Java 1.5 Tiger A Developers Notebook [Electronic resources] نسخه متنی

1.4 Overriding Return Types

One of the most annoying
features when you're using Java inheritance is
the inability to override return types. This is most commonly desired
when you've got a base class, and then a subclass adds a dimension
(either literally or figuratively) to the base class. Typically, you're unable
to return that extra dimension without defining a new method (and new
name), since the method that the base class used probably had a narrower
return type. Thankfully, you can solve this problem using Tiger.

1.4.1 How do I do that?

Example 1-4 is a simple class hierarchy that

demonstrates overriding the return type of a superclass's method.

NOTE

Keep in mind that
none of this code
compiles under
Java 1.4, or even in
Tiger without the
"-source 1.5" switch

Example 1-4. Overriding the methods of a superclass

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;
}
}
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; 
}
}
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;
}
}
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;
}
}

The key is the line public Point3D getLocation( ), which probably
looks pretty odd to you, but get used to it. This is called a covariant
return, and is
only allowed if the return type of the subclass is an extension
of the return type of the superclass. In this case, this is satisfied by
Point3D extending Point2D. It's accomplished through the annotation,
covered in detail in Chapter 6.