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CHAPTER 8
Class declarations define new reference types and describe how they are implemented (§8.1).
A nested class is any class whose declaration occurs within the body of another class or interface. A top level class is a class that is not a nested class.
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 (§8.1.3) and may implement interfaces (§8.1.4).
The body of a class declares members (fields and methods and nested classes and interfaces), instance and static initializers, and constructors (§8.1.5). The scope (§6.3) of a member (§8.2) is the entire 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 (§8.1.2).
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 the 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 (§14.18), thus allowing its activities to be synchronized with those of other threads (§17).
Method names may be overloaded (§8.4.7).
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 when it is first loaded (§12.4).
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 (§8.8.6).
ClassDeclaration: ClassModifiersopt class Identifier Superopt Interfacesopt ClassBodyThe Identifier in a class declaration specifies the name of the class. A compile-time error occurs if a class has the same simple name as any of its enclosing classes or interfaces.
ClassModifiers: ClassModifier ClassModifiers ClassModifier ClassModifier: one of public protected private abstract static final strictfpNot all modifiers are applicable to all kinds of class declarations. The access modifier public pertains only to top level classes (§7.6) and to member classes (§8.5, §9.5), and is discussed in §6.6, §8.5 and §9.5. The access modifiers protected and private pertain only to member classes within a directly enclosing class declaration (§8.5) and are discussed in §8.5.1. The access modifier static pertains only to member classes (§8.5, §9.5). A compile-time error occurs if the same modifier appears more than once in a class declaration.
arrays.doc.htmlIf two or more class modifiers appear in a class declaration, then it is customary, though not required, that they appear in the order consistent with that shown above in the production for ClassModifier.
abstract
Classesabstract
class is a class that is incomplete, or to be considered incomplete. Only abstract
classes may have abstract
methods (§8.4.3.1, §9.4), that is, methods that are declared but not yet implemented. If a class that is not abstract
contains an abstract
method, then a compile-time error occurs. A class C has abstract
methods if any of the following is true:
abstract
method (§8.4.3).
abstract
method that has not been implemented (§8.4.6.1) in C or any of its superclasses.
abstract
) and C neither declares nor inherits a method that implements it.
a classabstract class Point { int x = 1, y = 1; void move(int dx, int dy) { x += dx; y += dy; alert(); } abstract void alert(); } abstract class ColoredPoint extends Point { int color; } class SimplePoint extends Point { void alert() { } }
Point
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
.
A compile-time error occurs if an attempt is made to create an instance of an abstract
class using a class instance creation expression (§15.9).
Thus, continuing the example just shown, the statement:
would result in a compile-time error; the classPoint p = new Point();
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:
would be correct.Point p = new SimplePoint();
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. Thus, in the example just given, instantiation of a SimplePoint
causes the default constructor and field initializers for x
and y
of Point
to be executed.
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 different return types.
As an example, the declarations:
result in a compile-time error: it would be impossible for any subclass of classinterface Colorable { void setColor(int color); } abstract class Colored implements Colorable { abstract int setColor(int color); }
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 (§8.8.8) 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 . . . }
final
Classesfinal
if its definition is complete and no subclasses are desired or required. A compile-time error occurs if the name of a final
class appears in the extends
clause (§8.1.3) of another class
declaration; this implies that a final
class cannot have any subclasses. A compile-time error occurs 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 (§8.4.6.1).
strictfp
Classesstrictfp
modifier is to make all float
or double
expressions within the class declaration 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
.
Note also that all float
or double
expressions within all variable initializers, instance initializers, static initializers and constructors of the class will also be explicitly FP-strict.
To illustrate these rules, consider the example below:
Inner classes may inherit static members that are not compile-time constants even though they may not declare them. Nested classes that are not inner classes may declare static members freely, in accordance with the usual rules of the Java programming language. Member interfaces (§8.5) are always implicitly static so they are never considered to be inner classes.class HasStatic{ static int j = 100; } class Outer{ class Inner extends HasStatic{ static final x = 3; // ok - compile-time constant 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 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 (§8.8.5).
An inner class C is a direct inner class of a class O if O is the immediately lexically enclosing class of C and the declaration of C does not occur in a static context. A class C is an inner class of class O if it is either a direct inner class of O or an inner class of an inner class of O.
A class O is the zeroth lexically enclosing class of itself. A class O is the nth lexically enclosing class of a class C if it is the immediately enclosing class of the n - 1st lexically enclosing class of C.
An instance i of a direct inner class C of a class 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 nth lexically enclosing instance of an instance i if it is the immediately enclosing instance of the n - 1st lexically enclosing instance of i.
When an inner class refers to an instance variable that is a member of a lexically enclosing class, the variable of the corresponding lexically enclosing instance is used. A blank final (§4.5.4) field of a lexically enclosing class may not be assigned within an inner class.
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.
Furthermore, for every superclass S of C which is itself a direct inner class of a class 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.
Any local variable, formal method parameter or exception handler parameter used but not declared in an inner class must be declared final
, and must be definitely assigned (§16) before the body of the inner class.
Inner classes include local (§14.3), anonymous (§15.9.5) and non-static member classes (§8.5). Here are some examples:
The declaration of classclass 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; } } }
LocalInStaticContext
occurs in a static context-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 class. An instance variable is always defined with respect to an instance. In the case of instance variables of an enclosing class, the instance variable must be defined with respect to an enclosing instance of that class So, for example, the class Local
above has an enclosing instance of class Outer
. As a further example:
Here, every instance ofclass WithDeepNesting{ boolean toBe; WithDeepNesting(boolean b) { toBe = b;} class Nested { boolean theQuestion; class DeeplyNested { DeeplyNested(){ theQuestion = toBe || !toBe; } } } }
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).extends
clause in a class declaration specifies the direct superclass of the current class. A class is said to be a direct subclass of the class it extends. The direct superclass is the class from whose implementation the implementation of the current class is derived. The extends
clause must not appear in the definition of the class Object
, because it is the primordial class and has no direct superclass. If the class declaration for any other class has no extends
clause, then the class has the class Object
as its implicit direct superclass.
Super: extends ClassTypeThe following is repeated from §4.3 to make the presentation here clearer:
ClassType: TypeNameThe ClassType must name an accessible (§6.6) class type, or a compile-time error occurs. If the specified ClassType names a class that is
final
(§8.1.1.2), then a compile-time error occurs; final
classes are not allowed to have subclasses.the relationships are as follows:class Point { int x, y; } final class ColoredPoint extends Point { int color; } class Colored3DPoint extends ColoredPoint { int z; } // error
Point
is a direct subclass of Object
.
Object
is the direct superclass of the class Point
.
ColoredPoint
is a direct subclass of class Point
.
Point
is the direct superclass of class ColoredPoint
.
Colored3dPoint
causes a compile-time error because it attempts to extend the final
class ColoredPoint
.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:
the relationships are as follows:class Point { int x, y; } class ColoredPoint extends Point { int color; } final class Colored3dPoint extends ColoredPoint { int z; }
Point
is a superclass of class ColoredPoint
.
Point
is a superclass of class Colored3dPoint
.
ColoredPoint
is a subclass of class Point
.
ColoredPoint
is a superclass of class Colored3dPoint
.
Colored3dPoint
is a subclass of class ColoredPoint
.
Colored3dPoint
is a subclass of class Point
.
extends
or implements
clause of C either as a superclass or superinterface, or as a qualifier within a superclass or superinterface name. A class C depends on a reference type T if any of the following conditions hold:
causes a compile-time error.class Point extends ColoredPoint { int x, y; } class ColoredPoint extends Point { int color; }
If circularly declared classes are detected at run time, as classes are loaded (§12.2), then a ClassCircularityError
is thrown.
implements
clause in a class declaration lists the names of interfaces that are direct superinterfaces of the class being declared:
Interfaces: implements InterfaceTypeList InterfaceTypeList: InterfaceType InterfaceTypeList , InterfaceTypeThe following is repeated from §4.3 to make the presentation here clearer:
InterfaceType: TypeNameEach InterfaceType must name an accessible (§6.6) interface type, or a compile-time error occurs.
A compile-time error occurs if the same interface is mentioned two or more times in a single implements
clause.
This is true even if the interface is named in different ways; for example, the code:
results in a compile-time error because the names java.lang.class Redundant implements java.lang.Cloneable, Cloneable { int x; }
Cloneable
and Cloneable
refer to the same interface.An interface type I is a superinterface of class type C if any of the following is true:
the relationships are as follows:public interface Colorable { void setColor(int color); int getColor(); } public interface Paintable extends Colorable { int MATTE = 0, GLOSSY = 1; void setFinish(int finish); int 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 { int finish; public void setFinish(int finish) { this.finish = finish; } public int getFinish() { return finish; } }
Paintable
is a superinterface of class PaintedPoint
.
Colorable
is a superinterface of class ColoredPoint
and of class PaintedPoint
.
Paintable
is a subinterface of the interface Colorable
, and Colorable
is a superinterface of Paintable
, a
s defined in §9.1.2.
PaintedPoint
has Colorable
as a superinterface both because it is a superinterface of ColoredPoint
and because it is a superinterface of Paintable
.
Unless the class being declared is abstract
, the declarations of all the method members of each direct superinterface must be implemented either by a declaration in this class or by an existing method declaration inherited from the direct superclass, because a class that is not abstract
is not permitted to have abstract
methods (§8.1.1.1).
causes a compile-time error, becauseinterface Colorable { void setColor(int color); int getColor(); } class Point { int x, y; }; class ColoredPoint extends Point implements Colorable { int color; }
ColoredPoint
is not an abstract
class but it fails to provide an implementation of methods setColor
and getColor
of the interface Colorable
.It is permitted for a single method declaration in a class to implement methods of more than one superinterface. For example, in the code:
the methodinterface Fish { int getNumberOfScales(); } interface Piano { int getNumberOfScales(); } class Tuna implements Fish, Piano { // You can tune a piano, but can you tuna fish? int getNumberOfScales() { return 91; } }
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:
It is impossible to declare a method namedinterface 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; } }
getNumberOfScales
with the same signature and return type as those of both the methods declared in interface Fish
and in interface StringBass
, because a class can have only one method with a given signature (§8.4). Therefore, it is impossible for a single class to implement both interface Fish
and interface StringBass
(§8.4.6).
ClassBody: { ClassBodyDeclarationsopt } ClassBodyDeclarations: ClassBodyDeclaration ClassBodyDeclarations ClassBodyDeclaration ClassBodyDeclaration: ClassMemberDeclaration InstanceInitializer StaticInitializer ConstructorDeclaration ClassMemberDeclaration: FieldDeclaration MethodDeclaration ClassDeclaration InterfaceDeclaration ;The scope of a declaration of a member m declared in or inherited by a class type C is the entire body of C, including any nested type declarations.
If C itself is a nested class, there may be definitions of the same kind (variable, method, or type) for m in enclosing scopes. (The scopes may be blocks, classes, or packages.) In all such cases, the member m declared or inherited in C shadows (§6.3.1) the other definitions of m.
Object
, which has no direct superclass
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.
causes four compile-time errors: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 } }
ColoredPoint
has no constructor declared with two integer parameters, as requested by the use in main
. This illustrates the fact that ColoredPoint
does not inherit the constructors of its superclass Point
.
ColoredPoint
declares no constructors, and therefore a default constructor for it is automatically created (§8.8.7), and this default constructor is equivalent to:
which invokes the constructor, with no arguments, for the direct superclass of the classColoredPoint() { super(); }
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 (§8.8.5).
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.points
package declares two compilation units:
and:package points; public class Point { int x, y; public void move(int dx, int dy) { x += dx; y += dy; } }
and a third compilation unit, in another package, is:package points; public class Point3d extends Point { int z; public void move(int dx, int dy, int dz) { x += dx; y += dy; z += dz; } }
Here both classes in theimport 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 } }
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:
using theimport 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; } }
move
method of the superclass Point3d
to process dx
, dy
, and dz
. If Point4d
is written in this way it will compile without errors.Point
:
thepackage 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++; } }
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++; } }
the class variableclass 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++; } }
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
.public
, instances of the class might be available at run time to code outside the package in which it is declared if it has 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:
and another compilation unit of another package:package points; public class Point { public int x, y; public void move(int dx, int dy) { x += dx; y += dy; } }
An invocationpackage 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(); } }
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
:
then classpackage 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; } }
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
.
FieldDeclaration: FieldModifiersopt Type VariableDeclarators ; VariableDeclarators: VariableDeclarator VariableDeclarators , VariableDeclarator VariableDeclarator: VariableDeclaratorId VariableDeclaratorId = VariableInitializer VariableDeclaratorId: Identifier VariableDeclaratorId [ ] VariableInitializer: Expression ArrayInitializerThe FieldModifiers are described in §8.3.1. The Identifier in a FieldDeclarator may be used in a name to refer to the field. Fields are members; the scope (§6.3) of a field declaration is specified in §8.1.5. More than one field may be declared in a single field declaration by using more than one declarator; the FieldModifiers and Type apply to all the declarators in the declaration. Variable declarations involving array types are discussed in §10.2.
It is a compile-time error for the body of a class declaration to declare two fields with the same name. Methods, types, and fields may have the same name, since they are used in different contexts and are disambiguated by different lookup procedures (§6.5).
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. The field declaration also shadows (§6.3.1) declarations of any accessible fields in enclosing classes or interfaces, and any local variables, formal method parameters, and exception handler parameters with the same name in any enclosing blocks.
If a field declaration hides the declaration of another field, the two fields need not have the same type.
A class inherits from its direct superclass and direct superinterfaces all the non-private fields of the superclass and superinterfaces that are both accessible to code in the class and not hidden by a declaration in the class.
Note that 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 (§8.3.3.3). 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 hidden field can be accessed by using a qualified name (if it is static
) or by using a field access expression (§15.11) that contains the keyword super
or a cast to a superclass type. See §15.11.2 for discussion and an example.
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.
FieldModifiers: FieldModifier FieldModifiers FieldModifier FieldModifier: one of public protected private static final transient volatileThe access modifiers
public
, protected
, and private
are discussed in §6.6. A compile-time error occurs if the same modifier appears more than once in a field declaration, or if a field declaration has more than one of the access modifiers public
, protected
, and private
. 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.
static
Fieldsstatic
, 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, a new variable associated with that instance is created for every instance variable declared in that class or any of its superclasses. The example program:
prints:class Point { int x, y, useCount; Point(int x, int y) { this.x = x; this.y = y; } final static 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); } }
showing that changing the fields(2,2) 0 true 1
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):
isq.origin==Point.origin
true
. Further evidence is that the incrementation:
causes the value ofp.origin.useCount++;
q.origin.useCount
to be 1
; this is so because p.origin
and q.origin
refer to the same variable.final
Fieldsfinal
(§4.5.4). Both class and instance variables (static
and non-static
fields) may be declared final
.It is a compile-time error if a blank final (§4.5.4) class variable is not definitely assigned (§16.7) by a static initializer (§8.7) of the class in which it is declared.
A blank final instance variable must be definitely assigned (§16.8) at the end of every constructor (§8.8) of the class in which it is declared; otherwise a compile-time error occurs.
transient
Fieldstransient
to indicate that they are not part of the persistent state of an object.
If an instance of the class Point
:
were saved to persistent storage by a system service, then only the fieldsclass Point { int x, y; transient float rho, theta; }
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.volatile
FieldsThe Java programming language provides a second mechanism, volatile fields, that is more convenient for some purposes.
A field may be declared volatile
, in which case a thread must reconcile its working copy of the field with the master copy every time it accesses the variable. Moreover, operations on the master copies of one or more volatile variables on behalf of a thread are performed by the main memory in exactly the order that the thread requested.
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
:
then methodclass Test { static int i = 0, j = 0; static void one() { i++; j++; } static void two() { System.out.println("i=" + i + " j=" + j); } }
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, 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):
This prevents methodclass Test { static int i = 0, j = 0; static synchronized void one() { i++; j++; } static synchronized void two() { System.out.println("i=" + i + " j=" + j); } }
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
:
See §17 for more discussion and examples.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 methodone
and methodtwo
to be executed concurrently, but guarantees that accesses to the shared values fori
andj
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 forj
is never greater than that fori
, because each update toi
must be reflected in the shared value fori
before the update toj
occurs. It is possible, however, that any given invocation of methodtwo
might observe a value forj
that is much greater than the value observed fori
, because methodone
might be executed many times between the moment when methodtwo
fetches the value ofi
and the moment when methodtwo
fetches the value ofj
.
A compile-time error occurs if a final
variable is also declared volatile
.
static
field), then the variable initializer is evaluated and the assignment performed exactly once, when the class is initialized (§12.4).
static
), then the variable initializer is evaluated and the assignment performed each time an instance of the class is created (§12.5).
The example:
produces the output: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); } }
because the assignments to1, 5
x
and y
occur whenever a new Point
is created.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.
It is a compile-time error if the evaluation of a variable initializer for a static
field or for a instance variable of a named class (or of an interface) can complete abruptly with a checked exception (§11.2).
If the keyword this
(§15.8.3) or the keyword super
(§15.11.2, §15.12) occurs in an initialization expression for a class variable, then a compile-time error occurs.
One subtlety here is that, at run time, static
variables that are final
and that are initialized with compile-time constant values are initialized first. This also applies to such fields in interfaces (§9.3.1). These variables are "constants" that will never be observed to have their default initial values (§4.5.5), even by devious programs. See §12.4.2 and §13.4.8 for more discussion.
Use of class variables whose declarations appear textually after the use is sometimes restricted, even though these class variables are in scope. See §8.3.2.3 for the precise rules governing forward reference to class variables.
static
variable declared in or inherited by the class, even one whose declaration occurs textually later. compiles without error; it initializesclass Test { float f = j; static int j = 1; }
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.
Initialization expressions for instance variables are permitted to refer to the current object this
(§15.8.3) and to use the keyword super
(§15.11.2, §15.12).
Use of instance variables whose declarations appear textually after the use is sometimes restricted, even though these instance variables are in scope. See §8.3.2.3 for the precise rules governing forward reference to instance variables.
static
) field of a class or interface C and all of the following conditions hold:
static
) variable initializer of C or in an instance (respectively static
) initializer of C.
This means that a compile-time error results from the test program:
whereas the following example compiles without error:class Test { int i = j; // compile-time error: incorrect forward reference int j = 1; }
even though the constructor (§8.8) forclass Test { Test() { k = 2; } int j = 1; int i = j; int k; }
Test
refers to the field k
that is declared three lines later. These restrictions are designed to catch, at compile time, circular or otherwise malformed initializations. Thus, both:
and:class Z { static int i = j + 2; static int j = 4; }
result in compile-time errors. Accesses by methods are not checked in this way, so:class Z { static { i = j + 2; } static int i, j; static { j = 4; } }
produces the output: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); } }
because the variable initializer for0
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 (§4.5.5).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; 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; }
produces the output: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); } }
because the declaration of4.7 2
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:
then the fieldclass 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); } }
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
produces the output: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); } }
because the declaration of4.7 2 4.7 2
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 float
. 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 float
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:
then the fieldclass 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); } }
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
super
(§15.11.2) may be used to access such fields unambiguously. In the example:
the classinterface 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); } }
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
:
It compiles and prints: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); } }
Even if two distinct inherited fields have the same type, the same value, and are both2.5
final
, any reference to either field by simple name is considered ambiguous and results in a compile-time error. In the example:
it is not astonishing that the reference tointerface 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 } }
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.
the fieldspublic interface Colorable { int RED = 0xff0000, GREEN = 0x00ff00, BLUE = 0x0000ff; } public 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 { . . .RED
. . . }
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
.
MethodDeclaration: MethodHeader MethodBody MethodHeader: MethodModifiersopt ResultType MethodDeclarator Throwsopt ResultType: Type void MethodDeclarator: Identifer ( FormalParameterListopt )The MethodModifiers are described in §8.4.3, the Throws clause in §8.4.4, and the MethodBody in §8.4.5. A method declaration either specifies the type of value that the method returns or uses the keyword
void
to indicate that the method does not return a value.The Identifier in a MethodDeclarator may be used in a name to refer to the method. A class can declare a method with the same name as the class or a field, member class or member interface of the class.
For compatibility with older versions of the Java platform, a declaration form for a method that returns an array is allowed to place (some or all of) the empty bracket pairs that form the declaration of the array type after the parameter list. This is supported by the obsolescent production:
MethodDeclarator: MethodDeclarator [ ]but should not be used in new code.
It is a compile-time error for the body of a class to have as members two methods with the same signature (§8.4.2) (name, number of parameters, and types of any parameters). Methods and fields may have the same name, since they are used in different contexts and are disambiguated by the different lookup procedures (§6.5).
FormalParameterList: FormalParameter FormalParameterList , FormalParameter FormalParameter: finalopt Type VariableDeclaratorIdThe following is repeated from §8.3 to make the presentation here clearer:
VariableDeclaratorId: Identifier VariableDeclaratorId [ ]If a method or constructor has no parameters, only an empty pair of parentheses appears in the declaration of the method or constructor.
If two formal parameters of the same method or constructor are declared to have the same name (that is, their declarations mention the same Identifier), then a compile-time error occurs.
It is a compile-time error if a method or constructor parameter that is declared final is assigned to within the body of the method or constructor.
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.
The scope of a parameter of a method (§8.4.1) or constructor (§8.8.1) is the entire body of the method or constructor.
These parameter names may not be redeclared as local variables of the method, or as exception parameters of catch clauses in a try statement of the method or constructor. However, a parameter of a method or constructor may be shadowed anywhere inside a class declaration nested within that method or constructor. Such a nested class declaration could declare either a local class (§14.3) or an anonymous class (§15.9).
Formal parameters are referred to only using simple names, never by using qualified names (§6.6).
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 method invocation conversion (§5.3).
causes a compile-time error because it declares twoclass Point implements Move { int x, y; abstract void move(int dx, int dy); void move(int dx, int dy) { x += dx; y += dy; } }
move
methods with the same signature. This is an error even though one of the declarations is abstract
.MethodModifiers: MethodModifier MethodModifiers MethodModifier MethodModifier: one of public protected private abstract static final synchronized native strictfpThe access modifiers
public
, protected
, and private
are discussed in §6.6. A compile-time error occurs if the same modifier appears more than once in a method declaration, or if a method declaration has more than one of the access modifiers public
, protected
, and private
. A compile-time error occurs if a method declaration that contains the keyword abstract
also contains any one of the keywords private
, static
, final
, native
, strictfp
, or synchronized
. A compile-time error occurs if a method declaration that contains the keyword native
also contains strictfp
.If two or more 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.
abstract
Methodsabstract
method declaration introduces the method as a member, providing its signature (name and number and type of parameters), return type, and throws
clause (if any), but does not provide an implementation. The declaration of an abstract
method m must appear directly within an abstract
class (call it A); otherwise a compile-time error results. Every subclass of A that is not abstract
must provide an implementation for m, or a compile-time error occurs as specified in §8.1.1.1.
It is a compile-time error for a private
method to be declared abstract
.
It would be impossible for a subclass to implement a private
abstract
method, because private
methods are not inherited by subclasses; therefore such a method could never be used.
It is a compile-time error for a static
method to be declared abstract
.
It is a compile-time error for a final
method to be declared abstract
.
An abstract
class can override an abstract
method by providing another abstract
method declaration.
This can provide a place to put a documentation comment, or to declare that the set of checked exceptions (§11.2) that can be thrown by that method, when it is implemented by its subclasses, is to be more limited. For example, consider this code:
The overriding declaration of methodclass BufferEmpty extends Exception { BufferEmpty() { super(); } BufferEmpty(String s) { super(s); } } class BufferError extends Exception { BufferError() { super(); } BufferError(String s) { super(s); } } public interface Buffer { char get() throws BufferEmpty, BufferError; } public abstract class InfiniteBuffer implements Buffer { abstract char get() throws BufferError; }
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.
An instance method that is not abstract
can be overridden by an abstract
method.
For example, we can declare an abstract
class Point
that requires its subclasses to implement toString
if they are to be complete, instantiable classes:
Thisabstract class Point { int x, y; public abstract String toString(); }
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:
results in a compile-time error because the invocationclass ColoredPoint extends Point { int color; public String toString() { return super.toString() + ": color " + color; // error } }
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 } }
static
Methodsstatic
is called a class method. A class method is always invoked without reference to a particular object. An attempt to reference the current object using the keyword this
or the keyword super
in the body of a class method results in a compile-time error. It is a compile-time error for a static
method to be declared abstract
.
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.
final
Methodsfinal
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 in a final
class (§8.1.1.2) are implicitly final
, because it is impossible to override them. It is permitted but not required for the declarations of such methods to redundantly include the final
keyword.
It is a compile-time error for a final
method to be declared abstract
.
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. The 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.
Here, inlining the methodfinal 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); } } }
move
of class Point
in method main
would transform the for
loop to the form:
The loop might then be subject to further optimizations.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; }
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.
native
Methodsnative
is implemented in platform-dependent code, typically written in another programming language such as C, C++, FORTRAN, or assembly language. The body of a native
method is given as a semicolon only, indicating that the implementation is omitted, instead of a block.
A compile-time error occurs if a native
method is declared abstract
.
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; }
strictfp
Methodsstrictfp
modifier is to make all float
or double
expressions within the method body be explicitly FP-strict (§15.4).synchronized
Methodssynchronized
method acquires a lock (§17.1) before it executes. For a class (static)
method, the lock associated with the Class
object for the method's class is used. For an instance method, the lock associated with this
(the object for which the method was invoked) is used.
These are the same locks that can be used by the synchronized
statement (§14.18); thus, the code:
has exactly the same effect as:class Test { int count; synchronized void bump() { count++; } static int classCount; static synchronized void classBump() { classCount++; } }
The more elaborate example:class BumpTest { int count; void bump() { synchronized (this) { count++; } } static int classCount; static void classBump() { try { synchronized (Class.forName("BumpTest")) { classCount++; } } catch (ClassNotFoundException e) { ... } } }
defines a class which is designed for concurrent use. Each instance of the classpublic 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; } }
Box
has an instance variable contents
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.
See §17 for more discussion of threads and locks.
Throws: throws ClassTypeList ClassTypeList: ClassType ClassTypeList , ClassTypeA compile-time error occurs if any ClassType mentioned in a
throws
clause is not the class Throwable
or a subclass of Throwable
. It is permitted but not required to mention other (unchecked) exceptions in a throws
clause.
For each checked exception that can result from execution of the body of a method or constructor, a compile-time error occurs unless that exception type or a superclass of that exception type is mentioned in a throws
clause in the declaration of the method or constructor.
The requirement to declare checked exceptions allows the 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 will normally result in a compile-time error because of the lack of a proper exception type in a throws
clause. The Java programming language thus encourages a programming style where rare and otherwise truly exceptional conditions are documented in this way.
The predefined exceptions that are not checked in this way are those for which declaring every possible occurrence would be unimaginably inconvenient:
Error
, for example OutOfMemoryError
, are thrown due to a failure in or of the virtual machine. Many of these are the result of linkage failures and can occur at unpredictable points in the execution of a program. Sophisticated programs may yet wish to catch and attempt to recover from some of these conditions.
RuntimeException
, for example NullPointerException
, result from run--time integrity checks and are thrown either directly from the program or in library routines. It is beyond the scope of the Java programming language, and perhaps beyond the state of the art, to include sufficient information in the program to reduce to a manageable number the places where these can be proven not to occur.
abstract
methods defined in interfaces, may not be declared to throw more checked exceptions than the overridden or hidden method.
More precisely, suppose that B is a class or interface, and A is a superclass or superinterface of B, and a method declaration n in B overrides or hides a method declaration m in A. If n has a throws
clause that mentions any checked exception types, then m must have a throws
clause, and for every checked exception type listed in the throws
clause of n, that same exception class or one of its superclasses must occur in the throws
clause of m; otherwise, a compile-time error occurs.
See §11 for more information about exceptions and a large example.
abstract
(§8.4.3.1) or native
(§8.4.3.4).
MethodBody: Block ;A compile-time error occurs if a method declaration is either
abstract
or native
and has a block for its body. A compile-time error occurs 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 but the implementation requires no executable code, the method body should be written as a block that contains no statements: "{ }
".
If a method is declared void
, then its body must not contain any return
statement (§14.16) that has an Expression.
If a method is declared to have a return type, then every return
statement (§14.16) in its body must have an Expression. 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; it is not allowed to "drop off the end of its body."
Note that it is possible for a method to have a declared return type and yet contain no return statements. Here is one example:
class DizzyDean { int pitch() { throw new RuntimeException("90 mph?!"); } }
abstract
or not) of the superclass and superinterfaces that are accessible to code in the class and are neither overridden (§8.4.6.1) nor hidden (§8.4.6.2) by a declaration in the class.abstract
, then m1 is said to implement any and all declarations of abstract
methods that it overrides.
A compile-time error occurs 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
. Note that 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. See §15.12.4.9 for discussion and examples of this point.
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.
static
method, then the declaration of that method is said to hide any and all methods with the same signature in the superclasses and superinterfaces of the class that would otherwise be accessible to code in the class. A compile-time error occurs 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 (§6.3.1) and obscuring (§6.3.2).
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.
void
. Moreover, a method declaration must not have a throws
clause that conflicts (§8.4.4) with that of any method that it overrides or hides; otherwise, a compile-time error occurs. In these respects, overriding of methods differs from hiding of fields (§8.3), for it is permissible for a field to hide a field of another type.
The access modifier (§6.6) of an overriding or hiding method must provide at least as much access as the overridden or hidden method, or a compile-time error occurs. In more detail:
public
, then the overriding or hiding method must be public
; otherwise, a compile-time error occurs.
protected
, then the overriding or hiding method must be protected
or public
; otherwise, a compile-time error occurs.
private
; otherwise, a compile-time error occurs.
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.
abstract
, then there are two subcases:
abstract
is static
, a compile-time error occurs.
abstract
is considered to override, and therefore to implement, all the other methods on behalf of the class that inherits it. A compile-time error occurs if, comparing the method that is not abstract
with each of the other of the inherited methods, for any such pair, either they have different return types or one has a return type and the other is void
. Moreover, a compile-time error occurs if the inherited method that is not abstract
has a throws
clause that conflicts (§8.4.4) with that of any other of the inherited methods.
abstract
, then the class is necessarily an abstract
class and is considered to inherit all the abstract
methods. A compile-time error occurs if, for any two such inherited methods, either they have different return types or one has a return type and the other is void
. (The throws
clauses do not cause errors in this case.)
abstract
, because methods that are not abstract
are inherited only from the direct superclass, not from superinterfaces.There might be several paths by which the same method declaration might be inherited from an interface. This fact causes no difficulty and never, of itself, results in a compile-time error.
throws
clauses of two methods with the same name but different signatures.Methods are overridden on a signature-by-signature basis.
If, for example, a class declares two public
methods with the same name, and a subclass overrides one of them, the subclass still inherits the other method. In this respect, the Java programming language differs from C++.
When a method is invoked (§15.12), the number of actual 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).
the classclass 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; } }
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
.
the classclass 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; } }
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.
Here the classclass 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; } }
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
.
Here the overriding methodsclass 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); } }
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:
The output from this program is: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 first line of output illustrates the fact that an instance of(0, 0) (2.7182798, 3.14159) (2, 3) (2, 3)
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.
static
) method 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:
produces the output: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()); } }
because the invocation ofGoodnight, Dick
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.
This example produces the output: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"); } }
The classlbo print lbo
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 preexisting contract with subclasses. See the further discussion of binary compatibility in §13, especially §13.2.
BadPointException
:
This example results in a compile-time error, because the override of methodclass 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; } }
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
.
If the 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.
Within a class C, a declaration d of a member type named n shadows the declarations of any other types named n that are in scope at the point where d occurs.
If a member class or interface declared with simple name C is directly enclosed within the declaration of a class with fully qualified name N, then the member class or interface has the fully qualified name N.C.
A class may inherit two or more type declarations 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 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.
It is a compile-time error if a static class contains a usage of a non-static member of an enclosing class.
Member interfaces are always implicitly static. It is permitted but not required for the declaration of a member interface to explicitly list the static modifier.
InstanceInitializer: BlockAn instance initializer of a named class may not throw a checked exception unless that exception or one of its superclasses is explicitly declared in the throws clause of each constructor of its class and the class has at least one explicitly declared constructor. An instance initializer in an anonymous class (§15.9.5) can throw any exceptions.
The rules above distinguish between instance initializers in named and anonymous classes. This distinction is deliberate. A given anonymous class is only instantiated at a single point in a program. It is therefore possible to directly propagate information about what exceptions might be raised by an anonymous class' instance initializer to the surrounding expression. Named classes, on the other hand, can be instantiated in many places. Therefore the only way to propagate information about what exceptions might be raised by an instance initializer of a named class is through the throws clauses of its constructors. It follows that a more liberal rule can be used in the case of anonymous classes. Similar comments apply to instance variable initializers.
It is a compile-time error if an instance initializer cannot complete normally (§14.20). If a return statement (§14.16) appears anywhere within an instance initializer, then a compile-time error occurs.
Use of instance variables whose declarations appear textually after the use is sometimes restricted, even though these instance variables are in scope. See §8.3.2.3 for the precise rules governing forward reference to instance variables.
Instance initializers are permitted to refer to the current object this (§15.8.3) and to use the keyword super (§15.11.2, §15.12).
StaticInitializer: static BlockIt is a compile-time error for a static initializer to be able to complete abruptly (§14.1, §15.6) with a checked exception (§11.2). It is a compile-time error if a static initializer cannot complete normally (§14.20).
The static initializers and class variable initializers are executed in textual order.
Use of class variables whose declarations appear textually after the use is sometimes restricted, even though these class variables are in scope. See §8.3.2.3 for the precise rules governing forward reference to class variables.
If a return
statement (§14.16) appears anywhere within a static initializer, then a compile-time error occurs.
If the keyword this
(§15.8.3) or the keyword super
(§15.11, §15.12) appears anywhere within a static initializer, then a compile-time error occurs.
ConstructorDeclaration: ConstructorModifiersopt ConstructorDeclarator Throwsopt ConstructorBody ConstructorDeclarator: SimpleTypeName ( FormalParameterListopt )The SimpleTypeName in the ConstructorDeclarator must be the simple name of the class that contains the constructor declaration; otherwise a compile-time error occurs. In all other respects, the constructor declaration looks just like a method declaration that has no result type.
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 (§8.8.5). Constructors are never invoked by method invocation expressions (§15.12).class Point { int x, y; Point(int x, int y) { this.x = x; this.y = y; } }
Access to constructors is governed by access modifiers (§6.6).
This is useful, for example, in preventing instantiation by declaring an inaccessible constructor (§8.8.8). Constructor declarations are not members. They are never inherited and therefore are not subject to hiding or overriding.
ConstructorModifiers: ConstructorModifier ConstructorModifiers ConstructorModifier ConstructorModifier: one of public protected privateThe access modifiers
public
, protected
, and private
are discussed in §6.6. A compile-time error occurs if the same modifier appears more than once in a constructor declaration, or if a constructor declaration has more than one of the access modifiers public
, protected
, and private
.
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
and 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.
Note that a ConstructorModifier cannot be declared strictfp
. This difference in the definitions for ConstructorModifier and MethodModifier (§8.4.3) is an intentional language design choice; it effectively ensures that a constructor is FP-strict (§15.4) if and only if its class is FP-strict.
throws
clause for a constructor is identical in structure and behavior to the throws
clause for a method (§8.4.4).
ConstructorBody: { ExplicitConstructorInvocationopt BlockStatementsopt }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 is implicitly assumed by the compiler to begin 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, the body of a constructor is like the body of a method (§8.4.5). A return
statement (§14.16) may be used in the body of a constructor if it does not include an expression.
the first constructor ofclass 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; } }
ColoredPoint
invokes the second, providing an additional argument; the second constructor of ColoredPoint
invokes the constructor of its superclass Point
, passing along the coordinates.§12.5 and §15.9 describe the creation and initialization of new class instances.
ExplicitConstructorInvocation: this ( ArgumentListopt ) ; super ( ArgumentListopt ) ; Primary.super ( ArgumentListopt ) ;
Explicit constructor invocation statements can be divided into two kinds:
An explicit constructor invocation statement in a constructor body may not refer to any instance variables or instance methods declared in this class or any superclass, or useclass Outer { class Inner{} } class ChildOfInner extends Outer.Inner { ChildOfInner(){(new Outer()).super();} }
this
or super
in any expression; otherwise, a compile-time error occurs.
For example, if the first constructor of ColoredPoint
in the example above were changed to:
then a compile-time error would occur, because an instance variable cannot be used within a superclass constructor invocation.ColoredPoint(int x, int y) { this(x, y, color); }
If an anonymous class instance creation expression appears within an explicit constructor invocation statement, then the anonymous class may not refer to any of the enclosing instances of the class whose constructor is being invoked.
Let C be the class being instantiated, let S be the direct superclass of C, and let i be the instance being created. The evaluation of an explicit constructor invocation proceeds as follows:class Top { int x; class Dummy { Dummy(Object o) {} } class Inside extends Dummy { Inside() { super(new Object() { int r = x; }); // error } Inside(final int y) { super(new Object() { int r = y; }); // correct } } }
super
" is evaluated. If the primary expression 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. Let O be the immediately lexically enclosing class of S; it is a compile-time error if the type of p is not O or a subclass of O.
Object
, then the default constructor has an empty body.
A default constructor has no throws
clause.
It follows that if the nullary constructor of the superclass has a
It follows that is the nullary constructor of the superclass has a
throws
clause, then a compile-time error will occur.
If the class is declared public, then the default constructor is implicitly given the access modifier public (§6.6); if the class is declared protected, then the default constructor is implicitly given the access modifier protected (§6.6); if the class is declared private, then the default constructor is implicitly given the access modifier private (§6.6); otherwise, the default constructor has the default access implied by no access modifier.
is equivalent to the declaration:public class Point { int x, y; }
where the default constructor ispublic class Point { int x, y; public Point() { super(); } }
public
because the class Point
is public
."> The rule that the default constructor of a class has the same access modifier 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
The constructor forpackage p1; public class Outer { protected class Inner{} } package p2; class SonOfOuter extends p1.Outer { void foo() { new Inner(); // compile-time access error } }
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.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 declaring no constructor that is public
.the classclass ClassOnly { private ClassOnly() { } static String just = "only the lonely"; }
ClassOnly
cannot be instantiated, while in the example:
the classpackage just; public class PackageOnly { PackageOnly() { } String[] justDesserts = { "cheesecake", "ice cream" }; }
PackageOnly
can be instantiated only within the package just
, in which it is declared.
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