Java Reference
In-Depth Information
• It is the only class in Java that does not have a superclass.
• All Java classes inherit the methods of
Object
.
Because every class (except
Object
) has a superclass, classes in Java form a class
hierarchy, which can be represented as a tree with
Object
at its root.
Object
has no superclass, but every other class has exactly one
superclass. A subclass cannot extend more than one super‐
class. See
Chapter 4
for more information on how to achieve a
similar result.
m
g
O
Figure 3-1
shows a partial class hierarchy diagram that includes our
Circle
and
PlaneCircle
classes, as well as some of the standard classes from the Java API.
Figure 3-1. A class hierarchy diagram
Subclass Constructors
Look again at the
PlaneCircle()
constructor from
Example 3-3
:
public
PlaneCircle
(
double
r
,
double
x
,
double
y
)
{
super
(
r
);
// Invoke the constructor of the superclass, Circle()
this
.
cx
=
x
;
// Initialize the instance field cx
this
.
cy
=
y
;
// Initialize the instance field cy
}
Although this constructor explicitly initializes the
cx
and
cy
fields newly defined by
PlaneCircle
, it relies on the superclass
Circle()
constructor to initialize the