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.
134






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