为了运用多态性，当编写子类时，你肯定会覆写超类的方法。当你覆写超类的方法时，你是否想到过更改方法的返回类型。如果你没有，那祝贺你，你不曾为此烦恼过；如果你想到过，那你肯定是非常的郁闷。在Java 5.0以前，你是无法更改超类定义的方法的返回类型的。不过当你使用Java 5.0以后，你会惊喜地发现，此烦恼不复存在了。
下面的示例显示了这一新特性的用法：
package
com.jiang.tiger.chap1;
class
Point2D
{
protected
int
x, y;
public
Point2D( )
{
this
.x
=
0
;
this
.y
=
0
;
}
public
Point2D(
int
x,
int
y)
{
this
.x
=
x;
this
.y
=
y;
}
public
String toString()
{
return
"
the position is x =
"
+
x
+
"
,y =
"
+
y ;
}
}
class
Point3D
extends
Point2D
{
protected
int
z;
public
Point3D(
int
x,
int
y)
{
this
(x, y,
0
);
}
public
Point3D(
int
x,
int
y,
int
z)
{
this
.x
=
x;
this
.y
=
y;
this
.z
=
z;
}
public
String toString()
{
return
"
the position is x =
"
+
x
+
"
,y =
"
+
y
+
"
,z=
"
+
z;
}
}
class
Position2D
{
Point2D location;
public
Position2D( )
{
this
.location
=
new
Point2D( );
}
public
Position2D(
int
x,
int
y)
{
this
.location
=
new
Point2D(x, y);
}
public
Point2D getLocation( )
{
return
location;
}
}
public
class
Position3D
extends
Position2D
{
Point3D location;
public
Position3D(
int
x,
int
y,
int
z)
{
this
.location
=
new
Point3D(x, y, z);
}
public
Point3D getLocation( )
{
return
location;
}
public
static
void
main(String[] args)
{
Position2D position
=
new
Position3D(
12
,
23
,
36
);
Point3D clone
=
(Point3D)position.getLocation();
System.out.println(clone);
}
}
为验证功能是否正确，我们看看运行结果：
the position is x = 12,y = 23,z= 36