code: @SuppressWarnings("unchecked") // we know that the returning fragment is child of fragment.
      @TargetApi(Build.VERSION_CODES.HONEYCOMB)
      public static <F extends android.app.Fragment> F findFragmentById(android.app.FragmentManager manager, int id) {
          return (F) manager.findFragmentById(id);
      }
docs: Find a fragment that is under {@link android.app.FragmentManager}'s control by the id.
  @param manager the fragment manager.
  @param id the fragment id.
  @param <F> the concrete fragment class parameter.
  @return the fragment.