8.1.3

Superclasses and Subclasses

CLASSES

The class

Point

 is a direct subclass of

java.lang.Object

.

The class

java.lang.Object

 is the direct superclass of the class

Point

.

The class

ColoredPoint

 is a direct subclass of class

Point

.

The class

Point

 is the direct superclass of class

ColoredPoint

.

The declaration of class

Colored3dPoint

 causes a compile time error because it

attempts to extend the

final

 class

ColoredPoint

.

The

subclass

relationship is the transitive closure of the direct subclass rela 

tionship. A class

A

 is a subclass of class

C

 if either of the following is true:

A

 is the direct subclass of

C

.

There exists a class

B

 such that

A

 is a subclass of

B

, and

B

 is a subclass of

C

,

applying this definition recursively.

Class

C

 is said to be a

superclass

of class

A

 whenever

A

is a subclass of

C

.

In the example:

class Point { int x, y; }

class ColoredPoint extends Point { int color; }

final class Colored3dPoint extends ColoredPoint { int z; }

the relationships are as follows:

The class

Point

 is a superclass of class

ColoredPoint

.

The class

Point

 is a superclass of class

Colored3dPoint

.

The class

ColoredPoint

 is a subclass of class

Point

.

The class

ColoredPoint

 is a superclass of class

Colored3dPoint

.

The class

Colored3dPoint

 is a subclass of class

ColoredPoint

.

The class

Colored3dPoint

 is a subclass of class

Point

.

A compile time error occurs if a class is declared to be a subclass of itself. For

example:

class Point extends ColoredPoint { int x, y; }

class ColoredPoint extends Point { int color; }

causes a compile time error. If circularly declared classes are detected at run time,

as classes are loaded ( 12.2), then a

ClassCircularityError

 is thrown.

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