How do I ensure the type of a generic method in my implementation?

Question

This is my first time asking a question here so I hope I post everything correctly.

I'm working on my assignment and I'm a little stuck on this problem. There are four classes of shapes (circle, cone, sphere, and rectangle) that all implement the interface GeometricShape....

public interface GeometricShape {
    public void describe();  
}

The question says to add a new method called supersize() to the interface, which will take the current shape and return a shape of the same type that is double the size using generics. The hint says to generalize the interface as a start like this...

public interface GeometricShape<T extends GeometricShape<T>> {
    public void describe();
    public T supersize();
}

so that T can only be a geometric shape. But when done this way, it is possible for Rectangle.supersize() to return a circle. How can I make it so that this doesn't happen (ex. Rectangle.supersize() can only return Rectangle) by only modifying the interface code?

Answer 1

The trick is not in the interface definition but in the class declarations.

For rectangle, define it as such:

public class Rectangle implements GeometricShape<Rectangle> {
   public void describe() {// do stuff}
   public Rectangle supersize() {
      return new Rectangle()
      //this should fail since you have specified T
      //return new Circle()
   }
}