chapter-8.txt

Chapter 8. Classes

Table of Contents

8.1. Class Declarations
8.1.1. Class Modifiers
8.1.1.1. abstract Classes
8.1.1.2. sealed, non-sealed, and final Classes
8.1.1.3. strictfp Classes
8.1.1.4. static Classes
8.1.2. Generic Classes and Type Parameters
8.1.3. Inner Classes and Enclosing Instances
8.1.4. Superclasses and Subclasses
8.1.5. Superinterfaces
8.1.6. Permitted Direct Subclasses
8.1.7. Class Body and Member Declarations
8.1.8. Implicitly Declared Classes
8.2. Class Members
8.3. Field Declarations
8.3.1. Field Modifiers
8.3.1.1. static Fields
8.3.1.2. final Fields
8.3.1.3. transient Fields
8.3.1.4. volatile Fields
8.3.2. Field Initialization
8.3.3. Restrictions on Field References in Initializers
8.4. Method Declarations
8.4.1. Formal Parameters
8.4.2. Method Signature
8.4.3. Method Modifiers
8.4.3.1. abstract Methods
8.4.3.2. static Methods
8.4.3.3. final Methods
8.4.3.4. native Methods
8.4.3.5. strictfp Methods
8.4.3.6. synchronized Methods
8.4.4. Generic Methods
8.4.5. Method Result
8.4.6. Method Throws
8.4.7. Method Body
8.4.8. Inheritance, Overriding, and Hiding
8.4.8.1. Overriding (by Instance Methods)
8.4.8.2. Hiding (by Class Methods)
8.4.8.3. Requirements in Overriding and Hiding
8.4.8.4. Inheriting Methods with Override-Equivalent Signatures
8.4.9. Overloading
8.5. Member Class and Interface Declarations
8.6. Instance Initializers
8.7. Static Initializers
8.8. Constructor Declarations
8.8.1. Formal Parameters
8.8.2. Constructor Signature
8.8.3. Constructor Modifiers
8.8.4. Generic Constructors
8.8.5. Constructor Throws
8.8.6. The Type of a Constructor
8.8.7. Constructor Body
8.8.7.1. Constructor Invocations
8.8.8. Constructor Overloading
8.8.9. Default Constructor
8.8.10. Preventing Instantiation of a Class
8.9. Enum Classes
8.9.1. Enum Constants
8.9.2. Enum Body Declarations
8.9.3. Enum Members
8.10. Record Classes
8.10.1. Record Components
8.10.2. Record Body Declarations
8.10.3. Record Members
8.10.4. Record Constructor Declarations
8.10.4.1. Normal Canonical Constructors
8.10.4.2. Compact Canonical Constructors

A class declaration defines a new class and describes how it is implemented (§8.1).

A top level class (§7.6) is a class declared directly in a compilation unit.

A nested class is any class whose declaration occurs within the body of another class or interface declaration. A nested class may be a member class (§8.5, §9.5), a local class (§14.3), or an anonymous class (§15.9.5).

A nested class may be inner (§8.1.3), in which case it may (depending on precisely where its declaration occurs) be able to refer to enclosing class instances, local variables, and type variables.

An enum class (§8.9) is a class declared with abbreviated syntax that defines a small set of named class instances.

A record class (§8.10) is a class declared with abbreviated syntax that defines a simple aggregate of values.

This chapter discusses the common semantics of all classes. Details that are specific to particular kinds of classes are discussed in the sections dedicated to these constructs.

A class may be declared public (§8.1.1) so it can be referred to from code in any package of its module and potentially from code in other modules.

A class may be declared abstract (§8.1.1.1), and must be declared abstract if it is incompletely implemented; such a class cannot be instantiated, but can be extended by subclasses. The degree to which a class can be extended can be controlled explicitly (§8.1.1.2): it may be declared sealed to limit its subclasses, or it may be declared final to ensure no subclasses. Each class except Object is an extension of (that is, a subclass of) a single existing class (§8.1.4) and may implement interfaces (§8.1.5).

A class may be generic (§8.1.2), that is, its declaration may introduce type variables whose bindings differ among different instances of the class.

Class declarations may be decorated with annotations (§9.7) just like any other kind of declaration.

The body of a class declares members (fields, methods, classes, and interfaces), instance and static initializers, and constructors (§8.1.7). The scope (§6.3) of a member (§8.2) is the entire body of the declaration of the class to which the member belongs. Field, method, member class, member interface, and constructor declarations may include the access modifiers public, protected, or private (§6.6). The members of a class include both declared and inherited members (§8.2). Newly declared fields can hide fields declared in a superclass or superinterface. Newly declared member classes and member interfaces can hide member classes and member interfaces declared in a superclass or superinterface. Newly declared methods can hide, implement, or override methods declared in a superclass or superinterface.

Field declarations (§8.3) describe class variables, which are incarnated once, and instance variables, which are freshly incarnated for each instance of the class. A field may be declared final (§8.3.1.2), in which case it can be assigned to only once. Any field declaration may include an initializer.

Member class declarations (§8.5) describe nested classes that are members of the surrounding class. Member classes may be static, in which case they have no access to the instance variables of the surrounding class; or they may be inner classes.

Member interface declarations (§8.5) describe nested interfaces that are members of the surrounding class.

Method declarations (§8.4) describe code that may be invoked by method invocation expressions (§15.12). A class method is invoked relative to the class; an instance method is invoked with respect to some particular object that is an instance of a class. A method whose declaration does not indicate how it is implemented must be declared abstract. A method may be declared final (§8.4.3.3), in which case it cannot be hidden or overridden. A method may be implemented by platform-dependent native code (§8.4.3.4). A synchronized method (§8.4.3.6) automatically locks an object before executing its body and automatically unlocks the object on return, as if by use of a synchronized statement (§14.19), thus allowing its activities to be synchronized with those of other threads (§17 (Threads and Locks)).

Method names may be overloaded (§8.4.9).

Instance initializers (§8.6) are blocks of executable code that may be used to help initialize an instance when it is created (§15.9).

Static initializers (§8.7) are blocks of executable code that may be used to help initialize a class.

Constructors (§8.8) are similar to methods, but cannot be invoked directly by a method call; they are used to initialize new class instances. Like methods, they may be overloaded (§8.8.8).

8.1. Class Declarations

A class declaration specifies a class.

There are three kinds of class declarations: normal class declarations, enum declarations (§8.9), and record declarations (§8.10).

ClassDeclaration:
NormalClassDeclaration
EnumDeclaration
RecordDeclaration
NormalClassDeclaration:
{ClassModifier} class TypeIdentifier [TypeParameters] [ClassExtends] [ClassImplements] [ClassPermits] ClassBody

Some classes are implicitly declared by other constructs (§8.1.8).

The TypeIdentifier in a class declaration specifies the name of the class.

It is a compile-time error if a class has the same simple name as any of its enclosing classes or interfaces.

The scope and shadowing of a class declaration is specified in §6.3 and §6.4.1.

8.1.1. Class Modifiers

A class declaration may include class modifiers.

ClassModifier:
(one of)
Annotation public protected private
abstract static final sealed non-sealed strictfp

The rules concerning annotation modifiers for 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, §9.5), not to local classes (§14.3) or anonymous classes (§15.9.5).

The access modifiers protected and private pertain only to member classes.

The modifier static pertains only to member classes and local 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.

It is a compile-time error if a class declaration has more than one of the modifiers sealed, non-sealed, and final.

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.

8.1.1.1. abstract Classes

An abstract class is a class that is incomplete, or to be considered incomplete.

It is a compile-time error if an attempt is made to create an instance of an abstract class using a class instance creation expression (§15.9.1).

A subclass of an abstract class that is not itself abstract may be instantiated, resulting in the execution of a constructor for the abstract class and, therefore, the execution of the field initializers for instance variables of that class.

A normal class may have abstract methods, that is, methods that are declared but not yet implemented (§8.4.3.1), only if it is an abstract class. It is a compile-time error if a normal class that is not abstract has an abstract method.

A class C has abstract methods if either of the following is true:

Any of the member methods (§8.2) of C - either declared or inherited - is abstract.

Any of C's superclasses has an abstract method declared with package access, and there exists no method that overrides the abstract method from C or from a superclass of C.

It is a compile-time error to declare an abstract class type such that it is not possible to create a subclass that implements all of its abstract methods. This situation can occur if the class would have as members two abstract methods that have the same method signature (§8.4.2) but return types for which no type is return-type-substitutable with both (§8.4.5).

Example 8.1.1.1-1. Abstract Class Declaration

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() { }
}


Here, a class Point is declared that must be declared abstract, because it contains a declaration of an abstract method named alert. The subclass of Point named ColoredPoint inherits the abstract method alert, so it must also be declared abstract. On the other hand, the subclass of Point named SimplePoint provides an implementation of alert, so it need not be abstract.

The statement:

Point p = new Point();


would result in a compile-time error; the class Point cannot be instantiated because it is abstract. However, a Point variable could correctly be initialized with a reference to any subclass of Point, and the class SimplePoint is not abstract, so the statement:

Point p = new SimplePoint();


would be correct. Instantiation of a SimplePoint causes the default constructor and field initializers for x and y of Point to be executed.




Example 8.1.1.1-2. Abstract Class Declaration that Prohibits Subclasses

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


These declarations result in a compile-time error: it would be impossible for any subclass of class Colored to provide an implementation of a method named setColor, taking one argument of type int, that can satisfy both abstract method specifications, because the one in interface Colorable requires the same method to return no value, while the one in class Colored requires the same method to return a value of type int (§8.4).




A class type should be declared abstract only if the intent is that subclasses can be created to complete the implementation. If the intent is simply to prevent instantiation of a class, the proper way to express this is to declare a constructor (§8.8.10) of no arguments, make it private, never invoke it, and declare no other constructors. A class of this form usually contains class methods and variables.

The class Math is an example of a class that cannot be instantiated; its declaration looks like this:


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


8.1.1.2. sealed, non-sealed, and final Classes

A class can be declared sealed if all its direct subclasses are known when the class is declared (§8.1.6), and no other direct subclasses are desired or required.

Explicit and exhaustive control over a class's direct subclasses is useful when the class hierarchy is used to model the kinds of values in a domain, rather than as a mechanism for code inheritance and reuse. The direct subclasses may themselves be declared sealed in order to further control the class hierarchy.

A class can be declared final if its definition is complete and no subclasses are desired or required.

It is a compile-time error if a class is declared both final and abstract, because the implementation of such a class could never be completed (§8.1.1.1).

Because a final class never has any subclasses, the methods of a final class are never overridden (§8.4.8.1).

A class is freely extensible if its direct superclass is not sealed (§8.1.4), and none of its direct superinterfaces are sealed (§8.1.5), and it is neither sealed nor final itself.

A class that has a sealed direct superclass or a sealed direct superinterface is freely extensible if and only if it is declared non-sealed.

It is a compile-time error if a class has a sealed direct superclass or a sealed direct superinterface, and is not declared final, sealed, or non-sealed either explicitly or implicitly.

Thus, an effect of the sealed keyword is to force all direct subclasses to explicitly declare whether they are final, sealed, or non-sealed. This avoids accidentally exposing a sealed class hierarchy to unwanted subclassing.

An enum class is either implicitly final or implicitly sealed, so it can implement a sealed interface. Similarly, a record class is implicitly final, so it can also implement a sealed interface.

It is a compile-time error if a class is declared non-sealed but has neither a sealed direct superclass nor a sealed direct superinterface.

Thus, a subclass of a non-sealed class cannot itself be declared non-sealed.

8.1.1.3. strictfp Classes

The strictfp modifier on a class declaration is obsolete and should not be used in new code. Its presence or absence has no effect at compile time or run time.

8.1.1.4. static Classes

The static modifier specifies that a nested class is not an inner class (§8.1.3). Just as a static method of a class has no current instance of the class in its body, a static nested class has no immediately enclosing instance in its body.

References from a static nested class to type parameters, instance variables, local variables, formal parameters, exception parameters, or instance methods of lexically enclosing class, interface, or method declarations are disallowed (§6.5.5.1, §6.5.6.1, and §15.12.3).

The static modifier does not pertain to all nested classes. It pertains only to member classes, whose declarations may use the static modifier, and not to local classes or anonymous classes, whose declarations may not use the static modifier (§14.3, §15.9.5). However, some local classes are implicitly static, namely local enum classes and local record classes, because all nested enum classes and nested record classes are implicitly static (§8.9, §8.10).

8.1.2. Generic Classes and Type Parameters

A class is generic if the class declaration 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.

TypeParameters:
< TypeParameterList >
TypeParameterList:
TypeParameter {, TypeParameter}

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

TypeParameter:
{TypeParameterModifier} TypeIdentifier [TypeBound]
TypeParameterModifier:
Annotation
TypeBound:
extends TypeVariable
extends ClassOrInterfaceType {AdditionalBound}
AdditionalBound:
& InterfaceType

The rules concerning annotation modifiers for 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 §6.4.1.

References to a class's type parameter from a static context or a nested class or interface are restricted, as specified in §6.5.5.1.

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.

For instance, executing the code:


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



will result in the variable b holding the value true.

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.

Example 8.1.2-1. Mutually Recursive Type Variable Bounds

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(); }
}




Example 8.1.2-2. Nested Generic Classes

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>(
                Integer.valueOf(1),
                new Seq<Number>(Double.valueOf(1.5),
                                new Seq<Number>()));

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

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



8.1.3. Inner Classes and Enclosing Instances

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

An inner class is one of the following:

a member class that is not explicitly or implicitly static (§8.5)

a local class that is not implicitly static (§14.3)

an anonymous class (§15.9.5)

The following nested classes are implicitly static, so are not inner classes:

a member enum class (§8.9)

a local enum class (§14.3)

a member record class (§8.10)

a local record class (§14.3)

a member class of an interface (§9.5)

All of the rules that apply to nested classes apply to inner classes. In particular, an inner class may declare and inherit static members (§8.2), and declare static initializers (§8.7), even though the inner class itself is not static.

There are no "inner interfaces" because every nested interface is implicitly static (§9.1.1.3).

Example 8.1.3-1. Inner Class Declarations and Static Members

class HasStatic {
    static int j = 100;
}

class Outer {
    class Inner extends HasStatic {
        static {
            System.out.println("Hello from Outer.Inner");
        }

        static       int x = 3;
        static final int y = 4;

        static void hello() {
            System.out.println("Hello from Outer.Inner.hello");
        }

        static class VeryNestedButNotInner
            extends NestedButNotInner {}
    }

    static class NestedButNotInner {
        int z = Inner.x;
    }

    interface NeverInner {}  // Implicitly static, so never inner
}



Prior to Java SE 16, an inner class could not declare static initializers, and could only declare static members that were constant variables (§4.12.4).




A construct (statement, local variable declaration statement, local class declaration, local interface declaration, or expression) occurs in a static context if the innermost:

method declaration,

field declaration,

constructor declaration,

instance initializer, or

static initializer.

which encloses the construct is one of the following:

a static method declaration (§8.4.3.2, §9.4)

a static field declaration (§8.3.1.1, §9.3)

a static initializer (§8.7)

Note that a construct which appears in a constructor declaration or an instance initializer does not occur in a static context.

The purpose of a static context is to demarcate code for which there is no current instance defined of the class whose declaration lexically encloses the static context. Consequently, code that occurs in a static context is restricted in the following ways:

this expressions (both unqualified and qualified) are disallowed (§15.8.3, §15.8.4).

Field accesses, method invocations, and method references may not be qualified by super (§15.11.2, §15.12.3, §15.13.1).

Unqualified references to instance variables of any lexically enclosing class or interface declaration are disallowed (§6.5.6.1).

Unqualified invocations of instance methods of any lexically enclosing class or interface declaration are disallowed (§15.12.3).

References to type parameters of any lexically enclosing class or interface declarations are disallowed (§6.5.5.1).

References to type parameters, local variables, formal parameters, and exception parameters declared by methods or constructors of any lexically enclosing class or interface declaration that is outside the immediately enclosing class or interface declaration are disallowed (§6.5.5.1, §6.5.6.1).

Declarations of local normal classes (as opposed to local enum classes) and declarations of anonymous classes both specify classes that are inner, yet when instantiated have no immediately enclosing instances (§15.9.2).

Class instance creation expressions that instantiate inner member classes must be qualified (§15.9).

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

If an inner class is a local class or an anonymous class, it may be declared in a static context, and in that case is not considered an inner class of any enclosing class or interface.

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 class or interface declaration of an inner class to be an interface. This only occurs if the class is a local or anonymous class declared in a default or static method body (§9.4).

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

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

An instance i of a direct inner class C of a class or interface O may be 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 local class or an anonymous class whose declaration occurs in a static context has no immediately enclosing instance. Also, an instance of a static nested class (§8.1.1.4) has no immediately enclosing instance.

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 a constructor invocation (§8.8.7.1).

When an inner class contains a valid reference to an instance variable that is a member of a lexically enclosing class or interface 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 final or effectively final (§4.12.4), as specified in §6.5.6.1.

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 class or interface declaration may not be assigned within an inner class, or a compile-time error occurs.

Example 8.1.3-2. Inner Class Declarations

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;
        }
    }
}


The declaration of class LocalInStaticContext occurs in a static context due to being within the static method classMethod. Instance variables of class Outer are not available within the body of a static method. In particular, instance variables of Outer are not available inside the body of LocalInStaticContext. However, local variables from the surrounding method may be referred to without error (provided they are declared final or are effectively final).

Inner classes whose declarations do not occur in a static context may freely refer to the instance variables of their enclosing class declaration. An instance variable is always defined with respect to an instance. In the case of instance variables of an enclosing class declaration, the instance variable must be defined with respect to an enclosing instance of the inner class. For example, the class Local above has an enclosing instance of class Outer. As a further example:

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

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


Here, every instance of WithDeepNesting.Nested.DeeplyNested has an enclosing instance of class WithDeepNesting.Nested (its immediately enclosing instance) and an enclosing instance of class WithDeepNesting (its 2nd lexically enclosing instance).



8.1.4. Superclasses and Subclasses

The optional extends clause in a normal class declaration specifies the direct superclass type of the class being declared.

ClassExtends:
extends ClassType

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 type.

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

It is a compile-time error if the ClassType names a class that is sealed (§8.1.1.2) and the class being declared is not a permitted direct subclass of the named class (§8.1.6).

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, which can only be extended by an enum class (§8.9), or names the class Record, which can only be extended by a record class (§8.10).

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.

The direct superclass type of a class whose declaration lacks an extends clause is as follows:

The class Object has no direct superclass type.

For a class other than Object with a normal class declaration, the direct superclass type is Object.

For an enum class E, the direct superclass type is Enum<E>.

For a record class R, the direct superclass type is Record.

For an anonymous class, the direct superclass type is defined in §15.9.5.

The direct superclass of a class is the class named by its direct superclass type. The direct superclass is important because its implementation is used to derive the implementation of the class being declared.

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

A is the direct superclass of C.

Where a class B is the direct superclass of C, A is a superclass of B, applying this definition recursively.

A class is said to be a direct subclass of its direct superclass, and a subclass of each of its superclasses.

Example 8.1.4-1. Direct Superclasses and Subclasses

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


Here, the relationships are as follows:

The class Point is a direct subclass of Object.

The class Object is the direct superclass of the class Point.

The class ColoredPoint is a direct subclass of class Point.

The class Point is the direct superclass of class ColoredPoint.

The declaration of class Colored3dPoint causes a compile-time error because it attempts to extend the final class ColoredPoint.




Example 8.1.4-2. Superclasses and Subclasses

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


Here, the relationships are as follows:

The class Point is a superclass of class ColoredPoint.

The class Point is a superclass of class Colored3dPoint.

The class ColoredPoint is a subclass of class Point.

The class ColoredPoint is a superclass of class Colored3dPoint.

The class Colored3dPoint is a subclass of class ColoredPoint.

The class Colored3dPoint is a subclass of class Point.




A class C directly depends on a class or interface A if A 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 class or interface A if any of the following is true:

C directly depends on A.

C directly depends on an interface I that depends (§9.1.3) on A.

C directly depends on a class B that depends on A, applying this definition recursively.

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).

Example 8.1.4-3. Class Depends on Itself

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


This program causes a compile-time error because class Point depends on itself.



8.1.5. Superinterfaces

The optional implements clause in a class declaration specifies the direct superinterface types of the class being declared.

ClassImplements:
implements InterfaceTypeList
InterfaceTypeList:
InterfaceType {, InterfaceType}

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

It is a compile-time error if any InterfaceType names a interface that is sealed (§9.1.1.4) and the class being declared is not a permitted direct subclass of the named interface (§9.1.4).

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 named by a direct superinterface type more than once in a single implements clause. This is true even if the interface is named in different ways.

Example 8.1.5-1. Illegal Superinterfaces

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



This program results in a compile-time error because the names java.lang.Cloneable and Cloneable refer to the same interface.




A class whose declaration lacks an implements clause has no direct superinterface types, with one exception: an anonymous class may have a superinterface type (§15.9.5).

An interface is a direct superinterface of a class if the interface is named by one of the direct superinterface types of the class.

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

I is a direct superinterface of C.

C has some direct superinterface J for which I is a superinterface, using the definition of "superinterface of an interface" given in §9.1.3.

I is a superinterface of the direct superclass of C.

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

A class is said to directly implement its direct superinterfaces, and to implement all of its superinterfaces.

A class is said to be a direct subclass of its direct superinterfaces, and a subclass of all of its superinterfaces.

A class may not declare a direct superclass type and a direct superinterface type, or two direct superinterface types, which are, or which have supertypes (§4.10.2) which are, different parameterizations of the same generic interface (§9.1.2), or a parameterization of a generic interface and a raw type naming that same generic interface. In the case of such a conflict, a compile-time error occurs.

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

Example 8.1.5-2. Superinterfaces

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; }
}


Here, the relationships are as follows:

The interface Paintable is a superinterface of class PaintedPoint.

The interface Colorable is a superinterface of class ColoredPoint and of class PaintedPoint.

The interface Paintable is a subinterface of the interface Colorable, and Colorable is a superinterface of Paintable, as defined in §9.1.3.

The class PaintedPoint has Colorable as a superinterface both because it is a superinterface of ColoredPoint and because it is a superinterface of Paintable.




Example 8.1.5-3. Illegal Multiple Inheritance of an Interface

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


Class C causes a compile-time error because it attempts to be a subtype of both I<Integer> and 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.

Example 8.1.5-4. Implementing Methods of a Superinterface

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


This program causes a compile-time error, because ColoredPoint is not an abstract class but fails to provide an implementation of methods setColor and getColor of the interface Colorable.

In the following program:

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; }
}


the method getNumberOfScales in class Tuna has a name, signature, and return type that matches the method declared in interface Fish and also matches the method declared in interface Piano; it is considered to implement both.

On the other hand, in a situation such as this:

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; }
}


it is impossible to declare a method named getNumberOfScales whose signature and return type are compatible with those of both the methods declared in interface Fish and in interface StringBass, because a class cannot have multiple methods with the same signature and different primitive return types (§8.4). Therefore, it is impossible for a single class to implement both interface Fish and interface StringBass (§8.4.8).



8.1.6. Permitted Direct Subclasses

The optional permits clause in a normal class declaration specifies all the classes intended as direct subclasses of the class being declared (§8.1.1.2).

ClassPermits:
permits TypeName {, TypeName}

It is a compile-time error if a class declaration has a permits clause but no sealed modifier.

Every TypeName must name an accessible class (§6.6), or a compile-time error occurs.

It is a compile-time error if the same class is specified more than once in a permits clause. This is true even if the class is named in different ways.

The canonical name of a class does not need to be used in a permits clause, but a permits clause can only specify a class once. For example, the following program fails to compile:

package p;

sealed class A     permits B, C, p.B {}  // error

non-sealed class B extends A {}
non-sealed class C extends A {}


If a sealed class C is associated with a named module (§7.3), then every class specified in the permits clause of C's declaration must be associated with the same module as C, or a compile-time error occurs.

If a sealed class C is associated with an unnamed module (§7.7.5), then every class specified in the permits clause of C's declaration must belong to the same package as C, or a compile-time error occurs.

A sealed class and its direct subclasses need to refer to each other in a circular fashion, in permits and extends clauses, respectively. Therefore, in a modular codebase, they must be co-located in the same module, as classes in different modules cannot refer to each other in a circular fashion. Co-location is desirable in any case because a sealed class hierarchy should always be declared within a single maintenance domain, where the same developer or group of developers is responsible for maintaining the hierarchy. A named module typically represents a maintenance domain in a modular codebase.

If the declaration of a sealed class C has a permits clause, then the permitted direct subclasses of C are the classes specified by the permits clause.

Every permitted direct subclass specified by the permits clause must be a direct subclass of C (§8.1.4), or a compile-time error occurs.

If the declaration of a sealed class C lacks a permits clause, then the permitted direct subclasses of C are as follows:

If C is not an enum class, then its permitted direct subclasses are those classes declared in the same compilation unit as C (§7.3) which have a canonical name (§6.7) and whose direct superclass is C.

That is, the permitted direct subclasses are inferred as the classes in the same compilation unit that specify C as their direct superclass. The requirement for a canonical name means that no local classes or anonymous classes will be considered.

It is a compile-time error if the declaration of a sealed class C lacks a permits clause and C has no permitted direct subclasses.

If C is an enum class, then its permitted direct subclasses, if any, are specified in §8.9.

8.1.7. Class Body and Member Declarations

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

ClassBody:
{ {ClassBodyDeclaration} }
ClassBodyDeclaration:
ClassMemberDeclaration
InstanceInitializer
StaticInitializer
ConstructorDeclaration
ClassMemberDeclaration:
FieldDeclaration
MethodDeclaration
ClassDeclaration
InterfaceDeclaration
;

The scope and shadowing of a declaration of a member m declared in or inherited by a class C is specified in §6.3 and §6.4.1.

If C 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 the other definitions of the same kind and name.

8.1.8. Implicitly Declared Classes

Not all classes are specified by a class declaration. The following constructs implicitly declare classes:

a class instance creation expression that ends with a class body (§15.9.5)

an enum constant that ends with a class body (§8.9.1)

a compact compilation unit (§7.3)

In all cases, the members of any implicitly declared class, including any implicitly declared members, are subject to the usual rules for member declarations in a class.

The following production from §7.3 is shown here for convenience:

CompactCompilationUnit:
{ImportDeclaration} {ClassMemberDeclarationNoMethod} MethodDeclaration {ClassMemberDeclaration}
ClassMemberDeclarationNoMethod:
FieldDeclaration
ClassDeclaration
InterfaceDeclaration
;

The class implicitly declared by a compact compilation unit satisfies the following properties:

It is a top level class (§7.6).

Its name is a valid identifier (§3.8) determined by the host system.

In simple implementations of the Java SE Platform, where compilation units are stored in files, the name of this implicitly declared class would typically be the name of the file containing the compact compilation unit minus any extension (such as .java or .jav).

It is not abstract (§8.1.1.1).

It is final (§8.1.1.2).

It is a member of an unnamed package (§7.4.2) and has package access.

Its direct superclass is Object (§8.1.4).

It does not have any direct superinterfaces (§8.1.5).

The body of the class contains every class member declared in the compact compilation unit (these are declarations of fields (§8.3), methods (§8.4), member classes (§8.5), and member interfaces (§9.1.1.3)). It is not possible for a compact compilation unit to contain declarations of instance initializers (§8.6), static initializers (§8.7), or constructors (§8.8).

It has an implicitly declared default constructor (§8.8.9).

It is a compile-time error if this class does not declare at least one candidate main method (§12.1.4).

Note that an unnamed package may have multiple implicitly declared classes as members.

8.2. Class Members

The members of a class are all of the following:

Members inherited from its direct superclass type (§8.1.4), except in the class Object, which has no direct superclass type

Members inherited from any direct superinterface types (§8.1.5)

Members declared in the body of the class (§8.1.7)

Members of a class that are declared private are not inherited by subclasses of that class.

Only members of a class that are declared protected or public are inherited by subclasses declared in a package other than the one in which the class is declared.

Constructors, static initializers, and instance initializers are not members and therefore are not inherited.

We use the phrase the type of a member to denote:

For a field, its type.

For a method, an ordered 4-tuple (known as a method type) consisting of:

type parameters: the declarations of any type parameters of the method member (§8.4.4).

parameter types: a list of the types of the formal parameters of the method member (§8.4.1).

return type: the return type of the method member (§8.4.5).

throws clause: exception types declared in the throws clause of the method member (§8.4.6).

Fields, methods, member classes, and member interfaces of a class may have the same name, since they are used in different contexts and are disambiguated by different lookup procedures (§6.5). However, this is discouraged as a matter of style.

Example 8.2-1. Use of Class Members

class Point {
    int x, y;
    private Point() { reset(); }
    Point(int x, int y) { this.x = x; this.y = y; }
    private void reset() { this.x = 0; this.y = 0; }
}
class ColoredPoint extends Point {
    int color;
    void clear() { reset(); }  // error
}
class Test {
    public static void main(String[] args) {
        ColoredPoint c = new ColoredPoint(0, 0);  // error
        c.reset();  // error
    }
}


This program causes four compile-time errors.

One error occurs because ColoredPoint has no constructor declared with two int parameters, as requested by the use in main. This illustrates the fact that ColoredPoint does not inherit the constructors of its superclass Point.

Another error occurs because ColoredPoint declares no constructors, and therefore a default constructor for it is implicitly declared (§8.8.9), and this default constructor is equivalent to:

ColoredPoint() { super(); }


which invokes the constructor, with no arguments, for the direct superclass of the class ColoredPoint. The error is that the constructor for Point that takes no arguments is private, and therefore is not accessible outside the class Point, even through a superclass constructor invocation (§8.8.7).

Two more errors occur because the method reset of class Point is private, and therefore is not inherited by class ColoredPoint. The method invocations in method clear of class ColoredPoint and in method main of class Test are therefore not correct.




Example 8.2-2. Inheritance of Class Members with Package Access

Consider the example where the points package declares two compilation units:

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


and:

package points;
public class Point3d extends Point {
    int z;
    public void move(int dx, int dy, int dz) {
        x += dx; y += dy; z += dz;
    }
}


and a third compilation unit, in another package, is:

import points.Point3d;
class Point4d extends Point3d {
    int w;
    public void move(int dx, int dy, int dz, int dw) {
        x += dx; y += dy; z += dz; w += dw; // compile-time errors
    }
}


Here both classes in the points package compile. The class Point3d inherits the fields x and y of class Point, because it is in the same package as Point. The class Point4d, which is in a different package, does not inherit the fields x and y of class Point or the field z of class Point3d, and so fails to compile.

A better way to write the third compilation unit would be:


import points.Point3d;
class Point4d extends Point3d {
    int w;
    public void move(int dx, int dy, int dz, int dw) {
        super.move(dx, dy, dz); w += dw;
    }
}



using the move method of the superclass Point3d to process dx, dy, and dz. If Point4d is written in this way, it will compile without errors.




Example 8.2-3. Inheritance of public and protected Class Members

Given the class Point:

package points;
public class Point {
    public int x, y;
    protected int useCount = 0;
    static protected int totalUseCount = 0;
    public void move(int dx, int dy) {
        x += dx; y += dy; useCount++; totalUseCount++;
    }
}


the public and protected fields x, y, useCount, and totalUseCount are inherited in all subclasses of Point.

Therefore, this test program, in another package, can be compiled successfully:

class Test extends points.Point {
    public void moveBack(int dx, int dy) {
        x -= dx; y -= dy; useCount++; totalUseCount++;
    }
}




Example 8.2-4. Inheritance of private Class Members

class Point {
    int x, y;
    void move(int dx, int dy) {
        x += dx; y += dy; totalMoves++;
    }
    private static int totalMoves;
    void printMoves() { System.out.println(totalMoves); }
}
class Point3d extends Point {
    int z;
    void move(int dx, int dy, int dz) {
        super.move(dx, dy); z += dz; totalMoves++; // error
    }
}


Here, the class variable totalMoves can be used only within the class Point; it is not inherited by the subclass Point3d. A compile-time error occurs because method move of class Point3d tries to increment totalMoves.




Example 8.2-5. Accessing Members of Inaccessible Classes

Even though a class might not be declared public, instances of the class might be available at run time to code outside the package in which it is declared by means of a public superclass or superinterface. An instance of the class can be assigned to a variable of such a public type. An invocation of a public method of the object referred to by such a variable may invoke a method of the class if it implements or overrides a method of the public superclass or superinterface. (In this situation, the method is necessarily declared public, even though it is declared in a class that is not public.)

Consider the compilation unit:

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


and another compilation unit of another package:

package morePoints;
class Point3d extends points.Point {
    public int z;
    public void move(int dx, int dy, int dz) {
        super.move(dx, dy); z += dz;
    }
    public void move(int dx, int dy) {
        move(dx, dy, 0);
    }
}
public class OnePoint {
    public static points.Point getOne() {
        return new Point3d();
    }
}


An invocation morePoints.OnePoint.getOne() in yet a third package would return a Point3d that can be used as a Point, even though the type Point3d is not available outside the package morePoints. The two-argument version of method move could then be invoked for that object, which is permissible because method move of Point3d is public (as it must be, for any method that overrides a public method must itself be public, precisely so that situations such as this will work out correctly). The fields x and y of that object could also be accessed from such a third package.

While the field z of class Point3d is public, it is not possible to access this field from code outside the package morePoints, given only a reference to an instance of class Point3d in a variable p of type Point. This is because the expression p.z is not correct, as p has type Point and class Point has no field named z; also, the expression ((Point3d)p).z is not correct, because the class type Point3d cannot be referred to outside package morePoints.

The declaration of the field z as public is not useless, however. If there were to be, in package morePoints, a public subclass Point4d of the class Point3d:

package morePoints;
public class Point4d extends Point3d {
    public int w;
    public void move(int dx, int dy, int dz, int dw) {
        super.move(dx, dy, dz); w += dw;
    }
}


then class Point4d would inherit the field z, which, being public, could then be accessed by code in packages other than morePoints, through variables and expressions of the public type Point4d.



8.3. Field Declarations

The variables of a class are introduced by field declarations.

FieldDeclaration:
{FieldModifier} UnannType VariableDeclaratorList ;
VariableDeclaratorList:
VariableDeclarator {, VariableDeclarator}
VariableDeclarator:
VariableDeclaratorId [= VariableInitializer]
VariableDeclaratorId:
Identifier [Dims]
_
VariableInitializer:
Expression
ArrayInitializer
UnannType:
UnannPrimitiveType
UnannReferenceType
UnannPrimitiveType:
NumericType
boolean
UnannReferenceType:
UnannClassOrInterfaceType
UnannTypeVariable
UnannArrayType
UnannClassOrInterfaceType:
UnannClassType
UnannInterfaceType
UnannClassType:
TypeIdentifier [TypeArguments]
PackageName . {Annotation} TypeIdentifier [TypeArguments]
UnannClassOrInterfaceType . {Annotation} TypeIdentifier [TypeArguments]
UnannInterfaceType:
UnannClassType
UnannTypeVariable:
TypeIdentifier
UnannArrayType:
UnannPrimitiveType Dims
UnannClassOrInterfaceType Dims
UnannTypeVariable Dims

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

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

Each declarator in a FieldDeclaration declares one field. The declarator must include an Identifier, or a compile-time error occurs. The Identifier may be used in a name to refer to the field.

More than one field may be declared in a single FieldDeclaration by using more than one declarator; the FieldModifiers and UnannType apply to all the declarators in the declaration.

The FieldModifier clause is described in §8.3.1.

The declared type of a field is denoted by UnannType if no bracket pairs appear in UnannType and VariableDeclaratorId, and is specified by §10.2 otherwise.

The scope and shadowing of a field declaration is specified in §6.3 and §6.4.1.

It is a compile-time error for the body of a class declaration to declare two fields with the same name.

If a class declares a field with a certain name, then the declaration of that field is said to hide any and all accessible declarations of fields with the same name in superclasses, and superinterfaces of the class.

In this respect, hiding of fields differs from hiding of methods (§8.4.8.3), for there is no distinction drawn between static and non-static fields in field hiding whereas a distinction is drawn between static and non-static methods in method hiding.

A hidden field can be accessed by using a qualified name (§6.5.6.2) if it is static, or by using a field access expression that contains the keyword super (§15.11.2) or a cast to a superclass type.

In this respect, hiding of fields is similar to hiding of methods.

If a field declaration hides the declaration of another field, the two fields need not have the same type.

A class inherits from its direct superclass and direct superinterfaces all the non-private fields of the superclass and superinterfaces that are both accessible (§6.6) to code in the class and not hidden by a declaration in the class.

A private field of a superclass might be accessible to a subclass - for example, if both classes are members of the same class. Nevertheless, a private field is never inherited by a subclass.

It is possible for a class to inherit more than one field with the same name, either from its superclass and superinterfaces or from its superinterfaces alone. Such a situation does not in itself cause a compile-time error. However, any attempt within the body of the class to refer to any such field by its simple name will result in a compile-time error, because the reference is ambiguous.

There might be several paths by which the same field declaration is inherited from an interface. In such a situation, the field is considered to be inherited only once, and it may be referred to by its simple name without ambiguity.

Example 8.3-1. Multiply Inherited Fields

A class may inherit two or more fields with the same name, either from its superclass and a superinterface or from two superinterfaces. A compile-time error occurs on any attempt to refer to any ambiguously inherited field by its simple name. A qualified name or a field access expression that contains the keyword super (§15.11.2) may be used to access such fields unambiguously. In the program:

interface Frob  { float v = 2.0f; }
class SuperTest { int   v = 3; }
class Test extends SuperTest implements Frob {
    public static void main(String[] args) {
        new Test().printV();
    }
    void printV() { System.out.println(v); }
}


the class Test inherits two fields named v, one from its superclass SuperTest and one from its superinterface Frob. This in itself is permitted, but a compile-time error occurs because of the use of the simple name v in method printV: it cannot be determined which v is intended.

The following variation uses the field access expression super.v to refer to the field named v declared in class SuperTest and uses the qualified name Frob.v to refer to the field named v declared in interface Frob:

interface Frob  { float v = 2.0f; }
class SuperTest { int   v = 3; }
class Test extends SuperTest implements Frob {
    public static void main(String[] args) {
        new Test().printV();
    }
    void printV() {
        System.out.println((super.v + Frob.v)/2);
    }
}


It compiles and prints:

2.5


Even if two distinct inherited fields have the same type, the same value, and are both final, any reference to either field by simple name is considered ambiguous and results in a compile-time error. In the program:

interface Color        { int RED=0, GREEN=1,  BLUE=2;  }
interface TrafficLight { int RED=0, YELLOW=1, GREEN=2; }
class Test implements Color, TrafficLight {
    public static void main(String[] args) {
        System.out.println(GREEN);  // compile-time error
        System.out.println(RED);    // compile-time error
    }
}


it is not astonishing that the reference to GREEN should be considered ambiguous, because class Test inherits two different declarations for GREEN with different values. The point of this example is that the reference to RED is also considered ambiguous, because two distinct declarations are inherited. The fact that the two fields named RED happen to have the same type and the same unchanging value does not affect this judgment.




Example 8.3-2. Re-inheritance of Fields

If the same field declaration is inherited from an interface by multiple paths, the field is considered to be inherited only once. It may be referred to by its simple name without ambiguity. For example, in the code:

interface Colorable {
    int RED = 0xff0000, GREEN = 0x00ff00, BLUE = 0x0000ff;
}
interface Paintable extends Colorable {
    int MATTE = 0, GLOSSY = 1;
}
class Point { int x, y; }
class ColoredPoint extends Point implements Colorable {}
class PaintedPoint extends ColoredPoint implements Paintable {
    int p = RED;
}


the fields RED, GREEN, and BLUE are inherited by the class PaintedPoint both through its direct superclass ColoredPoint and through its direct superinterface Paintable. The simple names RED, GREEN, and BLUE may nevertheless be used without ambiguity within the class PaintedPoint to refer to the fields declared in interface Colorable.



8.3.1. Field Modifiers
FieldModifier:
(one of)
Annotation public protected private
static final transient volatile

The rules concerning annotation modifiers for 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.

8.3.1.1. static Fields

If a field is declared static, there exists exactly one incarnation of the field, no matter how many instances (possibly zero) of the class may eventually be created. A static field, sometimes called a class variable, is incarnated when the class is initialized (§12.4).

A field that is not declared static is called an instance variable, and sometimes called a non-static field. Whenever a new instance of a class is created (§12.5), a new variable associated with that instance is created for every instance variable declared in that class or any of its superclasses.

The declaration of a class variable introduces a static context (§8.1.3), which limits the use of constructs that refer to the current object. Notably, the keywords this and super are prohibited in a static context (§15.8.3, §15.11.2), as are unqualified references to instance variables, instance methods, and type parameters of lexically enclosing declarations (§6.5.5.1, §6.5.6.1, §15.12.3).

References to an instance variable from a static context or a nested class or interface are restricted, as specified in §6.5.6.1.

Example 8.3.1.1-1. static Fields

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);
    }
}


This program prints:

(2,2)
0
true
1


showing that changing the fields x, y, and useCount of p does not affect the fields of q, because these fields are instance variables in distinct objects. In this example, the class variable origin of the class Point is referenced both using the class name as a qualifier, in Point.origin, and using variables of the class type in field access expressions (§15.11), as in p.origin and q.origin. These two ways of accessing the origin class variable access the same object, evidenced by the fact that the value of the reference equality expression (§15.21.3):

q.origin==Point.origin


is true. Further evidence is that the incrementation:

p.origin.useCount++;


causes the value of q.origin.useCount to be 1; this is so because p.origin and q.origin refer to the same variable.




Example 8.3.1.1-2. Hiding of Class Variables

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);
    }
}


This program produces the output:

4.7 2


because the declaration of x in class Test hides the definition of x in class Point, so class Test does not inherit the field x from its superclass Point. Within the declaration of class Test, the simple name x refers to the field declared within class Test. Code in class Test may refer to the field x of class Point as super.x (or, because x is static, as Point.x). If the declaration of Test.x is deleted:

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);
    }
}


then the field x of class Point is no longer hidden within class Test; instead, the simple name x now refers to the field Point.x. Code in class Test may still refer to that same field as super.x. Therefore, the output from this variant program is:

2 2




Example 8.3.1.1-3. Hiding of Instance Variables

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);
    }
}


This program produces the output:

4.7 2
4.7 2


because the declaration of x in class Test hides the definition of x in class Point, so class Test does not inherit the field x from its superclass Point. It must be noted, however, that while the field x of class Point is not inherited by class Test, it is nevertheless implemented by instances of class Test. In other words, every instance of class Test contains two fields, one of type int and one of type double. Both fields bear the name x, but within the declaration of class Test, the simple name x always refers to the field declared within class Test. Code in instance methods of class Test may refer to the instance variable x of class Point as super.x.

Code that uses a field access expression to access field x will access the field named x in the class indicated by the type of reference expression. Thus, the expression sample.x accesses a double value, the instance variable declared in class Test, because the type of the variable sample is Test, but the expression ((Point)sample).x accesses an int value, the instance variable declared in class Point, because of the cast to type Point.

If the declaration of x is deleted from class Test, as in the program:

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);
    }
}


then the field x of class Point is no longer hidden within class Test. Within instance methods in the declaration of class Test, the simple name x now refers to the field declared within class Point. Code in class Test may still refer to that same field as super.x. The expression sample.x still refers to the field x within type Test, but that field is now an inherited field, and so refers to the field x declared in class Point. The output from this variant program is:

2 2
2 2



8.3.1.2. final Fields

A field can be declared final (§4.12.4). Both class and instance variables (static and non-static fields) may be declared final.

A blank final class variable must be definitely assigned by a static initializer of the class in which it is declared, or a compile-time error occurs (§8.7, §16.8).

A blank final instance variable must be definitely assigned and moreover not definitely unassigned at the end of every constructor of the class in which it is declared, or a compile-time error occurs (§8.8, §16.9).

8.3.1.3. transient Fields

Variables may be marked transient to indicate that they are not part of the persistent state of an object.

Example 8.3.1.3-1. Persistence of transient Fields

If an instance of the class Point:


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



were saved to persistent storage by a system service, then only the fields x and y would be saved. This specification does not specify details of such services; see the specification of java.io.Serializable for an example of such a service.



8.3.1.4. volatile Fields

The Java programming language allows threads to access shared variables (§17.1). As a rule, to ensure that shared variables are consistently and reliably updated, a thread should ensure that it has exclusive use of such variables by obtaining a lock that, conventionally, enforces mutual exclusion for those shared variables.

The Java programming language provides a second mechanism, volatile fields, that is more convenient than locking for some purposes.

A field may be declared volatile, in which case the Java Memory Model ensures that all threads see a consistent value for the variable (§17.4).

It is a compile-time error if a final variable is also declared volatile.

Example 8.3.1.4-1. volatile Fields

If, in the following example, one thread repeatedly calls the method one (but no more than Integer.MAX_VALUE times in all), and another thread repeatedly calls the method two:

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


then method two could occasionally print a value for j that is greater than the value of i, because the example includes no synchronization and, under the rules explained in §17.4, the shared values of i and j might be updated out of order.

One way to prevent this out-or-order behavior would be to declare methods one and two to be synchronized (§8.4.3.6):

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);
    }
}


This prevents method one and method two from being executed concurrently, and furthermore guarantees that the shared values of i and j are both updated before method one returns. Therefore method two never observes a value for j greater than that for i; indeed, it always observes the same value for i and j.

Another approach would be to declare i and j to be volatile:

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


This allows method one and method two to be executed concurrently, but guarantees that accesses to the shared values for i and j occur exactly as many times, and in exactly the same order, as they appear to occur during execution of the program text by each thread. Therefore, the shared value for j is never greater than that for i, because each update to i must be reflected in the shared value for i before the update to j occurs. It is possible, however, that any given invocation of method two might observe a value for j that is much greater than the value observed for i, because method one might be executed many times between the moment when method two fetches the value of i and the moment when method two fetches the value of j.

See §17.4 for more discussion and examples.



8.3.2. Field Initialization

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) (§8.3.1.1), then the following rules apply to its initializer:

The initializer may not refer to the current object using the keyword this or the keyword super, as specified in §15.8.3 and §15.11.2, nor refer by simple name to any instance variable or instance method, as specified in §6.5.6.1 and §15.12.3.

At run time, the initializer is evaluated and the assignment performed exactly once, when the class is initialized (§12.4.2).

Note that static fields that are constant variables (§4.12.4) are initialized before other static fields (§12.4.2, step 6). This also applies in interfaces (§9.3.1). When such fields are referenced by simple name, they will never be observed to have their default initial values (§4.12.5).

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

The initializer may refer to the current object using the keyword this or the keyword super, and may refer by simple name to any class variable declared in or inherited by the class, even one whose declaration occurs to the right of the initializer (§3.5).

At run time, the initializer is evaluated and the assignment performed each time an instance of the class is created (§12.5).

References from variable initializers to fields that may not yet be initialized are restricted, 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.

Example 8.3.2-1. Field Initialization

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);
    }
}


This program produces the output:

1, 5


because the assignments to x and y occur whenever a new Point is created.




Example 8.3.2-2. Forward Reference to a Class Variable

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


This program compiles without error; it initializes j to 1 when class Test is initialized, and initializes f to the current value of j every time an instance of class Test is created.



8.3.3. Restrictions on Field References in Initializers

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:

The reference appears either in a class variable initializer of C or in a static initializer of C (§8.7); and

The reference appears either in the initializer of f's own declarator or at a point to the left of f's declarator; and

The reference is not on the left hand side of an assignment expression (§15.26); and

The innermost class or interface enclosing the reference is C.

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

The reference appears either in an instance variable initializer of C or in an instance initializer of C (§8.6); and

The reference appears in the initializer of f's own declarator or at a point to the left of f's declarator; and

The reference is not on the left hand side of an assignment expression (§15.26); and

The innermost class enclosing the reference is C.

Example 8.3.3-1. Restrictions on Field References

A compile-time error occurs for this program:

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


whereas the following program compiles without error:

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


even though the constructor for Test2 (§8.8) refers to the field k that is declared three lines later.

The restrictions above are designed to catch, at compile time, circular or otherwise malformed initializations. Thus, both:

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


and:

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


result in compile-time errors. Accesses by methods are not checked in this way, so:

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);
    }
}


produces the output:

0


because the variable initializer for i uses the class method peek to access the value of the variable j before j has been initialized by its variable initializer, at which point it still has its default value (§4.12.5).

A more elaborate example is:

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;
}



8.4. Method Declarations

A method declares executable code that can be invoked, passing a fixed number of values as arguments.

MethodDeclaration:
{MethodModifier} MethodHeader MethodBody
MethodHeader:
Result MethodDeclarator [Throws]
TypeParameters {Annotation} Result MethodDeclarator [Throws]
MethodDeclarator:
Identifier ( [ReceiverParameter ,] [FormalParameterList] ) [Dims]
ReceiverParameter:
{Annotation} UnannType [Identifier .] this

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

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

The FormalParameterList clause is described in §8.4.1, the MethodModifier clause in §8.4.3, the TypeParameters clause in §8.4.4, the Result clause in §8.4.5, the Throws clause in §8.4.6, and the MethodBody in §8.4.7.

The Identifier in a MethodDeclarator may be used in a name to refer to the method (§6.5.7.1, §15.12).

The scope and shadowing of a method declaration is specified in §6.3 and §6.4.1.

The receiver parameter is an optional syntactic device for an instance method or an inner class's constructor. For an instance method, the receiver parameter represents the object for which the method is invoked. For an inner class's constructor, the receiver parameter represents the immediately enclosing instance of the newly constructed object. In both cases, the receiver parameter exists solely to allow the type of the represented object to be denoted in source code, so that the type may be annotated (§9.7.4). The receiver parameter is not a formal parameter; more precisely, it is not a declaration of any kind of variable (§4.12.3), it is never bound to any value passed as an argument in a method invocation expression or class instance creation expression, and it has no effect whatsoever at run time.

A receiver parameter may appear either in the MethodDeclarator of an instance method or in the ConstructorDeclarator of a constructor of an inner class where the inner class is not declared in a static context (§8.1.3). If a receiver parameter appears in any other kind of method or constructor, then a compile-time error occurs.

The type and name of a receiver parameter are constrained as follows:

In an instance method, the type of the receiver parameter must be the class or interface in which the method is declared, and the name of the receiver parameter must be this; otherwise, a compile-time error occurs.

In an inner class's constructor, the type of the receiver parameter must be the class or interface which is the immediately enclosing type declaration of the inner class, and the name of the receiver parameter must be Identifier . this where Identifier is the simple name of the class or interface which is the immediately enclosing type declaration of the inner class; otherwise, a compile-time error occurs.

It is a compile-time error for the body of a class declaration to declare as members two methods with override-equivalent signatures (§8.4.2).

The declaration of a method that returns an array is allowed to place some or all of the bracket pairs that denote the array type after the formal parameter list. This syntax is supported for compatibility with early versions of the Java programming language. It is very strongly recommended that this syntax is not used in new code.

8.4.1. Formal Parameters

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.

FormalParameterList:
FormalParameter {, FormalParameter}
FormalParameter:
{VariableModifier} UnannType VariableDeclaratorId
VariableArityParameter
VariableArityParameter:
{VariableModifier} UnannType {Annotation} ... Identifier
VariableModifier:
Annotation
final

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

VariableDeclaratorId:
Identifier [Dims]
_
Dims:
{Annotation} [ ] {{Annotation} [ ]}

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 of a method is a variable arity parameter, the method is a variable arity method. Otherwise, it is a fixed arity method.

The rules concerning annotation modifiers for a formal parameter declaration and for 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.

References to a formal parameter from a nested class or interface, or a lambda expression, are restricted, as specified in §6.5.6.1.

Every declaration of a formal parameter of a method or constructor must include an Identifier, otherwise a compile-time error occurs.

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 formal parameter is not a variable arity parameter, then the declared type is denoted by UnannType if no bracket pairs appear in UnannType and VariableDeclaratorId, and specified by §10.2 otherwise.

If the formal parameter is a variable arity parameter, then the declared type is an array type specified by §10.2.

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).

Here are some examples of receiver parameters in instance methods and inner classes' constructors:


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.
        }
    }
}



B's constructor and instance method show that the type of the receiver parameter may be denoted with a qualified TypeName like any other type; but that the name of the receiver parameter in an inner class's constructor must use the simple name of the enclosing class.

8.4.2. Method Signature

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 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:

m2 has the same signature as m1, or

the signature of m1 is the same as the erasure (§4.6) of the signature of m2.

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.

Example 8.4.2-1. Override-Equivalent Signatures

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


This program causes a compile-time error because it declares two move methods with the same (and hence, override-equivalent) signature. This is an error even though one of the declarations is abstract.




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.

Consider the example:


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



Now, assume this code was written before the introduction of generics, and now the author of class CollectionConverter decides to generify the code, thus:


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



Without special dispensation, Overrider.toList would no longer override CollectionConverter.toList. Instead, the code would be illegal. This would significantly inhibit the use of generics, since library writers would hesitate to migrate existing code.

8.4.3. Method Modifiers
MethodModifier:
(one of)
Annotation public protected private
abstract static final synchronized native strictfp

The rules concerning annotation modifiers for 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.

8.4.3.1. abstract Methods

An abstract method declaration introduces the method as a member, providing its signature (§8.4.2), result (§8.4.5), and throws clause if any (§8.4.6), but does not provide an implementation (§8.4.7). A method that is not abstract may be referred to as a concrete method.

The declaration of an abstract method m must appear directly within an abstract class (call it A) unless it occurs within an enum declaration (§8.9); otherwise, a compile-time error occurs.

Every subclass of A that is not abstract (§8.1.1.1) must provide an implementation for m, or a compile-time error occurs.

An abstract class can override an abstract method by providing another abstract method declaration.

This can provide a place to put a documentation comment, to refine the return type, or to declare that the set of checked exceptions that can be thrown by that method, when it is implemented by its subclasses, is to be more limited.

An instance method that is not abstract can be overridden by an abstract method.

Example 8.4.3.1-1. Abstract/Abstract Method Overriding

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;
}


The overriding declaration of method get in class InfiniteBuffer states that method get in any subclass of InfiniteBuffer never throws a BufferEmpty exception, putatively because it generates the data in the buffer, and thus can never run out of data.




Example 8.4.3.1-2. Abstract/Non-Abstract Overriding

We can declare an abstract class Point that requires its subclasses to implement toString if they are to be complete, instantiable classes:

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


This abstract declaration of toString overrides the non-abstract toString method of the class Object. (Object is the implicit direct superclass of class Point.) Adding the code:

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


results in a compile-time error because the invocation super.toString() refers to method toString in class Point, which is abstract and therefore cannot be invoked. Method toString of class Object can be made available to class ColoredPoint only if class Point explicitly makes it available through some other method, as in:

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
    }
}



8.4.3.2. static Methods

A method that is declared static is called a class method.

A class method is always invoked without reference to a particular object. The declaration of a class method introduces a static context (§8.1.3), which limits the use of constructs that refer to the current object. Notably, the keywords this and super are prohibited in a static context (§15.8.3, §15.11.2), as are unqualified references to instance variables, instance methods, and type parameters of lexically enclosing declarations (§6.5.5.1, §6.5.6.1, §15.12.3).

A method that is not declared static is called an instance method, and sometimes called a non-static method.

An instance method is always invoked with respect to an object, which becomes the current object to which the keywords this and super refer during execution of the method body.

References to an instance method from a static context or a nested class or interface are restricted, as specified in §15.12.3.

8.4.3.3. final Methods

A method can be declared final to prevent subclasses from overriding or hiding it.

It is a compile-time error to attempt to override or hide a final method.

A private method and all methods declared immediately within a final class (§8.1.1.2) behave as if they are final, since it is impossible to override them.

At run time, a machine-code generator or optimizer can "inline" the body of a final method, replacing an invocation of the method with the code in its body. The inlining process must preserve the semantics of the method invocation. In particular, if the target of an instance method invocation is null, then a NullPointerException must be thrown even if the method is inlined. A Java compiler must ensure that the exception will be thrown at the correct point, so that the actual arguments to the method will be seen to have been evaluated in the correct order prior to the method invocation.

Consider the example:

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);
        }
    }
}


Inlining the method move of class Point in method main would transform the for loop to the form:


    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;
    }



The loop might then be subject to further optimizations.

Such inlining cannot be done at compile time unless it can be guaranteed that Test and Point will always be recompiled together, so that whenever Point - and specifically its move method - changes, the code for Test.main will also be updated.

8.4.3.4. native Methods

A method that is native is implemented in platform-dependent code, typically written in another programming language such as C. The body of a native method is given as a semicolon only, indicating that the implementation is omitted, instead of a block (§8.4.7).

For example, the class RandomAccessFile of the package java.io might declare the following native methods:


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;
}


8.4.3.5. strictfp Methods

The strictfp modifier on a method declaration is obsolete and should not be used in new code. Its presence or absence has has no effect at run time.

8.4.3.6. synchronized Methods

A synchronized method acquires a monitor (§17.1) before it executes.

For a class (static) method, the monitor associated with the Class object for the method's class is used.

For an instance method, the monitor associated with this (the object for which the method was invoked) is used.

Example 8.4.3.6-1. synchronized Monitors

These are the same monitors that can be used by the synchronized statement (§14.19).

Thus, the code:

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


has exactly the same effect as:

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




Example 8.4.3.6-2. synchronized Methods

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;
    }
}


This program defines a class which is designed for concurrent use. Each instance of the class Box has an instance variable boxContents that can hold a reference to any object. You can put an object in a Box by invoking put, which returns false if the box is already full. You can get something out of a Box by invoking get, which returns a null reference if the box is empty.

If put and get were not synchronized, and two threads were executing methods for the same instance of Box at the same time, then the code could misbehave. It might, for example, lose track of an object because two invocations to put occurred at the same time.



8.4.4. Generic Methods

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 and §6.4.1.

References to a method's type parameter from a nested class or interface are restricted, as specified in §6.5.5.1.

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

M and N have same number of type parameters (possibly zero).

Where A1, ..., An are the type parameters of M and B1, ..., Bn are the type parameters of N, let θ=[B1:=A1, ..., Bn:=An]. Then, for all i (1 ≤ i ≤ n), the bound of Ai is the same type as θ applied to the bound of Bi.

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.

8.4.5. Method Result

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.

Result:
UnannType
void

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:

If R1 is void then R2 is void.

If R1 is a primitive type then R2 is identical to R1.

If R1 is a reference type then one of the following is true:

R1, adapted to the type parameters of d2 (§8.4.4), is a subtype of R2.

R1 can be converted to a subtype of R2 by unchecked conversion (§5.1.9).

d1 does not have the same signature as d2 (§8.4.2), and R1 = |R2|.

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.

8.4.6. Method Throws

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).

Throws:
throws ExceptionTypeList
ExceptionTypeList:
ExceptionType {, ExceptionType}
ExceptionType:
ClassType
TypeVariable

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.

Example 8.4.6-1. Type Variables as Thrown Exception Types

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 */ }
    }
}




8.4.7. Method Body

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

MethodBody:
Block
;

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:

It is a compile-time error if a method declaration is either abstract or native and has a block for its body.

It is a compile-time error if a method declaration is neither abstract nor native and has a semicolon for its body.

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.

It is possible for a method to have a return type and yet contain no return statements. Here is one example:

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

8.4.8. Inheritance, Overriding, and Hiding

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

m is a member of D.

m is public, protected, or declared with package access in the same package as C.

No method declared in C has a signature that is a subsignature (§8.4.2) of the signature of m as a member of D.

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

m is a member of the direct superclass type or a direct superinterface type of C, known in either case as D.

m is public, protected, or declared with package access in the same package as C.

No method declared in C has a signature that is a subsignature (§8.4.2) of the signature of m as a member of D.

No concrete method inherited by C from its direct superclass type has a signature that is a subsignature of the signature of m as a member of D.

There exists no method m' that is a member of the direct superclass type or a direct superinterface type of C, D' (m distinct from m', D distinct from D'), such that m' overrides from the class or interface of D' the declaration of the method m (§8.4.8.1, §9.4.1.1).

Inheritance for interfaces is defined in §9.1.3.

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

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.

Example 8.4.8-1. Inheritance

interface I1 {
    int foo();
}

interface I2 {
    int foo();
}

abstract class Test implements I1, I2 {}


Here, the abstract class Test inherits the abstract method foo from interface I1 and also the abstract method foo from interface I2. The key question in determining the inheritance of foo from I1 is: does the method foo in I2 override "from I2" (§9.4.1.1) the method foo in I1? No, because I1 and I2 are not subinterfaces of each other. Thus, from the viewpoint of class Test, the inheritance of foo from I1 is unfettered; similarly for the inheritance of foo from I2. Per §8.4.8.4, class Test can inherit both foo methods; obviously it must be declared abstract, or else override both abstract foo methods with a concrete method.




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.

8.4.8.1. Overriding (by Instance Methods)

An instance method mC declared in or inherited by class C, overrides from C another method mA declared in class A, iff all of the following are true:

C is a subclass of A.

C does not inherit mA.

The signature of mC is a subsignature (§8.4.2) of the signature of mA as a member of the supertype of C that names A.

One of the following is true:

mA is public.

mA is protected.

mA is declared with package access in the same package as C, and either C declares mC or mA is a member of the direct superclass type of C.

mA is declared with package access and mC overrides mA from some superclass of C.

mA is declared with package access and mC overrides a method m' from C (m' distinct from mC and mA), such that m' overrides mA from some superclass of C.

If mC is non-abstract and overrides from C an abstract method mA, then mC is said to implement mA from C.

It is a compile-time error if the overridden method, mA, is a static method.

In this respect, overriding of methods differs from hiding of fields (§8.3), for it is permissible for an instance variable to hide a static variable.

An instance method mC declared in or inherited by class C, overrides from C another method mI declared in interface I, iff all of the following are true:

I is a superinterface of C.

mI is not static.

C does not inherit mI.

The signature of mC is a subsignature (§8.4.2) of the signature of mI as a member of the supertype of C that names I.

mI is public.

The signature of an overriding method may differ from the overridden one if a formal parameter in one of the methods has a raw type, while the corresponding parameter in the other has a parameterized type. This accommodates migration of pre-existing code to take advantage of generics.

The notion of overriding includes methods that override another from some subclass of their declaring class. This can happen in two ways:

A concrete method in a generic superclass can, under certain parameterizations, have the same signature as an abstract method in that class. In this case, the concrete method is inherited and the abstract method is not (as described above). The inherited method should then be considered to override its abstract peer from C. (This scenario is complicated by package access: if C is in a different package, then mA would not have been inherited anyway, and should not be considered overridden.)

A method inherited from a class can override a superinterface method. (Happily, package access is not a concern here.)

An overridden method can be accessed by using a method invocation expression (§15.12) that contains the keyword super. A qualified name or a cast to a superclass type is not effective in attempting to access an overridden method.

In this respect, overriding of methods differs from hiding of fields.

The presence or absence of the strictfp modifier has absolutely no effect on the rules for overriding methods and implementing abstract methods. For example, it is permitted for a method that is not strictfp to override a strictfp method, and it is permitted for a strictfp method to override a method that is not strictfp.

Example 8.4.8.1-1. Overriding

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;
    }
}


Here, the class SlowPoint overrides the declarations of method move of class Point with its own move method, which limits the distance that the point can move on each invocation of the method. When the move method is invoked for an instance of class SlowPoint, the overriding definition in class SlowPoint will always be called, even if the reference to the SlowPoint object is taken from a variable whose type is Point.




Example 8.4.8.1-2. Overriding

Overriding makes it easy for subclasses to extend the behavior of an existing class, as shown in this example:

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

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");
    }
}



This program produces the output:

lbo
print
lbo


The class BufferOutput implements a very simple buffered version of an OutputStream, flushing the output when the buffer is full or flush is invoked. The subclass LineBufferOutput declares only a constructor and a single method putchar, which overrides the method putchar of BufferOutput. It inherits the methods putstr and flush from class BufferOutput.

In the putchar method of a LineBufferOutput object, if the character argument is a newline, then it invokes the flush method. The critical point about overriding in this example is that the method putstr, which is declared in class BufferOutput, invokes the putchar method defined by the current object this, which is not necessarily the putchar method declared in class BufferOutput.

Thus, when putstr is invoked in main using the LineBufferOutput object lbo, the invocation of putchar in the body of the putstr method is an invocation of the putchar of the object lbo, the overriding declaration of putchar that checks for a newline. This allows a subclass of BufferOutput to change the behavior of the putstr method without redefining it.

Documentation for a class such as BufferOutput, which is designed to be extended, should clearly indicate what is the contract between the class and its subclasses, and should clearly indicate that subclasses may override the putchar method in this way. The implementor of the BufferOutput class would not, therefore, want to change the implementation of putstr in a future implementation of BufferOutput not to use the method putchar, because this would break the pre-existing contract with subclasses. See the discussion of binary compatibility in §13 (Binary Compatibility), especially §13.2.



8.4.8.2. Hiding (by Class Methods)

If a class C declares or inherits a static method m, then m is said to hide any method m' declared in a class or interface A for which all of the following are true:

A is a superclass or superinterface of C.

If A is an interface, m' is an instance method.

m' is accessible to C (§6.6).

The signature of m is a subsignature (§8.4.2) of the signature of m' as a member of the supertype of C that names A.

It is a compile-time error if a static method hides an instance method.

In this respect, hiding of methods differs from hiding of fields (§8.3), for it is permissible for a static variable to hide an instance variable. Hiding is also distinct from shadowing (§6.4.1) and obscuring (§6.4.2).

A hidden method can be accessed by using a qualified name or by using a method invocation expression (§15.12) that contains the keyword super or a cast to a superclass type.

In this respect, hiding of methods is similar to hiding of fields.

Example 8.4.8.2-1. Invocation of Hidden Class Methods

A class (static) method that is hidden can be invoked by using a reference whose type is the type of the class that actually contains the declaration of the method. In this respect, hiding of static methods is different from overriding of instance methods. The example:

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());
    }
}


produces the output:

Goodnight, Dick


because the invocation of greeting uses the type of s, namely Super, to figure out, at compile time, which class method to invoke, whereas the invocation of name uses the class of s, namely Sub, to figure out, at run time, which instance method to invoke.



8.4.8.3. Requirements in Overriding and Hiding

If a method declaration d1 with return type R1 overrides or hides the declaration of another method d2 with return type R2, then d1 must be return-type-substitutable (§8.4.5) for d2, or a compile-time error occurs.

This rule allows for covariant return types - refining the return type of a method when overriding it.

If R1 is not a subtype of R2, then a compile-time unchecked warning occurs, unless suppressed by @SuppressWarnings (§9.6.4.5).

A method that overrides or hides another method, including methods that implement abstract methods defined in interfaces, may not be declared to throw more checked exceptions than the overridden or hidden method.

In this respect, overriding of methods differs from hiding of fields (§8.3), for it is permissible for a field to hide a field of another type.

More precisely, suppose that B is a class or interface, and A is a superclass or superinterface of B, and a method declaration m2 in B overrides or hides a method declaration m1 in A. Then:

If m2 has a throws clause that mentions any checked exception types, then m1 must have a throws clause, or a compile-time error occurs.

For every checked exception type listed in the throws clause of m2, that same exception class or one of its supertypes must occur in the erasure (§4.6) of the throws clause of m1; otherwise, a compile-time error occurs.

If the unerased throws clause of m1 does not contain a supertype of each exception type in the throws clause of m2 (adapted, if necessary, to the type parameters of m1), then a compile-time unchecked warning occurs, unless suppressed by @SuppressWarnings (§9.6.4.5).

It is a compile-time error if a class or interface C has a member method m1 and there exists a method m2 declared in C or a superclass or superinterface of C, A, such that all of the following are true:

m1 and m2 have the same name.

m2 is accessible (§6.6) from C.

The signature of m1 is not a subsignature (§8.4.2) of the signature of m2 as a member of the supertype of C that names A.

The declared signature of m1 or some method m1 overrides (directly or indirectly) has the same erasure as the declared signature of m2 or some method m2 overrides (directly or indirectly).

These restrictions are necessary because generics are implemented via erasure. The rule above implies that methods declared in the same class with the same name must have different erasures. It also implies that a class or interface cannot implement or extend two distinct parameterizations of the same generic interface.

The access modifier of an overriding or hiding method must provide at least as much access as the overridden or hidden method, as follows:

If the overridden or hidden method is public, then the overriding or hiding method must be public; otherwise, a compile-time error occurs.

If the overridden or hidden method is protected, then the overriding or hiding method must be protected or public; otherwise, a compile-time error occurs.

If the overridden or hidden method has package access, then the overriding or hiding method must not be private; otherwise, a compile-time error occurs.

Note that a private method cannot be overridden or hidden in the technical sense of those terms. This means that a subclass can declare a method with the same signature as a private method in one of its superclasses, and there is no requirement that the return type or throws clause of such a method bear any relationship to those of the private method in the superclass.

Example 8.4.8.3-1. Covariant Return Types

The following declarations are legal in the Java programming language from Java SE 5.0 onwards:

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


The relaxed rule for overriding also allows one to relax the conditions on abstract classes implementing interfaces.




Example 8.4.8.3-2. Unchecked Warning from Return Type

Consider:


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



and assume that someone subclasses StringSorter:


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



Now, at some point the author of StringSorter decides to generify the code:


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



An unchecked warning would be given when compiling Overrider against the new definition of StringSorter because the return type of Overrider.toList is List, which is not a subtype of the return type of the overridden method, List<String>.




Example 8.4.8.3-3. Incorrect Overriding because of throws

This program uses the usual and conventional form for declaring a new exception type, in its declaration of the class BadPointException:

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;
    }
}


The program results in a compile-time error, because the override of method move in class CheckedPoint declares that it will throw a checked exception that the move in class Point has not declared. If this were not considered an error, an invoker of the method move on a reference of type Point could find the contract between it and Point broken if this exception were thrown.

Removing the throws clause does not help:

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


A different compile-time error now occurs, because the body of the method move cannot throw a checked exception, namely BadPointException, that does not appear in the throws clause for move.




Example 8.4.8.3-4. Erasure Affects Overriding

A class cannot have two member methods with the same name and type erasure:


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



This is illegal since D.id(Object) is a member of D, C<String>.id(String) is declared in a supertype of D, and:

The two methods have the same name, id

C<String>.id(String) is accessible to D

The signature of D.id(Object) is not a subsignature of that of C<String>.id(String)

The two methods have the same erasure

Two different methods of a class may not override methods with the same erasure:


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) {...}
}



This is also illegal, since D.id(String) is a member of D, D.id(Integer) is declared in D, and:

The two methods have the same name, id

D.id(Integer) is accessible to D

The two methods have different signatures (and neither is a subsignature of the other)

D.id(String) overrides C<String>.id(String) and D.id(Integer) overrides I.id(Integer) yet the two overridden methods have the same erasure



8.4.8.4. Inheriting Methods with Override-Equivalent Signatures

It is possible for a class to inherit multiple methods with override-equivalent signatures (§8.4.2).

It is a compile-time error if a class C inherits a concrete method whose signature is override-equivalent with another method inherited by C.

It is a compile-time error if a class C inherits a default method whose signature is override-equivalent with another method inherited by C, unless there exists an abstract method declared in a superclass of C and inherited by C that is override-equivalent with the two methods.

This exception to the strict default-abstract and default-default conflict rules is made when an abstract method is declared in a superclass: the assertion of abstract-ness coming from the superclass hierarchy essentially trumps the default method, making the default method act as if it were abstract. However, the abstract method from a class does not override the default method(s), because interfaces are still allowed to refine the signature of the abstract method coming from the class hierarchy.

Note that the exception does not apply if all override-equivalent abstract methods inherited by C were declared in interfaces.

Otherwise, the set of override-equivalent methods consists of at least one abstract method and zero or more default methods; then the class is necessarily an abstract class and is considered to inherit all the methods.

One of the inherited methods must be return-type-substitutable for every other inherited method; otherwise, a compile-time error occurs. (The throws clauses do not cause errors in this case.)

There might be several paths by which the same method declaration is inherited from an interface. This fact causes no difficulty and never, of itself, results in a compile-time error.

Example 8.4.8.4-1. Inheritance of override-equivalent methods

The first compile-time error above, regarding a class C that inherits a concrete method, can happen if a superclass of C is generic, and the superclass has two methods that were distinct in the generic declaration but have the same signature in the parameterization (§4.5) used by C. For example:


class A<T> {
    void m(String s) {} // 1
    void m(T t) {} // 2
}
class C extends A<String> {}



C inherits two methods from its direct superclass type A<String>: the method m(String) marked at 1, and (due to C's parameterization of A) the method m(String) marked at 2. These methods have the same signature, so are override-equivalent with each other.



8.4.9. Overloading

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).

Example 8.4.9-1. Overloading

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+")"; }
}


Here, the class Point has two members that are methods with the same name, move. The overloaded move method of class Point chosen for any particular method invocation is determined at compile time by the overloading resolution procedure given in §15.12.

In total, the members of the class Point are the float instance variables x and y declared in Point, the two declared move methods, the declared toString method, and the members that Point inherits from its implicit direct superclass Object (§4.3.2), such as the method hashCode. Note that Point does not inherit the toString method of class Object because that method is overridden by the declaration of the toString method in class Point.




Example 8.4.9-2. Overloading, Overriding, and Hiding

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; }
}


Here, the class RealPoint hides the declarations of the int instance variables x and y of class Point with its own float instance variables x and y, and overrides the method move of class Point with its own move method. It also overloads the name move with another method with a different signature (§8.4.2).

In this example, the members of the class RealPoint include the instance variable color inherited from the class Point, the float instance variables x and y declared in RealPoint, and the two move methods declared in RealPoint.

Which of these overloaded move methods of class RealPoint will be chosen for any particular method invocation will be determined at compile time by the overloading resolution procedure described in §15.12.

This following program is an extended variation of the preceding program:

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; }
}


Here, the class Point provides methods getX and getY that return the values of its fields x and y; the class RealPoint then overrides these methods by declaring methods with the same signature. The result is two errors at compile time, one for each method, because the return types do not match; the methods in class Point return values of type int, but the wanna-be overriding methods in class RealPoint return values of type float.

This program corrects the errors of the preceding program:

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); }
}


Here, the overriding methods getX and getY in class RealPoint have the same return types as the methods of class Point that they override, so this code can be successfully compiled.

Consider, then, this test program:

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 + ")");
    }
}


The output from this program is:

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


The first line of output illustrates the fact that an instance of RealPoint actually contains the two integer fields declared in class Point; it is just that their names are hidden from code that occurs within the declaration of class RealPoint (and those of any subclasses it might have). When a reference to an instance of class RealPoint in a variable of type Point is used to access the field x, the integer field x declared in class Point is accessed. The fact that its value is zero indicates that the method invocation p.move(1, -1) did not invoke the method move of class Point; instead, it invoked the overriding method move of class RealPoint.

The second line of output shows that the field access rp.x refers to the field x declared in class RealPoint. This field is of type float, and this second line of output accordingly displays floating-point values. Incidentally, this also illustrates the fact that the method name show is overloaded; the types of the arguments in the method invocation dictate which of the two definitions will be invoked.

The last two lines of output show that the method invocations p.getX() and rp.getX() each invoke the getX method declared in class RealPoint. Indeed, there is no way to invoke the getX method of class Point for an instance of class RealPoint from outside the body of RealPoint, no matter what the type of the variable we may use to hold the reference to the object. Thus, we see that fields and methods behave differently: hiding is different from overriding.



8.5. Member Class and Interface Declarations

A member class is a class whose declaration is directly enclosed in the body of another class or interface declaration (§8.1.7, §9.1.5).

A member interface is an interface whose declaration is directly enclosed in the body of another class or interface declaration.

A member class may be a normal class (§8.1), an enum class (§8.9), or a record class (§8.10).

A member interface may be a normal interface (§9.1) or an annotation interface (§9.6).

The accessibility of a member class or interface declaration in the body of a class declaration is specified by its access modifier, or by §6.6 if lacking an access modifier.

The rules for modifiers of a member class declaration in the body of a class declaration are specified in §8.1.1.

The rules for modifiers of a member interface declaration in the body of a class declaration are specified in §9.1.1.

The scope and shadowing of a member class or interface is specified in §6.3 and §6.4.1.

If a class declares a member class or interface with a certain name, then the declaration of the member class or interface is said to hide any and all accessible declarations of member classes and interfaces with the same name in superclasses and superinterfaces of the class.

In this respect, hiding of member class and interfaces is similar to hiding of fields (§8.3).

A class inherits from its direct superclass and direct superinterfaces all the non-private member classes and interfaces of the superclass and superinterfaces that are both accessible to code in the class and not hidden by a declaration in the class.

It is possible for a class to inherit more than one member class or interface with the same name, either from its superclass and superinterfaces or from its superinterfaces alone. Such a situation does not in itself cause a compile-time error. However, any attempt within the body of the class to refer to any such member class or interface by its simple name will result in a compile-time error, because the reference is ambiguous.

There might be several paths by which the same member class or interface declaration is inherited from an interface. In such a situation, the member class or interface is considered to be inherited only once, and it may be referred to by its simple name without ambiguity.

8.6. Instance Initializers

An instance initializer declared in a class is executed when an instance of the class is created (§12.5, §15.9, §8.8.7.1).

InstanceInitializer:
Block

It is a compile-time error if an instance initializer cannot complete normally (§14.22).

It is a compile-time error if a return statement (§14.17) appears anywhere within an instance initializer.

An instance initializer is permitted to refer to the current object using the keyword this (§15.8.3) or the keyword super (§15.11.2, §15.12), and to use any type variables in scope.

Restrictions on how an instance initializer may refer to instance variables, even when the instance variables are in scope, are specified in §8.3.3.

Exception checking for an instance initializer is specified in §11.2.3.

8.7. Static Initializers

A static initializer declared in a class is executed when the class is initialized (§12.4.2). Together with any field initializers for class variables (§8.3.2), static initializers may be used to initialize the class variables of the class.

StaticInitializer:
static Block

It is a compile-time error if a static initializer cannot complete normally (§14.22).

It is a compile-time error if a return statement (§14.17) appears anywhere within a static initializer.

A static initializer introduces a static context (§8.1.3), which limits the use of constructs that refer to the current object. Notably, the keywords this and super are prohibited in a static context (§15.8.3, §15.11.2), as are unqualified references to instance variables, instance methods, and type parameters of lexically enclosing declarations (§6.5.5.1, §6.5.6.1, §15.12.3).

Restrictions on how a static initializer may refer to class variables, even when the class variables are in scope, are specified in §8.3.3.

Exception checking for a static initializer is specified in §11.2.3.

8.8. Constructor Declarations

A constructor is used in the creation of an object that is an instance of a class (§12.5, §15.9).

ConstructorDeclaration:
{ConstructorModifier} ConstructorDeclarator [Throws] ConstructorBody
ConstructorDeclarator:
[TypeParameters] SimpleTypeName ( [ReceiverParameter ,] [FormalParameterList] )
SimpleTypeName:
TypeIdentifier

The rules in this section apply to constructors in all class declarations, including enum declarations and record declarations. However, special rules apply to enum declarations with regard to constructor modifiers, constructor bodies, and default constructors; these rules are stated in §8.9.2. Special rules also apply to record declarations with regard to constructors, as stated in §8.10.4.

The SimpleTypeName in the ConstructorDeclarator must be the simple name of the class that contains the constructor declaration, or a compile-time error occurs.

In all other respects, a constructor declaration looks just like a method declaration that has no result (§8.4.5).

Constructor declarations are not members. They are never inherited and therefore are not subject to hiding or overriding.

Constructors are invoked by class instance creation expressions (§15.9), by the conversions and concatenations caused by the string concatenation operator + (§15.18.1), and by explicit constructor invocations from other constructors (§8.8.7). Access to constructors is governed by access modifiers (§6.6), so it is possible to prevent class instantiation by declaring an inaccessible constructor (§8.8.10).

Constructors are never invoked by method invocation expressions (§15.12).

Example 8.8-1. Constructor Declarations

class Point {
    int x, y;
    Point(int x, int y) { this.x = x; this.y = y; }
}



8.8.1. Formal Parameters

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

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

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:

In a class instance creation expression for a non-private inner member class, §15.9.2 specifies the immediately enclosing instance of the member class. The member class may have been emitted by a compiler which is different than the compiler of the class instance creation expression. Therefore, there must be a standard way for the compiler of the creation expression to pass a reference (representing the immediately enclosing instance) to the member class's constructor. Consequently, the Java programming language deems in this section that a non-private inner member class's constructor implicitly declares an initial parameter for the immediately enclosing instance. §15.9.3 specifies that the instance is passed to the constructor.

In a class instance creation expression for an inner local class or an anonymous class (not in a static context), §15.9.2 specifies the immediately enclosing instance of the local/anonymous class. The local/anonymous class is necessarily emitted by the same compiler as the class instance creation expression. That compiler can represent the immediately enclosing instance how ever it wishes. There is no need for the Java programming language to implicitly declare a parameter in the local/anonymous class's constructor.

In a class instance creation expression for an anonymous class, and where the anonymous class's superclass is an inner class (not in a static context), §15.9.2 specifies the anonymous class's immediately enclosing instance with respect to the superclass. This instance must be transmitted from the anonymous class to its superclass, where it will serve as the immediately enclosing instance. Since the superclass may have been emitted by a compiler which is different than the compiler of the class instance creation expression, it is necessary to transmit the instance in a standard way, by passing it as the first argument to the superclass's constructor. Note that the anonymous class itself is necessarily emitted by the same compiler as the class instance creation expression, so it would be possible for the compiler to transmit the immediately enclosing instance with respect to the superclass to the anonymous class how ever it wishes, before the anonymous class passes the instance to the superclass's constructor. However, for consistency, the Java programming language deems in §15.9.5.1 that, in some circumstances, an anonymous class's constructor implicitly declares an initial parameter for the immediately enclosing instance with respect to the superclass.

The fact that a non-private inner member class may be accessed by a different compiler than compiled it, whereas an inner local class or an 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 an inner local class or an anonymous class is not (§13.1).

8.8.2. Constructor Signature

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.

8.8.3. Constructor Modifiers
ConstructorModifier:
(one of)
Annotation public protected private

The rules concerning annotation modifiers for 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 not inherited, so there is no need to declare it final.

An abstract constructor could never be implemented.

A constructor is always invoked with respect to an object, so it makes no sense for a constructor to be static.

There is no practical need for a constructor to be synchronized, because it would lock the object under construction, which is normally not made available to other threads until all constructors for the object have completed their work.

The lack of native constructors is an arbitrary language design choice that makes it easy for an implementation of the Java Virtual Machine to verify that superclass constructors are always properly invoked during object creation.

The inability to declare a constructor as strictfp (in contrast to a method (§8.4.3)) is an intentional language design choice that stemmed from the (now obsolete) ability to declare a class as strictfp.

8.8.4. Generic Constructors

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 §6.4.1.

References to a constructor's type parameter from a constructor invocation or a nested class or interface are restricted, as specified in §6.5.5.1.

8.8.5. Constructor Throws

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

8.8.6. The Type of a Constructor

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

8.8.7. Constructor Body

A constructor body is a block of code that is executed as part of the process of creating a new instance of a class (§12.5). A constructor body may contain an explicit invocation of another constructor of the same class or of the direct superclass (§8.8.7.1).

ConstructorBody:
{ [BlockStatements] ConstructorInvocation [BlockStatements] }
{ [BlockStatements] }

If a constructor body contains an explicit constructor invocation, the BlockStatements preceding the constructor invocation are called the prologue of the constructor body. The prologue of a constructor body may be empty. The BlockStatements in a constructor with no explicit constructor invocation and the BlockStatements following a constructor invocation in a constructor body are called the epilogue. The epilogue of a constructor body may also be empty.

A construct (statement, local variable declaration statement, local class declaration, local interface declaration, or expression) occurs in the early construction context of a class C if it is contained in either the prologue of a constructor body of C, or it is nested in the constructor invocation (§8.8.7.1) of a constructor body of C.

If a constructor body does not contain an explicit constructor invocation and the constructor being declared is not part of the primordial class Object, then the constructor body (i) has an empty prologue, (ii) implicitly begins with a superclass constructor invocation "super();", an implicit invocation of the constructor of the direct superclass that takes no arguments, and (iii) the statements, if any, given in the constructor body are taken to form the epilogue of the constructor body.

Except for the possibility of explicit or implicit constructor invocations, and the prohibition on return statements (§14.17), the body of a constructor is like the body of a method (§8.4.7).

Note that a constructor body contains at most one constructor invocation. The grammar makes it impossible, for example, to place constructor invocations in different branches of an if statement.

Example 8.8.7-1. Constructor Bodies

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;
    }
}


Here, the first constructor of ColoredPoint invokes the second, providing an additional argument; the second constructor of ColoredPoint invokes the constructor of its superclass Point, passing along the coordinates.



8.8.7.1. Constructor Invocations
ConstructorInvocation:
[TypeArguments] this ( [ArgumentList] ) ;
[TypeArguments] super ( [ArgumentList] ) ;
ExpressionName . [TypeArguments] super ( [ArgumentList] ) ;
Primary . [TypeArguments] super ( [ArgumentList] ) ;

The following productions from §4.5.1 and §15.12 are shown here for convenience:

TypeArguments:
< TypeArgumentList >
ArgumentList:
Expression {, Expression}

Constructor invocations are divided into two kinds:

Alternate constructor invocations begin with the keyword this (possibly prefaced with explicit type arguments). They are used to invoke an alternate constructor of the same class.

Superclass constructor invocations begin with either the keyword super (possibly prefaced with explicit type arguments) or a Primary expression or an ExpressionName. They are used to invoke a constructor of the direct superclass. They are further divided:

Unqualified superclass constructor invocations begin with the keyword super (possibly prefaced with explicit type arguments).

Qualified superclass constructor invocations begin with a Primary expression or an ExpressionName. They allow a subclass constructor to explicitly specify the newly created object's immediately enclosing instance with respect to the direct superclass (§8.1.3). This may be necessary when the superclass is an inner class.

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

A constructor invocation introduces an early construction context (§8.8.7), which limits the use of constructs that refer to the current object. Notably, references to the current object using this and super are restricted in an early construction context (§15.8.3, §15.11.2), as are references to instance variables (§6.5.6.1) and instance methods (§6.5.7.1).

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

Let C be the class being instantiated, and let S be the direct superclass of C.

If a superclass constructor invocation is unqualified, then:

If S is an inner member class, but S is not a member of a class enclosing C, then a compile-time error occurs.

Otherwise, let O be the innermost enclosing class of C of which S is a member. C must be an inner class of O (§8.1.3), or a compile-time error occurs. If the superclass constructor invocation occurs in an early construction context of the class O, then a compile-time error occurs.

If S is an inner local class, and S does not occur in a static context, let O be the immediately enclosing class or interface declaration of S. C must be an inner class of O, or a compile-time error occurs.

If S is a inner local class whose declaration occurs in a static context, then let N be the nearest static method declaration, static field declaration, or static initializer that encloses the declaration of S. If N is not the nearest static method declaration, static field declaration, or static initializer that encloses the superclass constructor invocation, then a compile-time error occurs.

If a superclass constructor invocation is qualified, then:

If S is not an inner class, or if the declaration of S occurs in a static context, then a compile-time error occurs.

Otherwise, let p be the Primary expression or the ExpressionName immediately preceding ".super", and let O be the immediately enclosing class of S. It is a compile-time error if the type of p is not O or a subclass of O, or if the type of p is not accessible (§6.6).

The exception types that an explicit constructor invocation can throw are specified in §11.2.2.

Evaluation of an alternate constructor invocation proceeds by first evaluating the arguments to the constructor, left-to-right, as in an ordinary method invocation; and then invoking the constructor.

Let i be the instance being created by a superclass constructor invocation. The immediately enclosing instance of i with respect to S (if any) is determined as follows:

If S is not an inner class, or if the declaration of S occurs in a static context, then no immediately enclosing instance of i with respect to S exists.

Otherwise, if the superclass constructor invocation is unqualified, then S is necessarily an inner local class or an inner member class.

If S is an inner local class, let O be the immediately enclosing class or interface declaration of S.

If S is an inner member class, let O be the innermost enclosing class of C of which S is a member. If the superclass constructor invocation occurs in the early construction context of the class O, then a compile-time error occurs.

Let n be an integer (n ≥ 1) such that O is the n'th lexically enclosing class or interface declaration of C.

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

While it may be the case that S is a member of C due to inheritance, the zeroth lexically enclosing instance of this (that is, this itself) is never used as the immediately enclosing instance of i with respect to S.

Otherwise, if the superclass constructor invocation is qualified, then the immediately enclosing instance of i with respect to S is the object that is the value of the Primary expression or the ExpressionName.

If the superclass constructor invocation has an immediately enclosing instance then this instance is taken to be the first actual argument to the constructor invocation, and the subsequent actual arguments to the constructor invocation are taken to be the arguments in the argument list of superclass constructor invocation, if any, in the order that they appear. Otherwise the actual arguments to the constructor invocation are taken to be the arguments in the superclass constructor invocation, if any, in the order that they appear.

Evaluation of a superclass constructor invocation then proceeds as follows:

First, if the superclass constructor invocation is qualified, the Primary expression or the ExpressionName is evaluated. If it evaluates to null, a NullPointerException is raised and the superclass constructor invocation completes abruptly. If this evaluation completes abruptly, then the superclass constructor invocation completes abruptly for the same reason.

The actual arguments to the constructor invocation are evaluated, left-to-right, as in an ordinary method invocation; then the constructor is invoked.

Recall that the first actual argument to the constructor invocation may be an enclosing instance.

Finally, if the superclass constructor invocation completes normally, then all instance variable initializers of C and all instance initializers of C are executed. If an instance initializer or instance variable initializer I textually precedes another instance initializer or instance variable initializer J, then I is executed before J.

Execution of instance variable initializers and instance initializers is performed regardless of whether the superclass constructor invocation actually appears as an explicit constructor invocation or is provided implicitly. (An alternate constructor invocation does not perform this additional implicit execution.)

Example 8.8.7.1-1. Restrictions on Constructor Invocations

If the first constructor of ColoredPoint in the example from §8.8.7 were changed as follows:

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;
    }
}


then a compile-time error would occur, because the instance variable color cannot be used by a constructor invocation.




Example 8.8.7.1-2. Qualified Superclass Constructor Invocation

In the code below, ChildOfInner has no lexically enclosing class or interface declaration, so an instance of ChildOfInner has no enclosing instance. However, the superclass of ChildOfInner (Inner) has a lexically enclosing class declaration (Outer), and an instance of Inner must have an enclosing instance of Outer. The enclosing instance of Outer is set when an instance of Inner is created. Therefore, when we create an instance of ChildOfInner, which is implicitly an instance of Inner, we must provide the enclosing instance of Outer via a qualified superclass invocation in ChildOfInner's constructor. The instance of Outer is called the immediately enclosing instance of ChildOfInner with respect to Inner.

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


Perhaps surprisingly, the same instance of Outer may serve as the immediately enclosing instance of ChildOfInner with respect to Inner for multiple instances of ChildOfInner. These instances of ChildOfInner are implicitly linked to the same instance of Outer. The program below achieves this by passing an instance of Outer to the constructor of ChildOfInner, which uses the instance in a qualified superclass constructor invocation. The rules for a constructor invocation do not prohibit using formal parameters of the constructor that contains the invocation.

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());
    }
}



This program produces the output:

5
6


The effect is that manipulation of instance variables in the common instance of Outer is visible through references to different instances of ChildOfInner, even though such references are not aliases in the conventional sense.



8.8.8. Constructor Overloading

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).

8.8.9. Default Constructor

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 default constructor has the same access modifier as the class, unless the class lacks an access modifier, in which case the default constructor has package access (§6.6).

The default constructor has no formal parameters, except in a non-private inner member class, where the default constructor implicitly declares one formal parameter representing the immediately enclosing instance of the class (§8.8.1, §15.9.2, §15.9.3).

The default constructor has no throws clause.

If the class being declared is the primordial class Object, then the default constructor has an empty body. Otherwise, the default constructor simply invokes the superclass constructor with no arguments. This superclass constructor invocation must satisfy all the rules for an unqualified superclass constructor invocation (§8.8.7.1).

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.

Example 8.8.9-1. Default Constructors

The declaration:


public class Point {
    int x, y;
}



is equivalent to the declaration:


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



where the default constructor is public because the class Point is public.




Example 8.8.9-2. Accessibility of Constructors v. Classes

The rule that the default constructor of a class has the same accessibility as the class itself is simple and intuitive. Note, however, that this does not imply that the constructor is accessible whenever the class is accessible. Consider:

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


The default constructor for Inner is protected. However, the constructor is protected relative to Inner, while Inner is protected relative to Outer. So, Inner is accessible in SonOfOuter, since it is a subclass of Outer. Inner's constructor is not accessible in SonOfOuter, because the class SonOfOuter is not a subclass of Inner! Hence, even though Inner is accessible, its default constructor is not.



8.8.10. Preventing Instantiation of a Class

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).

Example 8.8.10-1. Preventing Instantiation via Constructor Accessibility


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



Here, the class ClassOnly cannot be instantiated, while in the following code:


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



the public class PackageOnly can be instantiated only within the package just, in which it is declared. This restriction would also apply if the constructor of PackageOnly was protected, although in that case, it would be possible for code in other packages to instantiate subclasses of PackageOnly.



8.9. Enum Classes

An enum declaration specifies a new enum class, a restricted kind of class that defines a small set of named class instances.

EnumDeclaration:
{ClassModifier} enum TypeIdentifier [ClassImplements] EnumBody

An enum declaration may specify a top level enum class (§7.6), a member enum class (§8.5, §9.5), or a local enum class (§14.3).

The TypeIdentifier in an enum declaration specifies the name of the enum class.

It is a compile-time error if an enum declaration has the modifier abstract, final, sealed, or non-sealed.

An enum class is either implicitly final or implicitly sealed, as follows:

An enum class is implicitly final if its declaration contains no enum constants that have a class body (§8.9.1).

An enum class E is implicitly sealed if its declaration contains at least one enum constant that has a class body. The permitted direct subclasses (§8.1.6) of E are the anonymous classes implicitly declared by the enum constants that have a class body.

A nested enum class is implicitly static. That is, every member enum class and local enum class is static. It is permitted for the declaration of a member enum class to redundantly specify the static modifier, but it is not permitted for the declaration of a local enum class (§14.3).

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

The direct superclass type of an enum class E is Enum<E> (§8.1.4).

An enum declaration does not have an extends clause, so it is not possible to explicitly declare a direct superclass type, even Enum<E>.

An enum class has no instances other than those defined by its enum constants. It is a compile-time error to attempt to explicitly instantiate an enum class (§15.9.1).

In addition to the compile-time error, three further mechanisms ensure that no instances of an enum class exist beyond those defined by its enum constants:

The final clone method in Enum ensures that enum constants can never be cloned.

Reflective instantiation of enum classes is prohibited.

Special treatment by the serialization mechanism ensures that duplicate instances are never created as a result of deserialization.

8.9.1. Enum Constants

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

EnumBody:
{ [EnumConstantList] [,] [EnumBodyDeclarations] }
EnumConstantList:
EnumConstant {, EnumConstant}
EnumConstant:
{EnumConstantModifier} Identifier [( [ArgumentList] )] [ClassBody]
EnumConstantModifier:
Annotation

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

ArgumentList:
Expression {, Expression}

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

The Identifier in an EnumConstant provides the name of an implicit field of the enum class (§8.9.3) that can be used to refer to the enum constant.

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 declares an anonymous class (§15.9.5) that (i) is a direct subclass of the immediately enclosing enum class (§8.1.4), and (ii) is final (§8.1.1.2). 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 class only if they override accessible methods in the enclosing enum class (§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.

8.9.2. Enum Body Declarations

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

EnumBodyDeclarations:
; {ClassBodyDeclaration}

The following productions from §8.1.7 are shown here for convenience:

ClassBodyDeclaration:
ClassMemberDeclaration
InstanceInitializer
StaticInitializer
ConstructorDeclaration
ClassMemberDeclaration:
FieldDeclaration
MethodDeclaration
ClassDeclaration
InterfaceDeclaration
;

Any constructor or member declarations in the body of an enum declaration apply to the enum class 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 (§8.8.7.1).

It is a compile-time error to refer to a static field of an enum class from a constructor, instance initializer, or instance variable initializer in the enum declaration of the class, 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 class by declaring String and int parameters in the default constructor of an enum class. 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 declaration knows how to instantiate the enum constants; other compilers can simply rely on the implicitly declared public static fields of the enum class (§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 class may never be finalized.

Example 8.9.2-1. Enum Body Declarations

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; }
}


Each enum constant arranges for a different value in the field value, passed in via a constructor. The field represents the value, in cents, of an American coin. Note that there are no restrictions on the parameters that may be declared by an enum class's constructor.




Example 8.9.2-2. Restriction On Enum Constant Self-Reference

Without the rule on static field access, apparently reasonable code would fail at run time due to the initialization circularity inherent in enum classes. (A circularity exists in any class with a "self-typed" static field.) Here is an example of the sort of code that would fail:

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

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

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



Static initialization of this enum would throw a NullPointerException because the static variable colorMap is uninitialized when the constructors for the enum constants run. The restriction above ensures that such code cannot be compiled. However, the code can easily be refactored to work properly:

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

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 refactored version is clearly correct, as static initialization occurs top to bottom.



8.9.3. Enum Members

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

Members declared in the body of the declaration of E.

Members inherited from Enum<E>.

For each enum constant c declared in the body of the declaration of E, E has an implicitly declared public static final field of type E that has the same name as c. The field has a variable initializer which instantiates E and passes any arguments of c to the constructor chosen for E. The field has the same annotations as c (if any).

These fields are implicitly declared in the same order as the corresponding enum constants, before any static fields explicitly declared in the body of the declaration of E.

An enum constant is said to be created when the corresponding implicitly declared field is initialized.

An implicitly declared method public static E[] values(), which returns an array containing the enum constants of E, in the same order as they appear in the body of the declaration of E.

An implicitly declared method public static E valueOf(String name), which returns the enum constant of E with the specified name.

It follows that the declaration of enum class 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>.

Example 8.9.3-1. Iterating Over Enum Constants With An Enhanced for Loop

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

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


This program produces the output:

WINTER
SPRING
SUMMER
FALL




Example 8.9.3-2. Switching Over Enum Constants

A switch statement (§14.11) is useful for simulating the addition of a method to an enum class from outside the class. This example "adds" a color method to the Coin class from §8.9.2, and prints a table of coins, their values, and their colors.

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));
    }
}


This program produces the output:

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




Example 8.9.3-3. Enum Constants with Class Bodies

Rather than using a switch statement to "add" behavior to an enum class from the outside, it is possible to use class bodies to attach behaviors to enum constants directly.

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));
    }
}



The program produces the output:

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


This pattern is much safer than using a switch statement because the pattern precludes the possibility of forgetting to add a behavior for a new constant (since the enum declaration would cause a compile-time error).




Example 8.9.3-4. Multiple Enum Classes

In the following program, a playing card class is built atop two simple enums.

import java.util.ArrayList;
import java.util.List;

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);
    }
}



The following program exercises the Card class. It takes two integer parameters on the command line, representing the number of hands to deal and the number of cards in each hand:

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

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;
    }
}



The program produces the output:

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]



8.10. Record Classes

A record declaration specifies a new record class, a restricted kind of class that defines a simple aggregate of values.

RecordDeclaration:
{ClassModifier} record TypeIdentifier [TypeParameters] RecordHeader [ClassImplements] RecordBody

A record declaration may specify a top level record class (§7.6), a member record class (§8.5, §9.5), or a local record class (§14.3).

The TypeIdentifier in a record declaration specifies the name of the record class.

It is a compile-time error if a record declaration has the modifier abstract, sealed, or non-sealed.

A record class is implicitly final. It is permitted for the declaration of a record class to redundantly specify the final modifier.

A nested record class is implicitly static. That is, every member record class and local record class is static. It is permitted for the declaration of a member record class to redundantly specify the static modifier, but it is not permitted for the declaration of a local record class (§14.3).

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

The direct superclass type of a record class is Record (§8.1.4).

A record declaration does not have an extends clause, so it is not possible to explicitly declare a direct superclass type, even Record.

The serialization mechanism treats instances of a record class differently than ordinary serializable or externalizable objects. In particular, a record object is deserialized using the canonical constructor (§8.10.4).

8.10.1. Record Components

The record components of a record class, if any, are specified in the header of a record declaration. Each record component consists of a type (optionally preceded by one or more annotations) and an identifier that specifies the name of the record component. A record component corresponds to two members of the record class: a private field declared implicitly, and a public accessor method declared explicitly or implicitly (§8.10.3).

If a record class has no record components, then an empty pair of parentheses appears in the header of the record declaration.

RecordHeader:
( [RecordComponentList] )
RecordComponentList:
RecordComponent {, RecordComponent}
RecordComponent:
{RecordComponentModifier} UnannType Identifier
VariableArityRecordComponent
VariableArityRecordComponent:
{RecordComponentModifier} UnannType {Annotation} ... Identifier
RecordComponentModifier:
Annotation

A record component may be a variable arity record component, indicated by an ellipsis following the type. At most one variable arity record component is permitted for a record class. It is a compile-time error if a variable arity record component appears anywhere in the list of record components except the last position.

The rules concerning annotation modifiers for a record component are specified in §9.7.4 and §9.7.5.

Annotations on a record component are available via reflection if their annotation interfaces are applicable in the record component context (§9.6.4.1). Independently, annotations on a record component are propagated to the declarations of members and constructors of the record class if their annotation interfaces are applicable in other contexts (§8.10.3, §8.10.4).

Every declaration of a record component must include an Identifier, otherwise a compile-time error occurs.

It is a compile-time error for a record declaration to have a record component with the name clone, finalize, getClass, hashCode, notify, notifyAll, toString, or wait.

These are the names of the no-args public and protected methods in Object. Disallowing them as the names of record components avoids confusion in a number of ways. First, every record class provides implementations of hashCode and toString that return representations of a record object as a whole; they cannot serve as accessor methods (§8.10.3) for record components called hashCode or toString, and there would be no way to access such record components from outside the record class. Similarly, some record classes may provide implementations of clone and (regrettably) finalize, so a record component called clone or finalize could not be accessed via an accessor method. Finally, the getClass, notify, notifyAll, and wait methods in Object are final, so record components with the same names could not have accessor methods. (The accessor methods would have the same signatures as the final methods, and would thus attempt, unsuccessfully, to override them.)

It is a compile-time error for a record declaration to have two record components with the same name.

The declared type of a record component depends on whether it is a variable arity record component:

If the record component is not a variable arity record component, then the declared type is denoted by UnannType.

If the record component is a variable arity record component, then the declared type is an array type specified by §10.2.

If the declared type of a variable arity record component has a non-reifiable element type (§4.7), then a compile-time unchecked warning occurs for the declaration of the variable arity record component, unless the canonical constructor (§8.10.4) is annotated with @SafeVarargs (§9.6.4.7) or the warning is suppressed by @SuppressWarnings (§9.6.4.5).

8.10.2. Record Body Declarations

The body of a record declaration may contain constructor and member declarations as well as static initializers.

RecordBody:
{ {RecordBodyDeclaration} }
RecordBodyDeclaration:
ClassBodyDeclaration
CompactConstructorDeclaration

The following productions from §8.1.7 are shown here for convenience:

ClassBodyDeclaration:
ClassMemberDeclaration
InstanceInitializer
StaticInitializer
ConstructorDeclaration
ClassMemberDeclaration:
FieldDeclaration
MethodDeclaration
ClassDeclaration
InterfaceDeclaration
;

The CompactConstructorDeclaration clause is described in §8.10.4.2.

It is a compile-time error for the body of a record declaration to contain a non-static field declaration (§8.3.1.1).

It is a compile-time error for the body of a record declaration to contain a method declaration that is abstract or native (§8.4.3.1, §8.4.3.4).

It is a compile-time error for the body of a record declaration to contain an instance initializer (§8.6).

8.10.3. Record Members

For each record component, a record class has a field with the same name as the record component and the same type as the declared type of the record component. This field, which is declared implicitly, is known as a component field.

A component field is private, final, and non-static.

A component field is annotated with the annotations, if any, that appear on the corresponding record component and whose annotation interfaces are applicable in the field declaration context, or in type contexts, or both (§9.7.4).

Furthermore, for each record component, a record class has a method with the same name as the record component and an empty formal parameter list. This method, which is declared explicitly or implicitly, is known as an accessor method.

If an accessor method for a record component is declared explicitly, then all of the following must be true, or a compile-time error occurs:

The return type of the accessor method (§8.4.5) must be the same as the declared type of the record component.

The accessor method must not be generic (§8.4.4).

The accessor method must be a public instance method with no formal parameters and no throws clause.

If a record class has a record component for which an accessor method is not declared explicitly, then an accessor method for that record component is declared implicitly, with the following properties:

Its name is the same as the name of the record component.

Its return type is the same as the declared type of the record component.

It is not generic.

It is a public instance method with no formal parameters and no throws clause.

It is annotated with the annotations, if any, that appear on the corresponding record component and whose annotation interfaces are applicable in the method declaration context, or in type contexts, or both (§9.7.4).

Its body returns the value of the corresponding component field.

The restrictions on record component names (§8.10.1) mean that no implicitly declared accessor method has a signature that is override-equivalent with a non-private method of class Object. An explicit method declaration that takes one of the restricted names, such as public void wait() {...}, is not an accessor method, since wait is never a record component name.

Annotations that appear on a record component are not propagated to an explicitly declared accessor method for that record component. In some situations, the programmer may need to duplicate a record component's annotations on an explicitly declared accessor method, but this is not generally necessary.

Annotations that are propagated to an implicitly declared accessor method must result in a legally annotated method. For example, in the following record declaration, the implicitly declared accessor method x() would be annotated with @SafeVarargs, but such an annotation is illegal on a fixed arity method (§9.6.4.7):

record BadRecord(@SafeVarargs int x) {}  // Error


The scope and shadowing of the component field and the accessor method are specified in §6.3 and §6.4.1. (The record component to which they correspond is not a declaration, so has no scope of its own.)

Record classes may explicitly declare instance methods other than accessor methods, but may not explicitly declare instance variables (§8.10.2). Explicit declarations of class methods and class variables are permitted.

All members of record classes, including implicitly declared members, are subject to the usual rules for member declarations in a class (§8.3, §8.4, §8.5).

All of the rules concerning inheritance that apply to normal classes apply to record classes. In particular, record classes may inherit members from superinterfaces, although a superinterface method will never be inherited as an accessor method because the record class will always declare, explicitly or implicitly, an accessor method that overrides the superinterface method.

For example, a record class can inherit default methods from its direct superinterfaces, although the default method bodies have no knowledge of the component fields of the record class. The following program prints Logged:

public class Test {
    interface Logging {
        default void logAction() {
            System.out.println("Logged");
        }
    }

    record Point(int i, int j) implements Logging {}

    public static void main(String[] args) {
        Point p = new Point(10, 20);
        p.logAction();
    }
}


A record class provides implementations of all the abstract methods declared in class Record. For each of the following methods, if a record class R does not explicitly declare a method with the same modifiers, name, and signature (§8.4.2), then the method is implicitly declared as follows:

A method public final boolean equals(Object) that returns true if and only if the argument is an instance of R, and the current instance is equal to the argument instance at every record component of R; otherwise false is returned.

Equality of an instance a of a record class R with another instance b of the same record class at a record component c is determined as follows:

If the type of the record component c is a reference type, equality is determined as follows: if the value of the component field c of both a and b is the null reference then true is returned; if the value of the component field c of either a or b, but not both, is the null reference then false is returned; otherwise equality is determined by invoking the equals method on the value of the component field c of a, with an argument that is the value of the component field c of b.

If the type of the record component c is a primitive type T, equality is determined as if by invoking the static method compare of the wrapper class corresponding to T (§5.1.7), with the first argument given by the value of the component field c of a, and the second argument given by the value of the component field c of b; if the method would return 0 then true is returned, otherwise false is returned.

The use of compare in wrapper classes ensures that the implicitly declared equals method is reflexive and behaves consistently with the implicitly declared hashCode method for record classes that have floating-point components.

A method public final int hashCode() that returns a hash code value derived from the hash code values at every record component of R.

The hash code value of an instance a of a record class at a record component c is as follows:

If the type of the record component c is a reference type, then the hash code value is determined as if by invoking the hashCode method on the value of the component field c of a.

If the type of the record component c is a primitive type T, then the hash code value is determined as if by subjecting the value of the component field c of a to boxing conversion (§5.1.7) and then invoking the method hashCode of the wrapper class corresponding to T on the resulting object.

A method public final String toString() that returns a string derived from the name of the record class and the names and string representations of every record component of R.

The string representation of a record component c of an instance a of a record class is as follows:

If the type of the record component c is a reference type, then the string representation is determined as if by invoking the toString method on the value of the component field c of a.

If the type of the record component c is a primitive type T, then the string representation is determined as if by subjecting the value of the component field c of a to boxing conversion (§5.1.7) and then invoking the method toString method of the wrapper class corresponding to T on the resulting object.

Note that equality, hash code values, and string representations are determined by looking at the values of component fields directly, rather than by invoking accessor methods.

Consider a record class R that has components c1, ..., cn, and an implicitly declared accessor method for every component, and an implicitly declared equals method. If an instance r1 of R is copied in the following way:

R r2 = new R(r1.c1(), r1.c2(), ..., r1.cn());


then, assuming r1 is not the null reference, it is always the case that the expression r1.equals(r2) will evaluate to true. Explicitly declared accessor methods and equals methods should respect this invariant. It is not generally possible for a compiler to check whether explicitly declared methods respect the invariant. The following record declaration is bad style because its accessor methods clip the x and y components and therefore prevent p3 from being equals to p1:

record SmallPoint(int x, int y) {
    public int x() { return this.x < 100 ? this.x : 100; }
    public int y() { return this.y < 100 ? this.y : 100; }

    public static void main(String[] args) {
        SmallPoint p1 = new SmallPoint(200,300);
        SmallPoint p2 = new SmallPoint(200,300);
        System.out.println(p1.equals(p2));  // prints true

        SmallPoint p3 = new SmallPoint(p1.x(), p1.y());
        System.out.println(p1.equals(p3));  // prints false
    }
}

8.10.4. Record Constructor Declarations

To ensure proper initialization of its record components, a record class does not implicitly declare a default constructor (§8.8.9). Instead, a record class has a canonical constructor, declared explicitly or implicitly, that initializes all the component fields of the record class.

There are two ways to explicitly declare a canonical constructor in a record declaration: by declaring a normal constructor with a suitable signature (§8.10.4.1) or by declaring a compact constructor (§8.10.4.2).

Given the signature of a normal constructor that qualifies as canonical, and the signature derived for a compact constructor, the rules of constructor signatures (§8.8.2) mean it is a compile-time error if a record declaration has both a normal constructor that qualifies as canonical and a compact constructor.

Either way, an explicitly declared canonical constructor must provide at least as much access as the record class, as follows:

If the record class is public, then the canonical constructor must be public; otherwise, a compile-time error occurs.

If the record class is protected, then the canonical constructor must be protected or public; otherwise, a compile-time error occurs.

If the record class has package access, then the canonical constructor must not be private; otherwise, a compile-time error occurs.

If the record class is private, then the canonical constructor may be declared with any accessibility.

An explicitly declared canonical constructor may be a fixed arity constructor or a variable arity constructor (§8.8.1).

If a canonical constructor is not explicitly declared in the declaration of a record class R, then a canonical constructor r is implicitly declared in R with the following properties:

The signature of r has no type parameters, and has formal parameters given by the derived formal parameter list of R, defined below.

r has the same access modifier as R, unless R lacks an access modifier, in which case r has package access.

r has no throws clause.

The body of r initializes each component field of the record class with the corresponding formal parameter of r, in the order that record components (corresponding to the component fields) appear in the record header.

The derived formal parameter list of a record class is formed by deriving a formal parameter from each record component in the record header, in order, as follows:

If the record component is not a variable arity record component, then the derived formal parameter has the same name and declared type as the record component.

If the record component is a variable arity record component, then the derived formal parameter is a variable arity parameter (§8.4.1) with the same name and declared type as the record component.

The derived formal parameter is annotated with the annotations, if any, that appear on the record component and whose annotation interfaces are applicable in the formal parameter context, or in type contexts, or both (§9.7.4).

A record declaration may contain declarations of constructors that are not canonical constructors. The body of every non-canonical constructor in a record declaration must contain an alternate constructor invocation (§8.8.7.1), or a compile-time error occurs.

8.10.4.1. Normal Canonical Constructors

A (non-compact) constructor in the declaration of record class R is the canonical constructor of R if its signature is override-equivalent (§8.4.2) to the derived constructor signature of R.

The derived constructor signature of a record class R is a signature that consists of the name R, no type parameters, and the formal parameter types derived from the record header of R by taking the declared type of each record component in order.

As a canonical constructor has a signature that is override-equivalent to the derived constructor signature of the record class, there can be only one canonical constructor declared explicitly in the record class.

The declaration of a (non-compact) canonical constructor must satisfy all of the following conditions, or a compile-time error occurs:

Each formal parameter in the formal parameter list must have the same name and declared type as the corresponding record component.

A formal parameter must be a variable arity parameter if and only if the corresponding record component is a variable arity record component.

The constructor must not be generic (§8.8.4).

The constructor must not have a throws clause.

The constructor body must not contain an explicit constructor invocation (§8.8.7.1).

All the other rules for constructor declarations in a normal class declaration must be satisfied (§8.8).

A consequence of these rules is that the annotations on a record component can differ from the annotations on the corresponding formal parameter of an explicitly declared canonical constructor. For example, the following record declaration is valid:

import java.lang.annotation.Target;
import java.lang.annotation.ElementType;

@interface Foo {}
@interface Bar {}

record Person(@Foo String name) {
    Person(@Bar String name) {
        this.name = name;
    }
}

8.10.4.2. Compact Canonical Constructors

A compact constructor declaration is a succinct form of constructor declaration, only available in a record declaration. It declares the canonical constructor of a record class without requiring the record components of the class to be manually repeated as formal parameters of the constructor.

CompactConstructorDeclaration:
{ConstructorModifier} SimpleTypeName ConstructorBody

The following productions from §8.8, §8.8.3, and §8.8.7 are shown here for convenience:

ConstructorModifier:
(one of)
Annotation public protected private
SimpleTypeName:
TypeIdentifier
ConstructorBody:
{ [BlockStatements] ConstructorInvocation [BlockStatements] }
{ [BlockStatements] }

It is a compile-time error for a record declaration to have more than one compact constructor declaration.

The formal parameters of a compact constructor of a record class are implicitly declared. They are given by the derived formal parameter list of the record class (§8.10.4).

The compact constructor of a record class is a variable arity constructor (§8.8.1) if the record class has a variable arity record component.

The signature of a compact constructor declaration is equal to the derived constructor signature of the record class (§8.10.4.1).

The body of a compact constructor declaration must satisfy all of the following conditions, or a compile-time error occurs:

The body must not contain a return statement (§14.17).

The body must not contain an explicit constructor invocation (§8.8.7.1).

The body must not contain an assignment to a component field of the record class.

All the other rules for a constructor in a normal class declaration must be satisfied (§8.8), except for the requirement that the component fields of the record class must be definitely assigned and moreover not definitely unassigned at the end of the compact constructor (§8.3.1.2).

If a record declaration has a record component named c, then the simple name c in the body of a compact constructor denotes the implicit formal parameter named c, and not the component field named c.

After the last statement, if any, in the body of the compact constructor has completed normally (§14.1), all component fields of the record class are implicitly initialized to the values of the corresponding formal parameters. The component fields are initialized in the order that the corresponding record components are declared in the record header.

The intent of a compact constructor declaration is that only code to validate or normalize parameters needs to be given in the constructor body; the remaining initialization code is supplied by the compiler. For example, the following record class has a compact constructor that simplifies a rational number:

record Rational(int num, int denom) {
    private static int gcd(int a, int b) {
        if (b == 0) return Math.abs(a);
        else return gcd(b, a % b);
    }

    Rational {
        int gcd = gcd(num, denom);
        num    /= gcd;
        denom  /= gcd;
    }
}


The compact constructor Rational {...} behaves the same as this normal constructor:

Rational(int num, int denom) {
    int gcd = gcd(num, denom);
    num    /= gcd;
    denom  /= gcd;
    this.num   = num;
    this.denom = denom;
}