Java extends class with inner class type

Anthony :

I have the following sample code and produce a "type parameter is not within its bound" error on the last line. Class C/D reuses lots of code from A/B thru the inheritance. How would I defined class Y to not have the error and still uses class D for type parameter? Is there a way I can define class D to use A.B but still has the signature of D extends S for class Y?

public abstract class S<E extends S<E>> extends somethingElse {}
public abstract class R<E extends S<E>> {}

public class A extends Z {
  public class B extends S<B> {
  }
}

public class C extends A {
  public class D extends A.B {
  }
}

public class X extends R<B> {}  // OK
public class Y extends R<D> {}  // Error: Type parameter D is not within its bound; should extends S<D>

Any help would be appreciated.

Jacob G. :

To get this to compile, you can change E extends S<E> to E extends S<? super E>:

public abstract class R<E extends S<? super E>> {}

The cause of this problem is similar to another question that I answered earlier today.

Without the bounded wildcard, D was extending S<B> instead of S<D>.