The problem

Suppose we now want a type for special 2D shapes that are circles.
We could represent circles by using a class called Circle that is derived from Shape.

Besides the iX and iY fields that Shapes have, Circles also need an iRadius field:

0288: public class Circle extends Shape                             // Circle.java
0289: {
0290:    private int iRadius;
0291:    public Circle(int pX, int pY, int pRadius) {
0292:       super(pX, pY);
0293:       iRadius = pRadius;
0294:    }
0295:    public int getRadius()
0296:    {
0297:       return iRadius;
0298:    }
0299:    public Shape translate(int pX, int pY)
0300:    {
0301:       return new Circle(getX() + pX, getY() + pY, iRadius);
0302:    }
0303:    public boolean equals(Object pObject)
0304:    { 
0305:       return super.equals(pObject) &&
0306:                 iRadius==((Circle)pObject).iRadius;
0307:    }
0308:    public String toString()
0309:    {
0310:       return super.toString() + ":" + iRadius;
0311:    }
0312: }

Note that the translate method creates a Circle object.
In Java 1.4, the method has to have a return type of Shape:
```
0299:    public Shape translate(int pX, int pY)
```

Because it returns a Shape, a client will have to cast if it wants to get a Circle:

0331:                Circle tCircle = new Circle(x, y, radius);
0332:                Circle tNewCircle = (Circle)tCircle.translate(1, 2);
0333:                System.out.println(tNewCircle.getRadius());