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81 Class Declaration
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A class declaration speci es a new named reference type There are two kinds of class declarations - normal class declarations and enum declarations: ClassDeclaration: NormalClassDeclaration EnumDeclaration
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The rules in this section apply to all class declarations unless this speci cation explicitly states otherwise In many cases, special restrictions apply to enum declarations Enum declarations are described in detail in 89 The Identi er in a class declaration speci es 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 811 Class Modi ers
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A class declaration may include class modi ers ClassModi ers: ClassModi er ClassModi ers ClassModi er
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ClassModi er: one of Annotation public protected private
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abstract static final strictfp
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Not all modi ers are applicable to all kinds of class declarations The access modi er public pertains only to top level classes ( 76) and to member classes ( 85, 95), and is discussed in 66, 85 and 95 The access modi ers protected and private pertain only to member classes within a directly enclosing class declaration ( 85) and are discussed in 851 The access modi er static pertains only to member classes ( 85, 95) A compile-time error occurs if the same modi er appears more than once in a class declaration If an annotation a on a class declaration corresponds to an annotation type T, and T has a (meta-)annotation m that corresponds to annotationTarget, then m must have an element whose value is annotationElementTypeTYPE, or a compile-time error occurs Annotation modi ers are described further in 97
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NormalClassDeclaration: ClassModi ersopt class Identi er TypeParametersopt Superopt Interfacesopt ClassBody
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If two or more class modi ers 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 ClassModi er 8111 abstract Classes An abstract class is a class that is incomplete, or to be considered incomplete Normal classes may have abstract methods ( 8431, 94), that is methods that are declared but not yet implemented, only if they are abstract classes If a normal class that is not abstract contains an abstract method, then a compile-time error occurs Enum types ( 89) must not be declared abstract; doing so will result in a compile-time error It is a compile-time error for an enum type E to have an abstract method m as a member unless E has one or more enum constants, and all of E s enum constants have class bodies that provide concrete implementations of m It is a compile-time error for the class body of an enum constant to declare an abstract method A class C has abstract methods if any of the following is true:
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C explicitly contains a declaration of an abstract method ( 843) Any of C s superclasses has an abstract method and C neither declares nor inherits a method that implements ( 8481) it A direct superinterface ( 815) of C declares or inherits a method (which is therefore necessarily abstract) and C neither declares nor inherits a method that implements it In the example:
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abstract class Point { int x = 1, y = 1; void move(int dx, int dy) { x += dx; y += dy; alert(); } abstract void alert(); }
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abstract class ColoredPoint extends Point { int color; }
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class SimplePoint extends Point { void alert() { } }
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Point p = new Point();
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would result in a compile-time error; the class Point cannot be instantiated because it is abstract However, a Point variable could correctly be initialized with a reference to any subclass of Point, and the class SimplePoint is not abstract, so the statement:
Point p = new SimplePoint();
interface Colorable { void setColor(int color); } abstract class Colored implements Colorable { abstract int setColor(int color); }
result in a compile-time error: it would be impossible for any subclass of class Colored to provide an implementation of a method named setColor, taking one argument of type int, that can satisfy both abstract method speci cations, 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 ( 84) 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
would be correct 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 eld initializers for instance variables of that class Thus, in the example just given, instantiation of a SimplePoint causes the default constructor and eld 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 ( 842) but incompatible return types As an example, the declarations:
a class Point is declared that must be declared abstract, because it contains a declaration of an abstract method named alert The subclass of Point named ColoredPoint inherits the abstract method alert, so it must also be declared abstract On the other hand, the subclass of Point named SimplePoint provides an implementation of alert, so it need not be abstract A compile-time error occurs if an attempt is made to create an instance of an abstract class using a class instance creation expression ( 159) Thus, continuing the example just shown, the statement: