/* Base fragment to ensure the parent activity implements a contract interface. */
public abstract class ContractFragment<T> extends Fragment {
    private T mContract;

    @Override
    public void onAttach(Activity activity) {
        try {
            mContract = (T)activity;
        } catch (ClassCastException e) {
            throw new IllegalStateException(activity.getClass().getSimpleName()
                + " does not implement " + getClass().getSimpleName() + "'s contract interface.", e);
        }
        super.onAttach(activity);
    }

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

    public final T getContract() {
        return mContract;
    }
}

/* Example fragment showing usage of ContractFragment. */
public class MyCoolFragment extends ContractFragment<MyCoolFragment.Contract> {
    public interface Contract {
        void sayHi(String message);
    }
    
    public void somethingHappened() {
        getContract().sayHi("Hi, Mom!");
    }
}