Java Reference
In-Depth Information
public void clear() {
super.clear();
color = null;
}
}
Pixel
extends both the
data
and
behavior
of its
Point
superclass.
Pixel
extends the data by adding a field named
color
.
Pixel
also extends the
behavior of
Point
by overriding
Point
's
clear
method.
Pixel
objects can be used by any code designed to work with
Point
ob-
jects. If a method expects a parameter of type
Point
, you can hand it a
Pixel
object and it just works. All the
Point
code can be used by anyone
with a
Pixel
in hand. This feature is known as
polymorphism
a single ob-
ject like
Pixel
can have many (
poly-
) forms (
-morph
) and can be used
as both a
Pixel
object and a
Point
object.
Pixel
's behavior extends
Point
's behavior. Extended behavior can be en-
tirely new (adding color in this example) or can be a restriction on old
behavior that follows all the original requirements. An example of re-
stricted behavior might be
Pixel
objects that live inside some kind of
Screen
object, restricting
x
and
y
to the dimensions of the screen. If the
original
Point
class did not forbid restrictions for coordinates, a class with
restricted range would not violate the original class's behavior.
An extended class often
overrides
the behavior of its superclass by
providing new implementations of one or more of the inherited methods.
To do this the extended class defines a method with the same signature
and return type as a method in the superclass. In the
Pixel
example,
we override
clear
to obtain the proper behavior that
Pixel
requires. The
clear
that
Pixel
inherited from
Point
knows only about
Point
's fields but
obviously can't know about the new
color
field declared in the
Pixel
sub-
class.
