Generic Restriction Hell: Bound Mismatch

Question

I'm working on a project that has an extensive tree of generic inheritance and dependencies. Go to edit to see better example. The basics look something like this:

class A {
  ...
}

class B {
  ...
}

class C extends B {
  ...
}

class D<T extends B> extends A {
  ...
}

class StringMap<T extends A> {
   HashMap<String, T> _elements;
   ...
}

So now I'm going to write a class that contains a specific StringMap type.

class X {
  StringMap<D<C>> _thing = new StringMap<D<C>>;
  ... 
}

So far this all works fine. D<C> is actually a very long name and the specific combination is going to show up very frequently in other parts of the code, so I decided to a class for the specific combination so it will be clearer and have a shorter name.

class DC extends D<C> {

}

//and go to update X
class X {
  StringMap<D<C>> _thing = new StringMap<D<C>>(); //still works fine
  StringMap<DC> _thing = new StringMap<DC>(); //error
  ... 
}

Eclipse gives the error of

Bound mismatch: The type DC is not a valid substitute for the bounded parameter <T extends A> of the type StringMap<T>

So the question is, why does this not just work? DC does nothing but extend D<C> and echo the constructors. Why does StringMap see DC as different when it is just a child class of something it excepts?

EDIT:
OK, reworked the example to be closer to what I'm actually doing. I tested it and it does produce the error. What I'm doing here is using the generic type to ensure that clone() returns the correct class for whoever implements it down the inheritance tree. Then in subclasses, I'm using B<T extends B<T>> to ensure that subclasses of B are passing in a subclass of B as the generic type T.

public abstract class Undoable<T> implements Comparable<T> {
  public abstract T clone();
  public abstract void updateFields(T modified);
}

abstract public class A<T extends A<T, U>, U extends Comparable<U>>
    extends Undoable<T> {
  abstract U getKey();

  @Override
  public int compareTo(T element)
  {
    return getKey().compareTo(element.getKey());
  }
}

public class B<T extends B<T>> extends A<T, String> {
  @Override
  public T clone()
  {
    // TODO Auto-generated method stub
    return null;
  }

  @Override
  public void updateFields(T modified)
  {
    // TODO Auto-generated method stub
  }

  @Override
  String getKey()
  {
    // TODO Auto-generated method stub
    return null;
  }
}

public class C extends B<C> {

}

public class D<T extends B<T>> extends A<D<T>, String> {
  @Override
  String getKey()
  {
    // TODO Auto-generated method stub
    return null;
  }

  @Override
  public D<T> clone()
  {
    // TODO Auto-generated method stub
    return null;
  }

  @Override
  public void updateFields(D<T> modified)
  {
    // TODO Auto-generated method stub
  }
}

public class DC extends D<C> {

}

public class StringMap<T extends Undoable<T>> {
  HashMap<String, T> _elements;

}

public class Main {
  public static void main(String[] args)
  {
    StringMap<D<C>> _thing = new StringMap<D<C>>(); //works
    StringMap<DC> _thing1 = new StringMap<DC>(); //error
//Bound mismatch: The type DC is not a valid substitute for
//the bounded parameter <T extends Undoable<T>> of the type StringMap<T>

  }
}

Answer 1

You must be doing wrong something else as the following works fine:

import java.util.HashMap;

public class Q {
    class A {
    }
    class B {
    }
    class C extends B {
    }
    class D<T extends B> extends A {
    }

    class StringMap<T extends A> {
        HashMap<String, T> _elements;
    }

    class DC extends D<C> {

    }

    //and go to update X
    class X {
        StringMap<D<C>> thing1 = new StringMap<D<C>>(); // still works fine
        StringMap<DC> thing2 = new StringMap<DC>(); // NO error!!!
    }
}

Try to post such a class reproducing your error.