Question

What is wrong with Class A below that won’t allow it to compile?

public class GenericsHell {
   interface Shape{} 
   interface Circle extends Shape {} 

   interface ShapeHelper<T extends Shape> {
      void draw(T shape);
   }

   class A<T extends Shape> {
      A(T shape, ShapeHelper<? extends Shape> helper) {
         helper.draw(shape); // Issues not-applicable argument error 
      }
   }

   class B {
      B(Circle circle, ShapeHelper<Circle> helper) {
         helper.draw(circle);
      }
   }
}   

Eclipse gives the following error:

The method draw(capture#1-of ? extends Shape) in the type ShapeHelper<capture#1-of ? extends Shape> is not applicable for the arguments (T)

Answer 1

The method draw(capture#1-of ? extends GenericsHell.Shape) in the type GenericsHell.ShapeHelper<capture#1-of ? extends GenericsHell.Shape> is not applicable for the arguments (T)

The problem is that in your declaration, shape is of type T, but you request a ShapeHelper of type <? extends Shape> which means that one could pass as argument a ShapeHelper where S and T are distinct.

You would then call helper<S>.draw(shape<T>); which doesn’t make sense.

A correct implementation for this method would be:

class A<T extends Shape> {
  A(T shape, ShapeHelper<T> helper) {
    helper.draw(shape); 
  }
}

Which ensures that the shape and the shape helper are of compatible types.