interface Foo<T> {

    Collection<T> blah();
  }

  class FooImpl<T> implements Foo<T> {

    @Override
    public List<T> blah() {
      return null;
    }
  }

  class FooExt<T> extends FooImpl<T> {

    @Override
    public ArrayList<T> blah() {
      return null;
    }
  }