code: public <T extends Widget & Checkable> boolean addOnCheckChangedListener
              (OnCheckChangedListener listener) {
          final boolean added;
          synchronized (mListeners) {
              added = mListeners.add(listener);
          }
          if (added) {
              List<T> c = getCheckableChildren();
              for (int i = 0; i < c.size(); ++i) {
                  listener.onCheckChanged(this, c.get(i), i);
              }
          }
          return added;
      }
docs: Add a listener to be invoked when the checked button changes in this group.
  @param listener
  @param <T>
  @return