public void addThumbnailChangeListener(ThumbnailChangeListener listener){
  if (!mListeners.contains(listener)) {
    mListeners.add(listener);
  }
}

public void addThumbnailChangeListener ( ThumbnailChangeListener listener ) { if ( ! mListeners . contains ( listener ) ) { mListeners . add ( listener ) ; } }
add a listener to thumbnail changes .
