I used to always call a setListener method on my fragment after it was instantiated, but recently this is how I've started attaching my fragment's listener. I like it because it doesn't require the client of the fragment to remember to set the listener, and throws a runtime exception if they don't implement the listener.

HowToAttachAFragmentListenerToActivity.java

private CategoryFragmentListener mListener = sCategoryListener;

private static CategoryFragmentListener sCategoryListener = new CategoryFragmentListener() {

    @Override
    public void onLeafCategoryChosen() {

    }
    
};

@Override
public void onAttach(Activity activity) {
    super.onAttach(activity);

    mListener = (CategoryFragmentListener) activity;
}