Class declarations define new reference types and describe how they are implemented (§8.1).
A top level class is a class that is not a nested class.
A nested class is any class whose declaration occurs within the body of another class or interface.
This chapter discusses the common semantics of all classes - top level (§7.6) and nested (including member classes (§8.5, §9.5), local classes (§14.3) and anonymous classes (§15.9.5)). Details that are specific to particular kinds of classes are discussed in the sections dedicated to these constructs.
A named 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. A class may be declared final (§8.1.1.2), in which case it cannot have subclasses. If a class is declared public, then it can be referred to from other packages. 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). Classes may be generic (§8.1.2), that is, they may declare type variables whose bindings may differ among different instances of the class.
Classes may be decorated with annotations (§9.7) just like any other kind of declaration.
The body of a class declares members (fields and methods and nested classes and interfaces), instance and static initializers, and constructors (§8.1.6). 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 (§6.6) public, protected, or private. 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 class members and interface members can hide class or interface members 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 (§8.1.3).
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 type; an instance method is invoked with respect to some particular object that is an instance of a class type. 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).
There are two kinds of class declarations: normal class declarations and enum declarations.
ClassDeclaration:
NormalClassDeclaration
EnumDeclaration
NormalClassDeclaration:
{ClassModifier} class Identifier [TypeParameters]
[Superclass] [Superinterfaces] ClassBody
The rules in this section apply to all class declarations, including enum declarations. However, special rules apply to enum declarations with regard to class modifiers, inner classes, and superclasses; these rules are stated in §8.9.
The Identifier 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.
A class declaration may include class modifiers.
ClassModifier: one of
Annotation public protected private
abstract static final strictfp
The rules for annotation modifiers on a class declaration are specified in §9.7.4 and §9.7.5.
The access modifier public (§6.6) pertains only to top level classes (§7.6) and member classes (§8.5), not to local classes (§14.3) or anonymous classes (§15.9.5).
The access modifiers protected and private pertain only to member classes within a directly enclosing class declaration (§8.5).
The modifier static pertains only to member classes (§8.5.1), not to top level or local or anonymous classes.
It is a compile-time error if the same keyword appears more than once as a modifier for a class declaration.
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.
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 . . .
}
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 the name of a final class appears in the extends clause (§8.1.4) of another class declaration; this implies that a final class cannot have any subclasses.
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).
The effect of the strictfp modifier is to make all float or double expressions within the class declaration (including within variable initializers, instance initializers, static initializers, and constructors) be explicitly FP-strict (§15.4).
This implies that all methods declared in the class, and all nested types declared in the class, are implicitly strictfp.
A class is generic if it declares one or more type variables (§4.4).
These type variables are known as the type parameters of the class. The type parameter section follows the class name and is delimited by angle brackets.
TypeParameters:
< TypeParameterList >
TypeParameterList:
TypeParameter {, TypeParameter}
The following productions from §4.4 are shown here for convenience:
TypeParameter:
{TypeParameterModifier} Identifier [TypeBound]
TypeParameterModifier:
Annotation
TypeBound:
extends TypeVariable
extends ClassOrInterfaceType {AdditionalBound}
AdditionalBound:
& InterfaceType
The rules for annotation modifiers on a type parameter declaration are specified in §9.7.4 and §9.7.5.
In a class’s type parameter section, a type variable T directly depends on a type variable S if S is the bound of T, while T depends on S if either T directly depends on S or T directly depends on a type variable U that depends on S (using this definition recursively). It is a compile-time error if a type variable in a class’s type parameter section depends on itself.
The scope and shadowing of a class’s type parameter is specified in §6.3 and §6.4.
A generic class declaration defines a set of parameterized types (§4.5), one for each possible parameterization of the type parameter section by type arguments. All of these parameterized types share the same class at run time.
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.
It is a compile-time error to refer to a type parameter of a generic class C in any of the following:
• the declaration of a static member of C (§8.3.1.1, §8.4.3.2, §8.5.1).
• the declaration of a static member of any type declaration nested within C.
• a static initializer of C (§8.7), or
• a static initializer of any class declaration nested within C.
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>(
new Integer(1),
new Seq<Number>(new Double(1.5),
new Seq<Number>()));
Seq<String>.Zipper<Number> zipper =
strs.new Zipper<Number>();
Seq<Pair<String,Number>> combined =
zipper.zip(nums);
}
}
An inner class is a nested class that is not explicitly or implicitly declared static.
An inner class may be a non-static member class (§8.5), a local class (§14.3), or an anonymous class (§15.9.5). A member class of an interface is implicitly static (§9.5) so is never considered to be an inner class.
It is a compile-time error if an inner class declares a static initializer (§8.7).
It is a compile-time error if an inner class declares a member that is explicitly or implicitly static, unless the member is a constant variable (§4.12.4).
An inner class may inherit static members that are not constant variables even though it cannot declare them.
A nested class that is not an inner class may declare static members freely, in accordance with the usual rules of the Java programming language.
Example 8.1.3-1. Inner Class Declarations and Static Members
class HasStatic {
static int j = 100;
}
class Outer {
class Inner extends HasStatic {
static final int x = 3; // OK: constant variable
static int y = 4; // Compile-time error: an inner class
}
static class NestedButNotInner{
static int z = 5; // OK: not an inner class
}
interface NeverInner {} // Interfaces are never inner
}
A statement or expression occurs in a static context if and only if the innermost method, constructor, instance initializer, static initializer, field initializer, or explicit constructor invocation statement enclosing the statement or expression is a static method, a static initializer, the variable initializer of a static variable, or an explicit constructor invocation statement (§8.8.7.1).
An inner class C is a direct inner class of a class or interface O if O is the immediately enclosing type declaration of C and the declaration of C does not occur in a static context.
A class C is an inner class of class or interface O if it is either a direct inner class of O or an inner class of an inner class of O.
It is unusual, but possible, for the immediately enclosing type declaration of an inner class to be an interface. This only occurs if the class is declared in a default method body (§9.4). Specifically, it occurs if an anonymous or local class is declared in a default method body, or a member class is declared in the body of an anonymous class that is declared in a default method body.
A class or interface O is the zeroth lexically enclosing type declaration of itself.
A class O is the n’th lexically enclosing type declaration of a class C if it is the immediately enclosing type declaration of the n-1’th lexically enclosing type declaration of C.
An instance i of a direct inner class C of a class or interface O is associated with an instance of O, known as the immediately enclosing instance of i. The immediately enclosing instance of an object, if any, is determined when the object is created (§15.9.2).
An object o is the zeroth lexically enclosing instance of itself.
An object o is the n’th lexically enclosing instance of an instance i if it is the immediately enclosing instance of the n-1’th lexically enclosing instance of i.
An instance of an inner class I whose declaration occurs in a static context has no lexically enclosing instances. However, if I is immediately declared within a static method or static initializer then I does have an enclosing block, which is the innermost block statement lexically enclosing the declaration of I.
For every superclass S of C which is itself a direct inner class of a class or interface SO, there is an instance of SO associated with i, known as the immediately enclosing instance of i with respect to S. The immediately enclosing instance of an object with respect to its class’ direct superclass, if any, is determined when the superclass constructor is invoked via an explicit constructor invocation statement (§8.8.7.1).
When an inner class (whose declaration does not occur in a static context) refers to an instance variable that is a member of a lexically enclosing type declaration, the variable of the corresponding lexically enclosing instance is used.
Any local variable, formal parameter, or exception parameter used but not declared in an inner class must either be declared final or be effectively final (§4.12.4), or a compile-time error occurs where the use is attempted.
Any local variable used but not declared in an inner class must be definitely assigned (§16 (Definite Assignment)) before the body of the inner class, or a compile-time error occurs.
Similar rules on variable use apply in the body of a lambda expression (§15.27.2).
A blank final field (§4.12.4) of a lexically enclosing type declaration may not be assigned within an inner class, or a compile-time error occurs.
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 marked final).
Inner classes whose declarations do not occur in a static context may freely refer to the instance variables of their enclosing type declaration. An instance variable is always defined with respect to an instance. In the case of instance variables of an enclosing type declaration, the instance variable must be defined with respect to an enclosing instance of that declared type. 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).
The optional extends clause in a normal class declaration specifies the direct superclass of the current class.
Superclass:
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.
The ClassType must name an accessible class type (§6.6), or a compile-time error occurs.
It is a compile-time error if the ClassType names a class that is final, because final classes are not allowed to have subclasses (§8.1.1.2).
It is a compile-time error if the ClassType names the class Enum or any invocation of Enum (§8.9).
If the ClassType has type arguments, it must denote a well-formed parameterized type (§4.5), and none of the type arguments may be wildcard type arguments, or a compile-time error occurs.
Given a (possibly generic) class declaration C<F1,...,Fn> (n ≥ 0, C ≠ Object), the direct superclass of the class type C<F1,...,Fn> is the type given in the extends clause of the declaration of C if an extends clause is present, or Object otherwise.
Given a generic class declaration C<F1,...,Fn> (n > 0), the direct superclass of the parameterized class type C<T1,...,Tn>, where Ti (1 ≤ i ≤ n) is a type, is D<U1 θ,...,Uk θ>, where D<U1,...,Uk> is the direct superclass of C<F1,...,Fn> and θ is the substitution [F1:=T1,...,Fn:=Tn].
A class is said to be a direct subclass of its direct superclass. The direct superclass is the class from whose implementation the implementation of the current class is derived.
The subclass relationship is the transitive closure of the direct subclass relationship. A class A is a subclass of class C if either of the following is true:
• A is the direct subclass of C
• There exists a class B such that A is a subclass of B, and B is a subclass of C, applying this definition recursively.
Class C is said to be a superclass of class A whenever A is a subclass of C.
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 type T if T is mentioned in the extends or implements clause of C either as a superclass or superinterface, or as a qualifier of a superclass or superinterface name.
A class C depends on a reference type T if any of the following is true:
• C directly depends on T.
• C directly depends on an interface I that depends (§9.1.3) on T.
• C directly depends on a class D that depends on T (using 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.
The optional implements clause in a class declaration lists the names of interfaces that are direct superinterfaces of the class being declared.
Superinterfaces:
implements InterfaceTypeList
InterfaceTypeList:
InterfaceType {, InterfaceType}
Each InterfaceType must name an accessible interface type (§6.6), or a compile-time error occurs.
If an InterfaceType has type arguments, it must denote a well-formed parameterized type (§4.5), and none of the type arguments may be wildcard type arguments, or a compile-time error occurs.
It is a compile-time error if the same interface is mentioned as a direct superinterface more than once in a single implements clause. This is true even if the interface is named in different ways.
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.
Given a (possibly generic) class declaration C<F1,...,Fn> (n ≥ 0, C ≠ Object), the direct superinterfaces of the class type C<F1,...,Fn> are the types given in the implements clause of the declaration of C, if an implements clause is present.
Given a generic class declaration C<F1,...,Fn> (n > 0), the direct superinterfaces of the parameterized class type C<T1,...,Tn>, where Ti (1 ≤ i ≤ n) is a type, are all types I<U1 θ,...,Uk θ>, where I<U1,...,Uk> is a direct superinterface of C<F1,...,Fn> and θ is the substitution [F1:=T1,...,Fn:=Tn].
An interface type I is a superinterface of class type C if any of the following is true:
• 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 implement all its superinterfaces.
A class may not at the same time be a subtype of two interface types which are different parameterizations of the same generic interface (§9.1.2), or a subtype of a parameterization of a generic interface and a raw type naming that same generic interface, or a compile-time error occurs.
This requirement was introduced in order to support translation by type erasure (§4.6).
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 8.1.5-3. 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).
A class body may contain declarations of members of the class, that is, fields (§8.3), methods (§8.4), classes (§8.5), and interfaces (§8.5).
A class body may also contain instance initializers (§8.6), static initializers (§8.7), and declarations of constructors (§8.8) for the class.
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 type C is specified in §6.3 and §6.4.
If C itself is a nested class, there may be definitions of the same kind (variable, method, or type) and name as m in enclosing scopes. (The scopes may be blocks, classes, or packages.) In all such cases, the member m declared in or inherited by C shadows (§6.4.1) the other definitions of the same kind and name.
The members of a class type are all of the following:
• Members inherited from its direct superclass (§8.1.4), except in class Object, which has no direct superclass
• Members inherited from any direct superinterfaces (§8.1.5)
• Members declared in the body of the class (§8.1.6)
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 consisting of:
- type parameters: the declarations of any type parameters of the method member.
- argument types: a list of the types of the arguments to the method member.
- return type: the return type of the method member.
- throws clause: exception types declared in the throws clause of the method member.
Fields, methods, and member types of a class type 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:
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.
The variables of a class type 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:
Identifier [TypeArguments]
UnannClassOrInterfaceType . {Annotation} Identifier [TypeArguments]
UnannInterfaceType:
UnannClassType
UnannTypeVariable:
Identifier
UnannArrayType:
UnannPrimitiveType Dims
UnannClassOrInterfaceType Dims
UnannTypeVariable Dims
The following production from §4.3 is shown here for convenience:
Dims:
{Annotation} [ ] {{Annotation} [ ]}
The FieldModifier clause is described in §8.3.1.
The Identifier in a FieldDeclaration may be used in a name to refer to the field.
More than one field may be declared in a single field declaration by using more than one declarator; the FieldModifiers and UnannType apply to all the declarators in the declaration.
The declared type of a field is denoted by the UnannType that appears in the field declaration, followed by any bracket pairs that follow the Identifier in the declarator.
It is a compile-time error for the body of a class declaration to declare two fields with the same name.
The scope and shadowing of a field declaration is specified in §6.3 and §6.4.
If the 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 nonprivate fields of the superclass and superinterfaces that are both accessible 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. 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 such a reference is ambiguous.
There might be several paths by which the same field declaration might be 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.
A value stored in a field of type float is always an element of the float value set (§4.2.3); similarly, a value stored in a field of type double is always an element of the double value set. It is not permitted for a field of type float to contain an element of the float-extended-exponent value set that is not also an element of the float value set, nor for a field of type double to contain an element of the double-extended-exponent value set that is not also an element of the double value set.
Example 8.3-1. Multiply Inherited Fields
A class may inherit two or more fields with the same name, either from two interfaces or from its superclass and an interface. 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.
FieldModifier: one of
Annotation public protected private
static final transient volatile
The rules for annotation modifiers on a field declaration are specified in §9.7.4 and §9.7.5.
It is a compile-time error if the same keyword appears more than once as a modifier for a field declaration.
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.
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 (sometimes called a non-static field) is called an instance variable. 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.
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
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 at the end of every constructor of the class in which it is declared, or a compile-time error occurs (§8.8, §16.9).
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.
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.
If a declarator in a field declaration has a variable initializer, then the declarator has the semantics of an assignment (§15.26) to the declared variable.
If the declarator is for a class variable (that is, a static field), then the following rules apply to its initializer:
• It is a compile-time error if a reference by simple name to any instance variable occurs in the initializer.
• It is a compile-time error if the keyword this (§15.8.3) or the keyword super (§15.11.2, §15.12) occurs in the initializer.
• 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). This also applies in interfaces (§9.3.1). Such fields will never be observed to have their default initial values (§4.12.5), even by devious programs.
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 use the simple name of any class variable declared in or inherited by the class, even one whose declaration occurs textually after the initializer.
• The initializer may refer to the current object this (§15.8.3) and may use the keyword super (§15.11.2, §15.12).
• At run time, the initializer is evaluated and the assignment performed each time an instance of the class is created (§12.5).
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.
Use of class variables whose declarations appear textually after the use is sometimes restricted, even though these class variables are in scope (§6.3). Specifically, it is a compile-time error if all of the following are true:
• The declaration of a class variable in a class or interface C appears textually after a use of the class variable;
• The use is a simple name in either a class variable initializer of C or a static initializer of C;
• The use is not on the left hand side of an assignment;
• C is the innermost class or interface enclosing the use.
Use of instance variables whose declarations appear textually after the use is sometimes restricted, even though these instance variables are in scope. Specifically, it is a compile-time error if all of the following are true:
• The declaration of an instance variable in a class or interface C appears textually after a use of the instance variable;
• The use is a simple name in either an instance variable initializer of C or an instance initializer of C;
• The use is not on the left hand side of an assignment;
• C is the innermost class or interface enclosing the use.
Example 8.3.3-1. Restrictions on Field Initialization
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;
}
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 ( [FormalParameterList] ) [Dims]
The following production from §4.3 is shown here for convenience:
Dims:
{Annotation} [ ] {{Annotation} [ ]}
The FormalParameterList 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).
It is a compile-time error for the body of a class to declare as members two methods with override-equivalent signatures (§8.4.2).
The scope and shadowing of a method declaration is specified in §6.3 and §6.4.
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.
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, only an empty pair of parentheses appears in the declaration of the method or constructor.
FormalParameterList:
FormalParameters , LastFormalParameter
LastFormalParameter
FormalParameters:
FormalParameter {, FormalParameter}
ReceiverParameter {, FormalParameter}
FormalParameter:
{VariableModifier} UnannType VariableDeclaratorId
VariableModifier: one of
Annotation final
ReceiverParameter:
{Annotation} UnannType [Identifier .] this
LastFormalParameter:
{VariableModifier} UnannType {Annotation} ... VariableDeclaratorId
FormalParameter
The following productions from §4.3 and §8.3 are shown here for convenience:
VariableDeclaratorId:
Identifier [Dims]
Dims:
{Annotation} [ ] {{Annotation} [ ]}
The last formal parameter of a method or constructor is special: it may be a variable arity parameter, indicated by an ellipsis following the type.
Note that the ellipsis (...) is a token unto itself (§3.11). It is possible to put whitespace between it and the type, but this is discouraged as a matter of style.
If the last formal parameter is a variable arity parameter, the method is a variable arity method. Otherwise, it is a fixed arity method.
The 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. Either way, 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. 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 qualified class instance creation expression, and it has no effect whatsoever at run time.
The rules for annotation modifiers on a formal parameter declaration and on a receiver parameter are specified in §9.7.4 and §9.7.5.
It is a compile-time error if final appears more than once as a modifier for a formal parameter declaration.
It is a compile-time error to use mixed array notation (§10.2) for a variable arity parameter.
The scope and shadowing of a formal parameter is specified in §6.3 and §6.4.
It is a compile-time error for a method or constructor to declare two formal parameters with the same name. (That is, their declarations mention the same Identifier.)
It is a compile-time error if a formal parameter that is declared final is assigned to within the body of the method or constructor.
A receiver parameter may appear only in the FormalParameters of an instance method or an inner class’s constructor; otherwise, a compile-time error occurs.
Where a receiver parameter is allowed, its type and name are specified 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.
The declared type of a formal parameter is denoted by the UnannType that appears in its parameter specifier, followed by any bracket pairs that follow the Identifier in the declarator, except for a variable arity parameter, whose declared type is an array type whose component type is the UnannType that appears in its parameter specifier.
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 unchecked 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 DeclaratorId may be used as a simple name in the body of the method or constructor to refer to the formal parameter.
Invocations of a variable arity method may contain more actual argument expressions than formal parameters. All the actual argument expressions that do not correspond to the formal parameters preceding the variable arity parameter will be evaluated and the results stored into an array that will be passed to the method invocation (§15.12.4.2).
A method or constructor parameter of type float always contains an element of the float value set (§4.2.3); similarly, a method or constructor parameter of type double always contains an element of the double value set. It is not permitted for a method or constructor parameter of type float to contain an element of the float-extended-exponent value set that is not also an element of the float value set, nor for a method parameter of type double to contain an element of the double-extended-exponent value set that is not also an element of the double value set.
Where an actual argument expression corresponding to a parameter variable is not FP-strict (§15.4), evaluation of that actual argument expression is permitted to use intermediate values drawn from the appropriate extended-exponent value sets. Prior to being stored in the parameter variable, the result of such an expression is mapped to the nearest value in the corresponding standard value set by being subjected to invocation conversion (§5.3).
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.
Two methods or constructors, M and N, have the same signature if they have the same name, the same type parameters (if any) (§8.4.4), and, after adapting the formal parameter types of N to the the type parameters of M, the same formal parameter types.
The signature of a method m1 is a subsignature of the signature of a method m2 if either:
• 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.
MethodModifier: one of
Annotation public protected private
abstract static final synchronized native strictfp
The rules for annotation modifiers on a method declaration are specified in §9.7.4 and §9.7.5.
It is a compile-time error if the same keyword appears more than once as a modifier for a method declaration.
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.
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 class Object. (Class 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
}
}
A method that is declared static is called a class method.
It is a compile-time error to use the name of a type parameter of any surrounding declaration in the header or body of a class method.
A class method is always invoked without reference to a particular object. It is a compile-time error to attempt to reference the current object using the keyword this (§15.8.3) or the keyword super (§15.11.2).
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.
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.
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;
}
The effect of the strictfp modifier is to make all float or double expressions within the method body be explicitly FP-strict (§15.4).
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.
A method is generic if it declares one or more type variables (§4.4).
These type variables are known as the type parameters of the method. The form of the type parameter section of a generic method is identical to the type parameter section of a generic class (§8.1.2).
A generic method declaration defines a set of methods, one for each possible invocation of the type parameter section by type arguments. Type arguments may not need to be provided explicitly when a generic method is invoked, as they can often be inferred (§18 (Type Inference)).
The scope and shadowing of a method’s type parameter is specified in §6.3.
Two methods or constructors M and N have the same type parameters if both of the following are true:
• 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.
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.
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.
A throws clause is used to declare 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 */ }
}
}
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?!"); }
}
A class C inherits from its direct superclass all concrete methods m (both static and instance) of the superclass for which all of the following are true:
• m is a member of the direct superclass of C.
• 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.
A class C inherits from its direct superclass and direct superinterfaces all abstract and default (§9.4) methods m for which all of the following are true:
• m is a member of the direct superclass or a direct superinterface, D, of C.
• 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.
• No concrete method inherited by C from its direct superclass has a signature that is a subsignature of the signature of m.
• There exists no method m’ that is a member of the direct superclass or a direct superinterface, D’, of C (m distinct from m’, D distinct from D’), such that m’ from D’ overrides the declaration of the method m.
A class does not inherit static methods from its superinterfaces.
Note that it is possible for an inherited concrete method to prevent the inheritance of an abstract or default method. (Later we will assert that the concrete method overrides the abstract or default method “from C”.) 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.
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.
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:
• A is a superclass of C.
• C does not inherit mA.
• The signature of mC is a subsignature (§8.4.2) of the signature of mA.
• One of the following is true:
- mA is public.
- 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 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 a non-abstract method mC overrides an abstract method mA from a class C, then mC is said to implement mA from C.
An instance method mC declared in or inherited by class C, overrides from C another method mI declared in an interface I, iff all of the following are true:
• I is a superinterface of C.
• mI is an abstract or default method.
• The signature of mC is a subsignature (§8.4.2) of the signature of mI.
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.)
It is a compile-time error if an instance method overrides 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 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 FP-strict to override an FP-strict method and it is permitted for an FP-strict method to override a method that is not FP-strict.
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.OutputStream;
import java.io.IOException;
class BufferOutput {
private OutputStream o;
BufferOutput(OutputStream o) { this.o = o; }
protected byte[] buf = new byte[512];
protected int pos = 0;
public void putchar(char c) throws IOException {
if (pos == buf.length) flush();
buf[pos++] = (byte)c;
}
public void putstr(String s) throws IOException {
for (int i = 0; i < s.length(); i++)
putchar(s.charAt(i));
}
public void flush() throws IOException {
o.write(buf, 0, pos);
pos = 0;
}
}
class LineBufferOutput extends BufferOutput {
LineBufferOutput(OutputStream o) { super(o); }
public void putchar(char c) throws IOException {
super.putchar(c);
if (c == '\n') flush();
}
}
class Test {
public static void main(String[] args) throws IOException {
LineBufferOutput lbo = new LineBufferOutput(System.out);
lbo.putstr("lbo\nlbo");
System.out.print("print\n");
lbo.putstr("\n");
}
}
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.
If a class C declares or inherits a static method m, then m is said to hide any method m', where the signature of m is a subsignature (§8.4.2) of the signature of m', in the superclasses and superinterfaces of C that would otherwise be accessible to code in C.
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 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.
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, a compile-time unchecked warning occurs unless suppressed by the SuppressWarnings annotation (§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), a compile-time unchecked warning occurs.
It is a compile-time error if a type declaration T has a member method m1 and there exists a method m2 declared in T or a supertype of T such that all of the following are true:
• m1 and m2 have the same name.
• m2 is accessible from T.
• The signature of m1 is not a subsignature (§8.4.2) of the signature of m2.
• The signature of m1 or some method m1 overrides (directly or indirectly) has the same erasure as the 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 type declaration cannot implement or extend two distinct invocations of the same generic interface.
The access modifier (§6.6) 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 hidden or overridden 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
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 overrideequivalent 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.
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.
A member class is a class whose declaration is directly enclosed in the body of another class or interface declaration (§8.1.6, §9.1.4).
A member interface is an interface whose declaration is directly enclosed in the body of another class or interface declaration (§8.1.6, §9.1.4).
The accessibility of a member type in a class or interface declaration is specified in §6.6.
It is a compile-time error if the same keyword appears more than once as a modifier for a member type declaration in a class.
The scope and shadowing of a member type is specified in §6.3 and §6.4.
If a class declares a member type with a certain name, then the declaration of that type is said to hide any and all accessible declarations of member types with the same name in superclasses and superinterfaces of the class.
In this respect, hiding of member types is similar to hiding of fields (§8.3).
A class inherits from its direct superclass and direct superinterfaces all the non-private member types of the superclass and superinterfaces that are both accessible to code in the class and not hidden by a declaration in the class.
A class may inherit two or more type declarations with the same name, either from two interfaces or from its superclass and an interface. It is a compile-time error to attempt to refer to any ambiguously inherited class or interface by its simple name.
If the same type declaration is inherited from an interface by multiple paths, the class or interface is considered to be inherited only once. It may be referred to by its simple name without ambiguity.
The static keyword may modify the declaration of a member type C within the body of a non-inner class or interface T. Its effect is to declare that C is not an inner class. Just as a static method of T has no current instance of T in its body, C also has no current instance of T, nor does it have any lexically enclosing instances.
It is a compile-time error if a static class contains a usage of a non-static member of an enclosing class.
A member interface is implicitly static (§9.1.1). It is permitted for the declaration of a member interface to redundantly specify the static modifier.
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 (§14.21).
It is a compile-time error if a return statement (§14.17) appears anywhere within an instance initializer.
Instance initializers are permitted to refer to the current object via the keyword this (§15.8.3), to use the keyword super (§15.11.2, §15.12), and to use any type variables in scope.
Use of instance variables whose declarations appear textually after the use is sometimes restricted, even though these instance variables are in scope. See §8.3.3 for the precise rules governing forward reference to instance variables.
Exception checking for an instance initializer is specified in §11.2.3.
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 (§14.21).
It is a compile-time error if a return statement (§14.17) appears anywhere within a static initializer.
It is a compile-time error if the keyword this (§15.8.3) or the keyword super (§15.11, §15.12) or any type variable declared outside the static initializer, appears anywhere within a static initializer.
Use of class variables whose declarations appear textually after the use is sometimes restricted, even though these class variables are in scope. See §8.3.3 for the precise rules governing forward reference to class variables.
Exception checking for a static initializer is specified in §11.2.3.
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 ( [FormalParameterList] )
SimpleTypeName:
Identifier
The rules in this section apply to constructors in all class declarations, including enum 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.
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 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; }
}
The formal parameters of a constructor are identical in syntax and semantics to those of a method (§8.4.1).
The constructor of a non-private inner member class implicitly declares, as the first formal parameter, a variable representing the immediately enclosing instance of the class (§15.9.2, §15.9.3).
The rationale for why only this kind of class has an implicitly declared constructor parameter is subtle. The following explanation may be helpful:
1. 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.
2. In a class instance creation expression for a local class (not in a static context) or anonymous class, §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.
3. In a class instance creation expression for an anonymous class, and where the anonymous class’s superclass is either inner or local (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 a local or anonymous class is always accessed by the same compiler that compiled it, explains why the binary name of a non-private inner member class is defined to be predictable but the binary name of a local or anonymous class is not (§13.1).
It is a compile-time error to declare two constructors with override-equivalent signatures (§8.4.2) in a class.
It is a compile-time error to declare two constructors whose signatures have the same erasure (§4.6) in a class.
ConstructorModifier: one of
Annotation public protected private
The rules for annotation modifiers on a constructor declaration are specified in §9.7.4 and §9.7.5.
It is a compile-time error if the same keyword appears more than once as a modifier in a constructor declaration.
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; it effectively ensures that a constructor is FP-strict if and only if its class is FP-strict (§15.4).
A constructor is generic if it declares one or more type variables (§4.4).
These type variables are known as the type parameters of the constructor. The form of the type parameter section of a generic constructor is identical to the type parameter section of a generic class (§8.1.2).
It is possible for a constructor to be generic independently of whether the class the constructor is declared in is itself generic.
A generic constructor declaration defines a set of constructors, one for each possible invocation of the type parameter section by type arguments. Type arguments may not need to be provided explicitly when a generic constructor is invoked, as they can often by inferred (§18 (Type Inference)).
The scope and shadowing of a constructor’s type parameter is specified in §6.3 and §6.4.
The throws clause for a constructor is identical in structure and behavior to the throws clause for a method (§8.4.6).
The type of a constructor consists of its signature and the exception types given by its throws clause.
The first statement of a constructor body may be an explicit invocation of another constructor of the same class or of the direct superclass (§8.8.7.1).
ConstructorBody:
{ [ExplicitConstructorInvocation] [BlockStatements] }
It is a compile-time error for a constructor to directly or indirectly invoke itself through a series of one or more explicit constructor invocations involving this.
If a constructor body does not begin with an explicit constructor invocation and the constructor being declared is not part of the primordial class Object, then the constructor body implicitly begins with a superclass constructor invocation “super();”, an invocation of the constructor of its direct superclass that takes no arguments.
Except for the possibility of explicit constructor invocations, and the prohibition on explicitly returning a value (§14.17), the body of a constructor is like the body of a method (§8.4.7).
A return statement (§14.17) may be used in the body of a constructor if it does not include an expression.
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.
ExplicitConstructorInvocation:
[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}
Explicit constructor invocation statements 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.
An explicit constructor invocation statement in a constructor body may not refer to any instance variables or instance methods or inner classes declared in this class or any superclass, or use this or super in any expression; otherwise, a compile-time error occurs.
This prohibition on using the current instance explains why an explicit constructor invocation statement is deemed to occur in a static context (§8.1.3).
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 statement is unqualified, then:
• If S is an inner member class, but S is not a member of a lexically enclosing type declaration of C, then a compile-time error occurs.
If a superclass constructor invocation statement 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 statement can throw are specified in §11.2.2.
Evaluation of an alternate constructor invocation statement proceeds by first evaluating the arguments to the constructor, left-to-right, as in an ordinary method invocation; and then invoking the constructor.
Evaluation of a superclass constructor invocation statement proceeds as follows:
1. Let i be the instance being created. The immediately enclosing instance of i with respect to S (if any) must be determined:
• 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.
• If the superclass constructor invocation is unqualified, then S is necessarily a local class or an inner member class.
Let O be the immediately enclosing class of S, and let n be an integer such that O is the n’th lexically enclosing type declaration of C.
The immediately enclosing instance of i with respect to S is the n’th lexically enclosing instance of this.
• If the superclass constructor invocation is qualified, then the Primary expression or the ExpressionName immediately preceding “.super”, p, is evaluated.
If p evaluates to null, a NullPointerException is raised, and the superclass constructor invocation completes abruptly.
Otherwise, the result of this evaluation is the immediately enclosing instance of i with respect to S.
2. After determining the immediately enclosing instance of i with respect to S (if any), evaluation of the superclass constructor invocation statement proceeds by evaluating the arguments to the constructor, left-to-right, as in an ordinary method invocation; and then invoking the constructor.
3. Finally, if the superclass constructor invocation statement 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 statement or is provided implicitly. (An alternate constructor invocation does not perform this additional implicit execution.)
Example 8.8.7.1-1. Restrictions on Explicit Constructor Invocation Statements
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 explicit constructor invocation statement.
Example 8.8.7.1-2. Qualified Superclass Constructor Invocation
In the code below, ChildOfInner has no lexically enclosing type declaration, so an instance of ChildOfInner has no enclosing instance. However, the superclass of ChildOfInner (Inner) has a lexically enclosing type 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 statement 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 statement. The rules for an explicit constructor invocation statement do not prohibit using formal parameters of the constructor that contains the statement.
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.
Overloading of constructors is identical in behavior to overloading of methods (§8.4.9). The overloading is resolved at compile time by each class instance creation expression (§15.9).
If a class contains no constructor declarations, then a default constructor is implicitly declared. The form of the default constructor for a top level class, member class, or local class is as follows:
• The default constructor has the same accessibility as the class (§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,
• The default constructor has no throws clauses.
• 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.
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.
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.
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.
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 class PackageOnly can be instantiated only within the package just, in which it is declared.
An enum declaration specifies a new enum type, a special kind of class type.
EnumDeclaration:
{ClassModifier} enum Identifier [Superinterfaces] EnumBody
It is a compile-time error if an enum declaration has the modifier abstract or final.
An enum declaration is implicitly final unless it contains at least one enum constant that has a class body (§8.9.1).
A nested enum type is implicitly static. It is permitted for the declaration of a nested enum type to redundantly specify the static modifier.
This implies that it is impossible to declare an enum type in the body of an inner class (§8.1.3), because an inner class cannot have static members except for constant variables.
It is a compile-time error if the same keyword appears more than once as a modifier for an enum declaration.
The direct superclass of an enum type E is Enum<E> (§8.1.4).
An enum type has no instances other than those defined by its enum constants. It is a compile-time error to attempt to explicitly instantiate an enum type (§15.9.1).
The final clone method in Enum ensures that enum constants can never be cloned, and the special treatment by the serialization mechanism ensures that duplicate instances are never created as a result of deserialization. Reflective instantiation of enums is prohibited. Together, these four things ensure that no instances of an enum exist beyond those defined by the enum constants.
The body of an enum declaration may contain enum constants. An enum constant defines an instance of the enum type.
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 for annotation modifiers on an enum constant declaration are specified in §9.7.4 and §9.7.5.
The Identifier in a EnumConstant may be used in a name to refer to the enum constant.
The scope and shadowing of an enum constant is specified in §6.3 and §6.4.
An enum constant may be followed by arguments, which are passed to the constructor of the enum when the constant is created during class initialization as described later in this section. The constructor to be invoked is chosen using the normal rules of overload resolution (§15.12.2). If the arguments are omitted, an empty argument list is assumed.
The optional class body of an enum constant implicitly defines an anonymous class declaration (§15.9.5) that extends the immediately enclosing enum type. The class body is governed by the usual rules of anonymous classes; in particular it cannot contain any constructors. Instance methods declared in these class bodies may be invoked outside the enclosing enum type only if they override accessible methods in the enclosing enum type (§8.4.8).
It is a compile-time error for the class body of an enum constant to declare an abstract method.
Because there is only one instance of each enum constant, it is permitted to use the == operator in place of the equals method when comparing two object references if it is known that at least one of them refers to an enum constant.
The equals method in Enum is a final method that merely invokes super.equals on its argument and returns the result, thus performing an identity comparison.
In addition to enum constants, the body of an enum declaration may contain constructor and member declarations as well as instance and static initializers.
EnumBodyDeclarations:
; {ClassBodyDeclaration}
The following productions from §8.1.6 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 type exactly as if they had been present in the body of a normal class declaration, unless explicitly stated otherwise.
It is a compile-time error if a constructor declaration in an enum declaration is public or protected (§6.6).
It is a compile-time error if a constructor declaration in an enum declaration contains a superclass constructor invocation statement (§8.8.7.1).
It is a compile-time error to reference a static field of an enum type from constructors, instance initializers, or instance variable initializer expressions of the enum type, unless the field is a constant variable (§4.12.4).
In an enum declaration, a constructor declaration with no access modifiers is private.
In an enum declaration with no constructor declarations, a default constructor is implicitly declared. The default constructor is private, has no formal parameters, and has no throws clause.
In practice, a compiler is likely to mirror the Enum type by declaring String and int parameters in the default constructor of an enum type. However, these parameters are not specified as “implicitly declared” because different compilers do not need to agree on the form of the default constructor. Only the compiler of an enum type knows how to instantiate the enum constants; other compilers can simply rely on the implicitly declared public static fields of the enum type (§8.9.3) without regard for how those fields were initialized.
It is a compile-time error if an enum declaration E has an abstract method m as a member, unless E has at least one enum constant and all of E’s enum constants have class bodies that provide concrete implementations of m.
It is a compile-time error for an enum declaration to declare a finalizer (§12.6). An instance of an enum type may never be finalized.
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 type’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 types. (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.Map;
import java.util.HashMap;
enum Color {
RED, GREEN, BLUE;
Color() { colorMap.put(toString(), this); }
static final Map<String,Color> colorMap =
new HashMap<String,Color>();
}
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.Map;
import java.util.HashMap;
enum Color {
RED, GREEN, BLUE;
static final Map<String,Color> colorMap =
new HashMap<String,Color>();
static {
for (Color c : Color.values())
colorMap.put(c.toString(), c);
}
}
The refactored version is clearly correct, as static initialization occurs top to bottom.
• 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 consisting of c, and is annotated by the same annotations as c.
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.
• The following implicitly declared methods:
/**
* Returns an array containing the constants of this enum
* type, in the order they're declared. This method may be
* used to iterate over the constants as follows:
*
* for(E c : E.values())
* System.out.println(c);
*
* @return an array containing the constants of this enum
* type, in the order they're declared
*/
public static E[] values();
/**
* Returns the enum constant of this type with the specified
* name.
* The string must match exactly an identifier used to declare
* an enum constant in this type. (Extraneous whitespace
* characters are not permitted.)
*
* @return the enum constant with the specified name
* @throws IllegalArgumentException if this enum type has no
* constant with the specified name
*/
public static E valueOf(String name);
It follows that the declaration of enum type E cannot contain fields that conflict with the implicitly declared fields corresponding to E’s enum constants, nor contain methods that conflict with implicitly declared methods or override final methods of class Enum<E>.
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 type from outside the type. This example “adds” a color method to the Coin type 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
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));
}
}
Class bodies attach behaviors to the enum constants. 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 in the base type (Operation), as 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 Types
In the following program, a playing card class is built atop two simple enums.
import java.util.List;
import java.util.ArrayList;
class Card implements Comparable<Card>,
java.io.Serializable {
public enum Rank { DEUCE, THREE, FOUR, FIVE, SIX, SEVEN,
EIGHT, NINE, TEN,JACK, QUEEN, KING, ACE }
public enum Suit { CLUBS, DIAMONDS, HEARTS, SPADES }
private final Rank rank;
private final Suit suit;
public Rank rank() { return rank; }
public Suit suit() { return suit; }
private Card(Rank rank, Suit suit) {
if (rank == null || suit == null)
throw new NullPointerException(rank + ", " + suit);
this.rank = rank;
this.suit = suit;
}
public String toString() { return rank + " of " + suit; }
// Primary sort on suit, secondary sort on rank
public int compareTo(Card c) {
int suitCompare = suit.compareTo(c.suit);
return (suitCompare != 0 ?
suitCompare :
rank.compareTo(c.rank));
}
private static final List<Card> prototypeDeck =
new ArrayList<Card>(52);
static {
for (Suit suit : Suit.values())
for (Rank rank : Rank.values())
prototypeDeck.add(new Card(rank, suit));
}
// Returns a new deck
public static List<Card> newDeck() {
return new ArrayList<Card>(prototypeDeck);
}
}
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.List;
import java.util.ArrayList;
import java.util.Collections;
class Deal {
public static void main(String args[]) {
int numHands = Integer.parseInt(args[0]);
int cardsPerHand = Integer.parseInt(args[1]);
List<Card> deck = Card.newDeck();
Collections.shuffle(deck);
for (int i=0; i < numHands; i++)
System.out.println(dealHand(deck, cardsPerHand));
}
/**
* Returns a new ArrayList consisting of the last n
* elements of deck, which are removed from deck.
* The returned list is sorted using the elements'
* natural ordering.
*/
public static <E extends Comparable<E>>
ArrayList<E> dealHand(List<E> deck, int n) {
int deckSize = deck.size();
List<E> handView = deck.subList(deckSize - n, deckSize);
ArrayList<E> hand = new ArrayList<E>(handView);
handView.clear();
Collections.sort(hand);
return hand;
}
}
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]