Illustration of boilerplate introduced by the interface-listener pattern used in Android apps to communication between fragment & activity

interface_listener_boilerplate.java

public class FragmentA extends Fragment{
    OnFragmentInteractionListener listener;

    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);
        try {
            listener = (OnFragmentInteractionListener) activity;
        } catch (ClassCastException e) {
            throw new ClassCastException(activity.toString() + " must implement OnFragmentInteractionListener");
        }
    }

    @Override
    public void onDetach() {
        super.onDetach();
        listener = null;
    }

    public interface OnFragmentInteractionListener{
        void foo();
        void bar(); 
    }

    /* 
     * Calls to interface methods could involve not null checks as all the 
     * possible containing activities may not implement the interface
    */
    if(listener != null){
        listener.foo();
    }
}

public class ActivityA extends Activity implements FragmentA.OnFragmentInteractionListener, 
                                        FragmentB.OnFragmentInteractionListener,
                                        FragmentC.OnFragmentInteractionListener, 
                                        FragmentD.OnFragmentInteractionListener

....