct inner class of a class or interface SO, there is an instance of SO associated with i, known as the immediately enclosing instance of i with respect to S. The immediately enclosing instance of an object with respect to its class' direct superclass, if any, is determined when the superclass constructor is invoked via an explicit constructor invocation statement (§8.8.7.1).

When an inner class (whose declaration does not occur in a static context) refers to an instance variable that is a member of a lexically enclosing type declaration, the variable of the corresponding lexically enclosing instance is used.

Any local variable, formal parameter, or exception parameter used but not declared in an inner class 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 an inner class must be definitely assigned (§16 (Definite Assignment)) before the body of the inner class, or a compile-time error occurs.

Similar rules on variable use apply in the body of a lambda expression (§15.27.2).

A blank final field (§4.12.4) of a lexically enclosing type declaration may not be assigned within an inner class, or a compile-time error occurs.

class Outer {
    int i = 100;
    static void classMethod() {
        final int l = 200;
        class LocalInStaticContext {
            int k = i;  // Compile-time error
            int m = l;  // OK
        }
    }
    void foo() {
        class Local {  // A local class
            int j = i;
        }
    }
}

class WithDeepNesting {
    boolean toBe;
    WithDeepNesting(boolean b) { toBe = b; }

    class Nested {
        boolean theQuestion;
        class DeeplyNested {
            DeeplyNested(){
                theQuestion = toBe || !toBe;
            }
        }
    }
}

The optional extends clause in a normal class declaration specifies the direct superclass of the current class.

The extends clause must not appear in the definition of the class Object, or a compile-time error occurs, because it is the primordial class and has no direct superclass.

The ClassType must name an accessible class type (§6.6), or a compile-time error occurs.

It is a compile-time error if the ClassType names a class that is final, because final classes are not allowed to have subclasses (§8.1.1.2).

It is a compile-time error if the ClassType names the class Enum or any invocation of Enum (§8.9).

If the ClassType has type arguments, it must denote a well-formed parameterized type (§4.5), and none of the type arguments may be wildcard type arguments, or a compile-time error occurs.

Given a (possibly generic) class declaration C<F₁,...,F_n> (n ≥ 0, C ≠ Object), the direct superclass of the class type C<F₁,...,F_n> is the type given in the extends clause of the declaration of C if an extends clause is present, or Object otherwise.

Given a generic class declaration C<F₁,...,F_n> (n > 0), the direct superclass of the parameterized class type C<T₁,...,T_n>, where T_i (1 ≤ i ≤ n) is a type, is D<U₁ θ,...,U_k θ>, where D<U₁,...,U_k> is the direct superclass of C<F₁,...,F_n> and θ is the substitution [F1:=T1,...,Fn:=Tn].

A class is said to be a direct subclass of its direct superclass. The direct superclass is the class from whose implementation the implementation of the current class is derived.

The subclass relationship is the transitive closure of the direct subclass relationship. A class A is a subclass of class C if either of the following is true:

Class C is said to be a superclass of class A whenever A is a subclass of C.

class Point { int x, y; }
final class ColoredPoint extends Point { int color; }
class Colored3DPoint extends ColoredPoint { int z; }  // error

class Point { int x, y; }
class ColoredPoint extends Point { int color; }
final class Colored3dPoint extends ColoredPoint { int z; }

A class C directly depends on a type T if T is mentioned in the extends or implements clause of C either as a superclass or superinterface, or as a qualifier in the fully qualified form of a superclass or superinterface name.

A class C depends on a reference type T if any of the following is true:

It is a compile-time error if a class depends on itself.

If circularly declared classes are detected at run time, as classes are loaded, then a ClassCircularityError is thrown (§12.2.1).

class Point extends ColoredPoint { int x, y; }
class ColoredPoint extends Point { int color; }

The optional implements clause in a class declaration lists the names of interfaces that are direct superinterfaces of the class being declared.

Each InterfaceType must name an accessible interface type (§6.6), or a compile-time error occurs.

If an InterfaceType has type arguments, it must denote a well-formed parameterized type (§4.5), and none of the type arguments may be wildcard type arguments, or a compile-time error occurs.

It is a compile-time error if the same interface is mentioned as a direct superinterface more than once in a single implements clause. This is true even if the interface is named in different ways.

class Redundant implements java.lang.Cloneable, Cloneable {
    int x;
}

Given a (possibly generic) class declaration C<F₁,...,F_n> (n ≥ 0, C ≠ Object), the direct superinterfaces of the class type C<F₁,...,F_n> are the types given in the implements clause of the declaration of C, if an implements clause is present.

Given a generic class declaration C<F₁,...,F_n> (n > 0), the direct superinterfaces of the parameterized class type C<T₁,...,T_n>, where T_i (1 ≤ i ≤ n) is a type, are all types I<U₁ θ,...,U_k θ>, where I<U₁,...,U_k> is a direct superinterface of C<F₁,...,F_n> and θ is the substitution [F1:=T1,...,Fn:=Tn].

An interface type I is a superinterface of class type C if any of the following is true:

A class can have a superinterface in more than one way.

A class is said to implement all its superinterfaces.

A class may not at the same time be a subtype of two interface types which are different parameterizations of the same generic interface (§9.1.2), or a subtype of a parameterization of a generic interface and a raw type naming that same generic interface, or a compile-time error occurs.

This requirement was introduced in order to support translation by type erasure (§4.6).

interface Colorable {
    void setColor(int color);
    int getColor();
}
enum Finish { MATTE, GLOSSY }
interface Paintable extends Colorable {
    void setFinish(Finish finish);
    Finish getFinish();
}

class Point { int x, y; }
class ColoredPoint extends Point implements Colorable {
    int color;
    public void setColor(int color) { this.color = color; }
    public int getColor() { return color; }
}
class PaintedPoint extends ColoredPoint implements Paintable {
    Finish finish;
    public void setFinish(Finish finish) {
        this.finish = finish;
    }
    public Finish getFinish() { return finish; }
}

interface I<T> {}
class B implements I<Integer> {}
class C extends B implements I<String> {}

Unless the class being declared is abstract, all the abstract member methods of each direct superinterface must be implemented (§8.4.8.1) either by a declaration in this class or by an existing method declaration inherited from the direct superclass or a direct superinterface, because a class that is not abstract is not permitted to have abstract methods (§8.1.1.1).

Each default method (§9.4.3) of a superinterface of the class may optionally be overridden by a method in the class; if not, the default method is typically inherited and its behavior is as specified by its default body.

It is permitted for a single method declaration in a class to implement methods of more than one superinterface.

interface Colorable {
    void setColor(int color);
    int getColor();
}
class Point { int x, y; };
class ColoredPoint extends Point implements Colorable {
    int color;
}

interface Fish  { int getNumberOfScales(); }
interface Piano { int getNumberOfScales(); }
class Tuna implements Fish, Piano {
    // You can tune a piano, but can you tuna fish?
    public int getNumberOfScales() { return 91; }
}

interface Fish       { int    getNumberOfScales(); }
interface StringBass { double getNumberOfScales(); }
class Bass implements Fish, StringBass {
    // This declaration cannot be correct,
    // no matter what type is used.
    public ?? getNumberOfScales() { return 91; }
}

A class body may contain declarations of members of the class, that is, fields (§8.3), methods (§8.4), classes (§8.5), and interfaces (§8.5).

A class body may also contain instance initializers (§8.6), static initializers (§8.7), and declarations of constructors (§8.8) for the class.

The scope and shadowing of a declaration of a member m declared in or inherited by a class type C is specified in §6.3 and §6.4.

If C itself is a nested class, there may be definitions of the same kind (variable, method, or type) and name as m in enclosing scopes. (The scopes may be blocks, classes, or packages.) In all such cases, the member m declared in or inherited by C shadows (§6.4.1) the other definitions of the same kind and name.

The rules for annotation modifiers on a field declaration are specified in §9.7.4 and §9.7.5.

It is a compile-time error if the same keyword appears more than once as a modifier for a field declaration, or if a field declaration has more than one of the access modifiers public, protected, and private (§6.6).

If two or more (distinct) field modifiers appear in a field declaration, it is customary, though not required, that they appear in the order consistent with that shown above in the production for FieldModifier.

class Point {
    int x, y, useCount;
    Point(int x, int y) { this.x = x; this.y = y; }
    static final Point origin = new Point(0, 0);
}
class Test {
    public static void main(String[] args) {
        Point p = new Point(1,1);
        Point q = new Point(2,2);
        p.x = 3;
        p.y = 3;
        p.useCount++;
        p.origin.useCount++;
        System.out.println("(" + q.x + "," + q.y + ")");
        System.out.println(q.useCount);
        System.out.println(q.origin == Point.origin);
        System.out.println(q.origin.useCount);
    }
}

(2,2)
0
true
1

q.origin==Point.origin

p.origin.useCount++;

class Point {
    static int x = 2;
}
class Test extends Point {
    static double x = 4.7;
    public static void main(String[] args) {
        new Test().printX();
    }
    void printX() {
        System.out.println(x + " " + super.x);
    }
}

4.7 2

class Point {
    static int x = 2;
}
class Test extends Point {
    public static void main(String[] args) {
        new Test().printX();
    }
    void printX() {
        System.out.println(x + " " + super.x);
    }
}

2 2

class Point {
    int x = 2;
}
class Test extends Point {
    double x = 4.7;
    void printBoth() {
        System.out.println(x + " " + super.x);
    }
    public static void main(String[] args) {
        Test sample = new Test();
        sample.printBoth();
        System.out.println(sample.x + " " + ((Point)sample).x);
    }
}

4.7 2
4.7 2

class Point {
    static int x = 2;
}
class Test extends Point {
    void printBoth() {
        System.out.println(x + " " + super.x);
    }
    public static void main(String[] args) {
        Test sample = new Test();
        sample.printBoth();
        System.out.println(sample.x + " " +	((Point)sample).x);
    }
}

2 2
2 2

class Point {
    int x, y;
    transient float rho, theta;
}

class Test {
    static int i = 0, j = 0;
    static void one() { i++; j++; }
    static void two() {
        System.out.println("i=" + i + " j=" + j);
    }
}

class Test {
    static int i = 0, j = 0;
    static synchronized void one() { i++; j++; }
    static synchronized void two() {
        System.out.println("i=" + i + " j=" + j);
    }
}

class Test {
    static volatile int i = 0, j = 0;
    static void one() { i++; j++; }
    static void two() {
        System.out.println("i=" + i + " j=" + j);
    }
}

If a declarator in a field declaration has a variable initializer, then the declarator has the semantics of an assignment (§15.26) to the declared variable.

If the declarator is for a class variable (that is, a static field), then the following rules apply to its initializer:

If the declarator is for an instance variable (that is, a field that is not static), then the following rules apply to its initializer:

References from variable initializers to fields that may not yet be initialized are subject to additional restrictions, as specified in §8.3.3 and §16 (Definite Assignment).

Exception checking for a variable initializer in a field declaration is specified in §11.2.3.

Variable initializers are also used in local variable declaration statements (§14.4), where the initializer is evaluated and the assignment performed each time the local variable declaration statement is executed.

class Point {
    int x = 1, y = 5;
}
class Test {
    public static void main(String[] args) {
        Point p = new Point();
        System.out.println(p.x + ", " + p.y);
    }
}

1, 5

class Test {
    float f = j;
    static int j = 1;
}

References to a field are sometimes restricted, even through the field is in scope. The following rules constrain forward references to a field (where the use textually precedes the field declaration) as well as self-reference (where the field is used in its own initializer).

For a reference by simple name to a class variable f declared in class or interface C, it is a compile-time error if:

For a reference by simple name to an instance variable f declared in class C, it is a compile-time error if:

class Test1 {
    int i = j;  // compile-time error:
                // incorrect forward reference
    int j = 1;
}

class Test2 {
    Test2() { k = 2; }
    int j = 1;
    int i = j;
    int k;
}

class Z {
    static int i = j + 2; 
    static int j = 4;
}

class Z {
    static { i = j + 2; }
    static int i, j;
    static { j = 4; }
}

class Z {
    static int peek() { return j; }
    static int i = peek();
    static int j = 1;
}
class Test {
    public static void main(String[] args) {
        System.out.println(Z.i);
    }
}

0

class UseBeforeDeclaration {
    static {
        x = 100;
          // ok - assignment
        int y = x + 1;
          // error - read before declaration
        int v = x = 3;
          // ok - x at left hand side of assignment
        int z = UseBeforeDeclaration.x * 2;
          // ok - not accessed via simple name

        Object o = new Object() { 
            void foo() { x++; }
              // ok - occurs in a different class
            { x++; }
              // ok - occurs in a different class
        };
    }

    {
        j = 200;
          // ok - assignment
        j = j + 1;
          // error - right hand side reads before declaration
        int k = j = j + 1;
          // error - illegal forward reference to j
        int n = j = 300;
          // ok - j at left hand side of assignment
        int h = j++;
          // error - read before declaration
        int l = this.j * 3;
          // ok - not accessed via simple name

        Object o = new Object() { 
            void foo(){ j++; }
              // ok - occurs in a different class
            { j = j + 1; }
              // ok - occurs in a different class
        };
    }

    int w = x = 3;
      // ok - x at left hand side of assignment
    int p = x;
      // ok - instance initializers may access static fields

    static int u =
        (new Object() { int bar() { return x; } }).bar();
	    // ok - occurs in a different class

    static int x;

    int m = j = 4;
      // ok - j at left hand side of assignment
    int o =
        (new Object() { int bar() { return j; } }).bar(); 
        // ok - occurs in a different class
    int j;
}

The formal parameters of a method or constructor, if any, are specified by a list of comma-separated parameter specifiers. Each parameter specifier consists of a type (optionally preceded by the final modifier and/or one or more annotations) and an identifier (optionally followed by brackets) that specifies the name of the parameter.

If a method or constructor has no formal parameters, and no receiver parameter, then an empty pair of parentheses appears in the declaration of the method or constructor.

The following productions from §8.3 and §4.3 are shown here for convenience:

A formal parameter of a method or constructor may be a variable arity parameter, indicated by an ellipsis following the type. At most one variable arity parameter is permitted for a method or constructor. It is a compile-time error if a variable arity parameter appears anywhere in the list of parameter specifiers except the last position.

In the grammar for VariableArityParameter, note that the ellipsis (...) is a token unto itself (§3.11). It is possible to put whitespace between it and the type, but this is discouraged as a matter of style.

If the last formal parameter is a variable arity parameter, the method is a variable arity method. Otherwise, it is a fixed arity method.

The rules for annotation modifiers on a formal parameter declaration and on a receiver parameter 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.

The scope and shadowing of a formal parameter is specified in §6.3 and §6.4.

It is a compile-time error for a method or constructor 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 formal parameter that is declared final is assigned to within the body of the method or constructor.

The declared type of a formal parameter depends on whether it is a variable arity parameter:

If the declared type of a variable arity parameter has a non-reifiable element type (§4.7), then a compile-time unchecked warning occurs for the declaration of the variable arity method, unless the method is annotated with @SafeVarargs (§9.6.4.7) or the warning is suppressed by @SuppressWarnings (§9.6.4.5).

When the method or constructor is invoked (§15.12), the values of the actual argument expressions initialize newly created parameter variables, each of the declared type, before execution of the body of the method or constructor. The Identifier that appears in the FormalParameter may be used as a simple name in the body of the method or constructor to refer to the formal parameter.

Invocations of a variable arity method may contain more actual argument expressions than formal parameters. All the actual argument expressions that do not correspond to the formal parameters preceding the variable arity parameter will be evaluated and the results stored into an array that will be passed to the method invocation (§15.12.4.2).

A method's or constructor's formal parameter of type float always contains an element of the float value set (§4.2.3); similarly, a method's or constructor's formal parameter of type double always contains an element of the double value set. It is not permitted for a method's or constructor's formal 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 method's or constructor's formal 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.

Where an actual argument expression corresponding to a parameter variable is not FP-strict (§15.4), evaluation of that actual argument expression is permitted to use intermediate values drawn from the appropriate extended-exponent value sets. Prior to being stored in the parameter variable, the result of such an expression is mapped to the nearest value in the corresponding standard value set by being subjected to invocation conversion (§5.3).

class Test {
    Test(/* ?? ?? */) {}
      // No receiver parameter is permitted in the constructor of
      // a top level class, as there is no conceivable type or name.

    void m(Test this) {}
      // OK: receiver parameter in an instance method

    static void n(Test this) {}
      // Illegal: receiver parameter in a static method                         

    class A {
        A(Test Test.this) {}
          // OK: the receiver parameter represents the instance
          // of Test which immediately encloses the instance
          // of A being constructed.

        void m(A this) {}
          // OK: the receiver parameter represents the instance 
          // of A for which A.m() is invoked.

        class B {
            B(Test.A A.this) {}
              // OK: the receiver parameter represents the instance 
              // of A which immediately encloses the instance of B 
              // being constructed.

            void m(Test.A.B this) {}
              // OK: the receiver parameter represents the instance 
              // of B for which B.m() is invoked.
        }
    }
}

Two methods or constructors, M and N, have the same signature if they have the same name, the same type parameters (if any) (§8.4.4), and, after adapting the formal parameter types of N to the the type parameters of M, the same formal parameter types.

The signature of a method m1 is a subsignature of the signature of a method m2 if either:

Two method signatures m1 and m2 are override-equivalent iff either m1 is a subsignature of m2 or m2 is a subsignature of m1.

It is a compile-time error to declare two methods with override-equivalent signatures in a class.

class Point {
    int x, y;
    abstract void move(int dx, int dy);
    void move(int dx, int dy) { x += dx; y += dy; }
}

The notion of subsignature is designed to express a relationship between two methods whose signatures are not identical, but in which one may override the other. Specifically, it allows a method whose signature does not use generic types to override any generified version of that method. This is important so that library designers may freely generify methods independently of clients that define subclasses or subinterfaces of the library.

class CollectionConverter {
    List toList(Collection c) {...}
}
class Overrider extends CollectionConverter {
    List toList(Collection c) {...}
}

class CollectionConverter {
    <T> List<T> toList(Collection<T> c) {...}
}

The rules for annotation modifiers on a method declaration are specified in §9.7.4 and §9.7.5.

It is a compile-time error if the same keyword appears more than once as a modifier for a method declaration, or if a method declaration has more than one of the access modifiers public, protected, and private (§6.6).

It is a compile-time error if a method declaration that contains the keyword abstract also contains any one of the keywords private, static, final, native, strictfp, or synchronized.

It is a compile-time error if a method declaration that contains the keyword native also contains strictfp.

If two or more (distinct) method modifiers appear in a method declaration, it is customary, though not required, that they appear in the order consistent with that shown above in the production for MethodModifier.

class BufferEmpty extends Exception {
    BufferEmpty() { super(); }
    BufferEmpty(String s) { super(s); }
}
class BufferError extends Exception {
    BufferError() { super(); }
    BufferError(String s) { super(s); }
}
interface Buffer {
    char get() throws BufferEmpty, BufferError;
}
abstract class InfiniteBuffer implements Buffer {
    public abstract char get() throws BufferError;
}

abstract class Point {
    int x, y;
    public abstract String toString();
}

class ColoredPoint extends Point {
    int color;
    public String toString() {
        return super.toString() + ": color " + color;  // error
    }
}

abstract class Point {
    int x, y;
    public abstract String toString();
    protected String objString() { return super.toString(); }
}
class ColoredPoint extends Point {
    int color;
    public String toString() {
        return objString() + ": color " + color;  // correct
    }
}

final class Point {
    int x, y;
    void move(int dx, int dy) { x += dx; y += dy; }
}
class Test {
    public static void main(String[] args) {
        Point[] p = new Point[100];
        for (int i = 0; i < p.length; i++) {
            p[i] = new Point();
            p[i].move(i, p.length-1-i);
        }
    }
}

    for (int i = 0; i < p.length; i++) {
        p[i] = new Point();
        Point pi = p[i];
        int j = p.length-1-i;
        pi.x += i;
        pi.y += j;
    }

package java.io;
public class RandomAccessFile
    implements DataOutput, DataInput {
    . . .
    public native void open(String name, boolean writeable)
        throws IOException;
    public native int readBytes(byte[] b, int off, int len)
        throws IOException;
    public native void writeBytes(byte[] b, int off, int len)
        throws IOException;
    public native long getFilePointer() throws IOException;
    public native void seek(long pos) throws IOException;
    public native long length() throws IOException;
    public native void close() throws IOException;
}

class Test {
    int count;
    synchronized void bump() {
        count++;
    }
    static int classCount;
    static synchronized void classBump() {
        classCount++;
    }
}

class BumpTest {
    int count;
    void bump() {
        synchronized (this) { count++; }
    }
    static int classCount;
    static void classBump() {
        try {
            synchronized (Class.forName("BumpTest")) {
                classCount++;
            }
        } catch (ClassNotFoundException e) {}
    }
}

public class Box {
    private Object boxContents;
    public synchronized Object get() {
        Object contents = boxContents;
        boxContents = null;
        return contents;
    }
    public synchronized boolean put(Object contents) {
        if (boxContents != null) return false;
        boxContents = contents;
        return true;
    }
}

A method is generic if it declares one or more type variables (§4.4).

These type variables are known as the type parameters of the method. The form of the type parameter section of a generic method is identical to the type parameter section of a generic class (§8.1.2).

A generic method declaration defines a set of methods, one for each possible invocation of the type parameter section by type arguments. Type arguments may not need to be provided explicitly when a generic method is invoked, as they can often be inferred (§18 (Type Inference)).

The scope and shadowing of a method's type parameter is specified in §6.3.

Two methods or constructors M and N have the same type parameters if both of the following are true:

Where two methods or constructors M and N have the same type parameters, a type mentioned in N can be adapted to the type parameters of M by applying θ, as defined above, to the type.

The result of a method declaration either declares the type of value that the method returns (the return type), or uses the keyword void to indicate that the method does not return a value.

If the result is not void, then the return type of a method is denoted by UnannType if no bracket pairs appear after the formal parameter list, and is specified by §10.2 otherwise.

Return types may vary among methods that override each other if the return types are reference types. The notion of return-type-substitutability supports covariant returns, that is, the specialization of the return type to a subtype.

A method declaration d1 with return type R₁ is return-type-substitutable for another method d2 with return type R₂ iff any of the following is true:

An unchecked conversion is allowed in the definition, despite being unsound, as a special allowance to allow smooth migration from non-generic to generic code. If an unchecked conversion is used to determine that R₁ is return-type-substitutable for R₂, then R₁ is necessarily not a subtype of R₂ and the rules for overriding (§8.4.8.3, §9.4.1) will require a compile-time unchecked warning.

A throws clause is used to denote any checked exception classes (§11.1.1) that the statements in a method or constructor body can throw (§11.2.2).

It is a compile-time error if an ExceptionType mentioned in a throws clause is not a subtype (§4.10) of Throwable.

Type variables are allowed in a throws clause even though they are not allowed in a catch clause (§14.20).

It is permitted but not required to mention unchecked exception classes (§11.1.1) in a throws clause.

The relationship between a throws clause and the exception checking for a method or constructor body is specified in §11.2.3.

Essentially, for each checked exception that can result from execution of the body of a method or constructor, a compile-time error occurs unless its exception type or a supertype of its exception type is mentioned in a throws clause in the declaration of the method or constructor.

The requirement to declare checked exceptions allows a Java compiler to ensure that code for handling such error conditions has been included. Methods or constructors that fail to handle exceptional conditions thrown as checked exceptions in their bodies will normally cause compile-time errors if they lack proper exception types in their throws clauses. The Java programming language thus encourages a programming style where rare and otherwise truly exceptional conditions are documented in this way.

The relationship between the throws clause of a method and the throws clauses of overridden or hidden methods is specified in §8.4.8.3.

import java.io.FileNotFoundException;
interface PrivilegedExceptionAction<E extends Exception> { 
    void run() throws E;
} 
class AccessController {
    public static <E extends Exception> 
    Object doPrivileged(PrivilegedExceptionAction<E> action) throws E {
        action.run();
        return "success";
    }
}
class Test {
    public static void main(String[] args) {
        try {
            AccessController.doPrivileged(
              new PrivilegedExceptionAction<FileNotFoundException>() {
                  public void run() throws FileNotFoundException {
                      // ... delete a file ...
                  } 
              }); 
        } catch (FileNotFoundException f) { /* Do something */ }
    }
}

A method body is either a block of code that implements the method or simply a semicolon, indicating the lack of an implementation.

The body of a method must be a semicolon if the method is abstract or native (§8.4.3.1, §8.4.3.4). More precisely:

If an implementation is to be provided for a method declared void, but the implementation requires no executable code, the method body should be written as a block that contains no statements: "{ }".

The rules for return statements in a method body are specified in §14.17.

If a method is declared to have a return type (§8.4.5), then a compile-time error occurs if the body of the method can complete normally (§14.1).

In other words, a method with a return type must return only by using a return statement that provides a value return; the method is not allowed to "drop off the end of its body". See §14.17 for the precise rules about return statements in a method body.

class DizzyDean {
    int pitch() { throw new RuntimeException("90 mph?!"); }
}

A class C inherits from its direct superclass all concrete methods m (both static and instance) of the superclass for which all of the following are true:

A class C inherits from its direct superclass and direct superinterfaces all abstract and default (§9.4) methods m for which all of the following are true:

A class does not inherit private or static methods from its superinterfaces.

Note that methods are overridden or hidden on a signature-by-signature basis. If, for example, a class declares two public methods with the same name (§8.4.9), and a subclass overrides one of them, the subclass still inherits the other method.

interface I1 {
    int foo();
}

interface I2 {
    int foo();
}

abstract class Test implements I1, I2 {} 

Note that it is possible for an inherited concrete method to prevent the inheritance of an abstract or default method. (The concrete method will override the abstract or default method "from C", per §8.4.8.1 and §9.4.1.1.) Also, it is possible for one supertype method to prevent the inheritance of another supertype method if the former "already" overrides the latter - this is the same as the rule for interfaces (§9.4.1), and prevents conflicts in which multiple default methods are inherited and one implementation is clearly meant to supersede the other.

class Point {
    int x = 0, y = 0;
    void move(int dx, int dy) { x += dx; y += dy; }
}
class SlowPoint extends Point {
    int xLimit, yLimit;
    void move(int dx, int dy) {
        super.move(limit(dx, xLimit), limit(dy, yLimit));
    }
    static int limit(int d, int limit) {
        return d > limit ? limit : d < -limit ? -limit : d;
    }
}

import java.io.OutputStream;
import java.io.IOException;

class BufferOutput {
    private OutputStream o;
    BufferOutput(OutputStream o) { this.o = o; }
    protected byte[] buf = new byte[512];
    protected int pos = 0;
    public void putchar(char c) throws IOException {
        if (pos == buf.length) flush();
        buf[pos++] = (byte)c;
    }
    public void putstr(String s) throws IOException {
        for (int i = 0; i < s.length(); i++)
            putchar(s.charAt(i));
    }
    public void flush() throws IOException {
        o.write(buf, 0, pos);
        pos = 0;
    }
}
class LineBufferOutput extends BufferOutput {
    LineBufferOutput(OutputStream o) { super(o); }
    public void putchar(char c) throws IOException {
        super.putchar(c);
        if (c == '\n') flush();
    }
}
class Test {
    public static void main(String[] args) throws IOException {
        LineBufferOutput lbo = new LineBufferOutput(System.out);
        lbo.putstr("lbo\nlbo");
        System.out.print("print\n");
        lbo.putstr("\n");
    }
}

lbo
print
lbo

class Super {
    static String greeting() { return "Goodnight"; }
    String name() { return "Richard"; }
}
class Sub extends Super {
    static String greeting() { return "Hello"; }
    String name() { return "Dick"; }
}
class Test {
    public static void main(String[] args) {
        Super s = new Sub();
        System.out.println(s.greeting() + ", " + s.name());
    }
}

Goodnight, Dick

class C implements Cloneable { 
    C copy() throws CloneNotSupportedException {
        return (C)clone();
    } 
}
class D extends C implements Cloneable { 
    D copy() throws CloneNotSupportedException {
        return (D)clone();
    } 
}

class StringSorter {
    // turns a collection of strings into a sorted list
    List toList(Collection c) {...} 
}

class Overrider extends StringSorter {
    List toList(Collection c) {...}
}

class StringSorter {
    // turns a collection of strings into a sorted list
    List<String> toList(Collection<String> c) {...}
}

class BadPointException extends Exception {
    BadPointException() { super(); }
    BadPointException(String s) { super(s); }
}
class Point {
    int x, y;
    void move(int dx, int dy) { x += dx; y += dy; }
}
class CheckedPoint extends Point {
    void move(int dx, int dy) throws BadPointException {
        if ((x + dx) < 0 || (y + dy) < 0)
            throw new BadPointException();
        x += dx; y += dy;
    }
}

class CheckedPoint extends Point {
    void move(int dx, int dy) {
        if ((x + dx) < 0 || (y + dy) < 0)
            throw new BadPointException();
        x += dx; y += dy;
    }
}

class C<T> {
    T id (T x) {...}
}
class D extends C<String> {
    Object id(Object x) {...}
}

class C<T> {
    T id(T x) {...}
}
interface I<T> {
    T id(T x);
}
class D extends C<String> implements I<Integer> {
   public String  id(String x)  {...}
   public Integer id(Integer x) {...}
}

If two methods of a class (whether both declared in the same class, or both inherited by a class, or one declared and one inherited) have the same name but signatures that are not override-equivalent, then the method name is said to be overloaded.

This fact causes no difficulty and never of itself results in a compile-time error. There is no required relationship between the return types or between the throws clauses of two methods with the same name, unless their signatures are override-equivalent.

When a method is invoked (§15.12), the number of actual arguments (and any explicit type arguments) and the compile-time types of the arguments are used, at compile time, to determine the signature of the method that will be invoked (§15.12.2). If the method that is to be invoked is an instance method, the actual method to be invoked will be determined at run time, using dynamic method lookup (§15.12.4).

class Point {
    float x, y;
    void move(int dx, int dy) { x += dx; y += dy; }
    void move(float dx, float dy) { x += dx; y += dy; }
    public String toString() { return "("+x+","+y+")"; }
}

class Point {
    int x = 0, y = 0;
    void move(int dx, int dy) { x += dx; y += dy; }
    int color;
}
class RealPoint extends Point {
    float x = 0.0f, y = 0.0f;
    void move(int dx, int dy) { move((float)dx, (float)dy); }
    void move(float dx, float dy) { x += dx; y += dy; }
}

class Point {
    int x = 0, y = 0, color;
    void move(int dx, int dy) { x += dx; y += dy; }
    int getX() { return x; }
    int getY() { return y; }
}
class RealPoint extends Point {
    float x = 0.0f, y = 0.0f;
    void move(int dx, int dy) { move((float)dx, (float)dy); }
    void move(float dx, float dy) { x += dx; y += dy; }
    float getX() { return x; }
    float getY() { return y; }
}

class Point {
    int x = 0, y = 0;
    void move(int dx, int dy) { x += dx; y += dy; }
    int getX() { return x; }
    int getY() { return y; }
    int color;
}
class RealPoint extends Point {
    float x = 0.0f, y = 0.0f;
    void move(int dx, int dy) { move((float)dx, (float)dy); }
    void move(float dx, float dy) { x += dx; y += dy; }
    int getX() { return (int)Math.floor(x); }
    int getY() { return (int)Math.floor(y); }
}

class Test {
    public static void main(String[] args) {
        RealPoint rp = new RealPoint();
        Point p = rp;
        rp.move(1.71828f, 4.14159f);
        p.move(1, -1);
        show(p.x, p.y);
        show(rp.x, rp.y);
        show(p.getX(), p.getY());
        show(rp.getX(), rp.getY());
    }
    static void show(int x, int y) {
        System.out.println("(" + x + ", " + y + ")");
    }
    static void show(float x, float y) {
        System.out.println("(" + x + ", " + y + ")");
    }
}

(0, 0)
(2.7182798, 3.14159)
(2, 3)
(2, 3)

The static keyword may modify the declaration of a member type C within the body of a non-inner class or interface T. Its effect is to declare that C is not an inner class. Just as a static method of T has no current instance of T in its body, C also has no current instance of T, nor does it have any lexically enclosing instances.

It is a compile-time error if a static class contains a usage of a non-static member of an enclosing class.

A member interface is implicitly static (§9.1.1). It is permitted for the declaration of a member interface to redundantly specify the static modifier.

The formal parameters of a constructor are identical in syntax and semantics to those of a method (§8.4.1).

The constructor of a non-private inner member class implicitly declares, as the first formal parameter, a variable representing the immediately enclosing instance of the class (§15.9.2, §15.9.3).

The rationale for why only this kind of class has an implicitly declared constructor parameter is subtle. The following explanation may be helpful:

The fact that a non-private inner member class may be accessed by a different compiler than compiled it, whereas a local or anonymous class is always accessed by the same compiler that compiled it, explains why the binary name of a non-private inner member class is defined to be predictable but the binary name of a local or anonymous class is not (§13.1).

It is a compile-time error to declare two constructors with override-equivalent signatures (§8.4.2) in a class.

It is a compile-time error to declare two constructors whose signatures have the same erasure (§4.6) in a class.

The rules for annotation modifiers on a constructor declaration are specified in §9.7.4 and §9.7.5.

It is a compile-time error if the same keyword appears more than once as a modifier in a constructor declaration, or if a constructor declaration has more than one of the access modifiers public, protected, and private (§6.6).

In a normal class declaration, a constructor declaration with no access modifiers has package access.

If two or more (distinct) method modifiers appear in a method declaration, it is customary, though not required, that they appear in the order consistent with that shown above in the production for MethodModifier.

Unlike methods, a constructor cannot be abstract, static, final, native, strictfp, or synchronized:

A constructor is generic if it declares one or more type variables (§4.4).

These type variables are known as the type parameters of the constructor. The form of the type parameter section of a generic constructor is identical to the type parameter section of a generic class (§8.1.2).

It is possible for a constructor to be generic independently of whether the class the constructor is declared in is itself generic.

A generic constructor declaration defines a set of constructors, one for each possible invocation of the type parameter section by type arguments. Type arguments may not need to be provided explicitly when a generic constructor is invoked, as they can often by inferred (§18 (Type Inference)).

The scope and shadowing of a constructor's type parameter is specified in §6.3 and §6.4.

The throws clause for a constructor is identical in structure and behavior to the throws clause for a method (§8.4.6).

The type of a constructor consists of its signature and the exception types given by its throws clause.

The first statement of a constructor body may be an explicit invocation of another constructor of the same class or of the direct superclass (§8.8.7.1).

It is a compile-time error for a constructor to directly or indirectly invoke itself through a series of one or more explicit constructor invocations involving this.

If a constructor body does not begin with an explicit constructor invocation and the constructor being declared is not part of the primordial class Object, then the constructor body implicitly begins with a superclass constructor invocation "super();", an invocation of the constructor of its direct superclass that takes no arguments.

Except for the possibility of explicit constructor invocations, and the prohibition on explicitly returning a value (§14.17), the body of a constructor is like the body of a method (§8.4.7).

A return statement (§14.17) may be used in the body of a constructor if it does not include an expression.

class Point {
    int x, y;
    Point(int x, int y) { this.x = x; this.y = y; }
}
class ColoredPoint extends Point {
    static final int WHITE = 0, BLACK = 1;
    int color;
    ColoredPoint(int x, int y) {
        this(x, y, WHITE);
    }
    ColoredPoint(int x, int y, int color) {
        super(x, y);
        this.color = color;
    }
}

class Point {
    int x, y;
    Point(int x, int y) { this.x = x; this.y = y; }
}
class ColoredPoint extends Point {
    static final int WHITE = 0, BLACK = 1;
    int color;
    ColoredPoint(int x, int y) {
        this(x, y, color);  // Changed to color from WHITE
    }
    ColoredPoint(int x, int y, int color) {
        super(x, y);
        this.color = color;
    }
}

class Outer {
    class Inner {}
}
class ChildOfInner extends Outer.Inner {
    ChildOfInner() { (new Outer()).super(); }
}

class Outer {
    int secret = 5;
    class Inner {
        int  getSecret()      { return secret; }
        void setSecret(int s) { secret = s; }
    }
}
class ChildOfInner extends Outer.Inner {
    ChildOfInner(Outer x) { x.super(); }
}

public class Test {
    public static void main(String[] args) {
        Outer x = new Outer();
        ChildOfInner a = new ChildOfInner(x);
        ChildOfInner b = new ChildOfInner(x);
        System.out.println(b.getSecret());
        a.setSecret(6);
        System.out.println(b.getSecret());
    }
}

5
6

Overloading of constructors is identical in behavior to overloading of methods (§8.4.9). The overloading is resolved at compile time by each class instance creation expression (§15.9).

If a class contains no constructor declarations, then a default constructor is implicitly declared. The form of the default constructor for a top level class, member class, or local class is as follows:

The form of the default constructor for an anonymous class is specified in §15.9.5.1.

It is a compile-time error if a default constructor is implicitly declared but the superclass does not have an accessible constructor that takes no arguments and has no throws clause.

public class Point {
    int x, y;
}

public class Point {
    int x, y;
    public Point() { super(); }
}

package p1;
public class Outer {
    protected class Inner {}
}
package p2;
class SonOfOuter extends p1.Outer {
    void foo() {
        new Inner();  // compile-time access error
    }
}

A class can be designed to prevent code outside the class declaration from creating instances of the class by declaring at least one constructor, to prevent the creation of a default constructor, and by declaring all constructors to be private (§6.6.1).

A public class can likewise prevent the creation of instances outside its package by declaring at least one constructor, to prevent creation of a default constructor with public access, and by declaring no constructor that is public or protected (§6.6.2).

class ClassOnly {
    private ClassOnly() { }
    static String just = "only the lonely";
}

package just;
public class PackageOnly {
    PackageOnly() { }
    String[] justDesserts = { "cheesecake", "ice cream" };
}

The body of an enum declaration may contain enum constants. An enum constant defines an instance of the enum type.

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

The rules for annotation modifiers on an enum constant declaration are specified in §9.7.4 and §9.7.5.

The Identifier in a EnumConstant may be used in a name to refer to the enum constant.

The scope and shadowing of an enum constant is specified in §6.3 and §6.4.

An enum constant may be followed by arguments, which are passed to the constructor of the enum when the constant is created during class initialization as described later in this section. The constructor to be invoked is chosen using the normal rules of overload resolution (§15.12.2). If the arguments are omitted, an empty argument list is assumed.

The optional class body of an enum constant implicitly defines an anonymous class declaration (§15.9.5) that extends the immediately enclosing enum type. The class body is governed by the usual rules of anonymous classes; in particular it cannot contain any constructors. Instance methods declared in these class bodies may be invoked outside the enclosing enum type only if they override accessible methods in the enclosing enum type (§8.4.8).

It is a compile-time error for the class body of an enum constant to declare an abstract method.

Because there is only one instance of each enum constant, it is permitted to use the == operator in place of the equals method when comparing two object references if it is known that at least one of them refers to an enum constant.

The equals method in Enum is a final method that merely invokes super.equals on its argument and returns the result, thus performing an identity comparison.

In addition to enum constants, the body of an enum declaration may contain constructor and member declarations as well as instance and static initializers.

The following productions from §8.1.6 are shown here for convenience:

Any constructor or member declarations in the body of an enum declaration apply to the enum type exactly as if they had been present in the body of a normal class declaration, unless explicitly stated otherwise.

It is a compile-time error if a constructor declaration in an enum declaration is public or protected (§6.6).

It is a compile-time error if a constructor declaration in an enum declaration contains a superclass constructor invocation statement (§8.8.7.1).

It is a compile-time error to refer to a static field of an enum type from a constructor, instance initializer, or instance variable initializer of the enum type, unless the field is a constant variable (§4.12.4).

In an enum declaration, a constructor declaration with no access modifiers is private.

In an enum declaration with no constructor declarations, a default constructor is implicitly declared. The default constructor is private, has no formal parameters, and has no throws clause.

In practice, a compiler is likely to mirror the Enum type by declaring String and int parameters in the default constructor of an enum type. However, these parameters are not specified as "implicitly declared" because different compilers do not need to agree on the form of the default constructor. Only the compiler of an enum type knows how to instantiate the enum constants; other compilers can simply rely on the implicitly declared public static fields of the enum type (§8.9.3) without regard for how those fields were initialized.

It is a compile-time error if an enum declaration E has an abstract method m as a member, unless E has at least one enum constant and all of E's enum constants have class bodies that provide concrete implementations of m.

It is a compile-time error for an enum declaration to declare a finalizer (§12.6). An instance of an enum type may never be finalized.

enum Coin {
    PENNY(1), NICKEL(5), DIME(10), QUARTER(25);
    Coin(int value) { this.value = value; }

    private final int value;
    public int value() { return value; }
}

import java.util.Map;
import java.util.HashMap;

enum Color {
    RED, GREEN, BLUE;
    Color() { colorMap.put(toString(), this); }

    static final Map<String,Color> colorMap =
        new HashMap<String,Color>();
}

import java.util.Map;
import java.util.HashMap;

enum Color {
    RED, GREEN, BLUE;

    static final Map<String,Color> colorMap =
        new HashMap<String,Color>();
    static {
        for (Color c : Color.values())
            colorMap.put(c.toString(), c);
    }
}

The members of an enum type E are all of the following:

It follows that the declaration of enum type E cannot contain fields that conflict with the implicitly declared fields corresponding to E's enum constants, nor contain methods that conflict with implicitly declared methods or override final methods of class Enum<E>.

public class Test {
    enum Season { WINTER, SPRING, SUMMER, FALL }

    public static void main(String[] args) {
        for (Season s : Season.values())
            System.out.println(s);
    }
}

WINTER
SPRING
SUMMER
FALL

class Test {
    enum CoinColor { COPPER, NICKEL, SILVER }

    static CoinColor color(Coin c) {
        switch (c) {
            case PENNY:
                return CoinColor.COPPER;
            case NICKEL:
                return CoinColor.NICKEL;
            case DIME: case QUARTER:
                return CoinColor.SILVER;
            default:
                throw new AssertionError("Unknown coin: " + c);
        }
    }

    public static void main(String[] args) {
        for (Coin c : Coin.values())
            System.out.println(c + "\t\t" +
                               c.value() + "\t" + color(c));
    }
}

PENNY           1       COPPER
NICKEL          5       NICKEL
DIME            10      SILVER
QUARTER         25      SILVER

enum Operation {
    PLUS {
        double eval(double x, double y) { return x + y; }
    },
    MINUS {
        double eval(double x, double y) { return x - y; }
    },
    TIMES {
        double eval(double x, double y) { return x * y; }
    },
    DIVIDED_BY {
        double eval(double x, double y) { return x / y; }
    };

    // Each constant supports an arithmetic operation
    abstract double eval(double x, double y);

    public static void main(String args[]) {
        double x = Double.parseDouble(args[0]);
        double y = Double.parseDouble(args[1]);
        for (Operation op : Operation.values())
            System.out.println(x + " " + op + " " + y +
                               " = " + op.eval(x, y));
    }
}

java Operation 2.0 4.0
2.0 PLUS 4.0 = 6.0
2.0 MINUS 4.0 = -2.0
2.0 TIMES 4.0 = 8.0
2.0 DIVIDED_BY 4.0 = 0.5

import java.util.List;
import java.util.ArrayList;
class Card implements Comparable<Card>,
                      java.io.Serializable {
    public enum Rank { DEUCE, THREE, FOUR, FIVE, SIX, SEVEN,
                       EIGHT, NINE, TEN,JACK, QUEEN, KING, ACE }

    public enum Suit { CLUBS, DIAMONDS, HEARTS, SPADES }

    private final Rank rank;
    private final Suit suit;
    public Rank rank() { return rank; }
    public Suit suit() { return suit; }

    private Card(Rank rank, Suit suit) {
        if (rank == null || suit == null)
            throw new NullPointerException(rank + ", " + suit);
        this.rank = rank;
        this.suit = suit;
    }

    public String toString() { return rank + " of " + suit; }

    // Primary sort on suit, secondary sort on rank
    public int compareTo(Card c) {
        int suitCompare = suit.compareTo(c.suit);
        return (suitCompare != 0 ?
                    suitCompare :
                    rank.compareTo(c.rank));
    }

    private static final List<Card> prototypeDeck =
        new ArrayList<Card>(52);

    static {
        for (Suit suit : Suit.values())
            for (Rank rank : Rank.values())
                prototypeDeck.add(new Card(rank, suit));
    }

    // Returns a new deck
    public static List<Card> newDeck() {
        return new ArrayList<Card>(prototypeDeck);
    }
}

import java.util.List;
import java.util.ArrayList;
import java.util.Collections;
class Deal {
    public static void main(String args[]) {
        int numHands     = Integer.parseInt(args[0]);
        int cardsPerHand = Integer.parseInt(args[1]);
        List<Card> deck  = Card.newDeck();
        Collections.shuffle(deck);
        for (int i=0; i < numHands; i++)
            System.out.println(dealHand(deck, cardsPerHand));
    }

    /**
     * Returns a new ArrayList consisting of the last n
     * elements of deck, which are removed from deck.
     * The returned list is sorted using the elements'
     * natural ordering.
     */
    public static <E extends Comparable<E>>
    ArrayList<E> dealHand(List<E> deck, int n) {
        int deckSize = deck.size();
        List<E> handView = deck.subList(deckSize - n, deckSize);
        ArrayList<E> hand = new ArrayList<E>(handView);
        handView.clear();
        Collections.sort(hand);
        return hand;
    }
}

java Deal 4 3
[DEUCE of CLUBS, SEVEN of CLUBS, QUEEN of DIAMONDS]
[NINE of HEARTS, FIVE of SPADES, ACE of SPADES]
[THREE of HEARTS, SIX of HEARTS, TEN of SPADES]
[TEN of CLUBS, NINE of DIAMONDS, THREE of SPADES]