CLASSES

Superclasses and Subclasses

8.1.3

8.1.2.2

final

Classes

A class can be declared

final

 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 ( 8.1.2.1).

Because a

final

 class never has any subclasses, the methods of a

final

 class

are never overridden ( 8.4.6.1).

8.1.3   Superclasses and Subclasses

The optional

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 defini 

tion of the class

java.lang.Object

 ( 20.1), 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

java.lang.Object

 as its implicit

direct superclass.

Super:

extends

ClassType

The following is repeated from  4.3 to make the presentation here clearer:

ClassType:

TypeName

The

ClassType

 must name an accessible ( 6.6) class type, or a compile time

error occurs. All classes in the current package are accessible. Classes in other

packages are accessible if the host system permits access to the package ( 7.2)

and the class is declared

public

. If the specified

ClassType

names a class that is

final

 ( 8.1.2.2), then a compile time error occurs;

final

 classes are not allowed

to have subclasses.

In the example:

class Point { int x, y; }

final class ColoredPoint extends Point { int color; }

class Colored3DPoint extends ColoredPoint { int z; } //

error

the relationships are as follows:

133