code: public void addListener(InstallEventListener listener, String notificationType) {
          if (listener == null || notificationType == null)
              return;
          if (notificationType.isEmpty())
              return;
          if (listenersMap == null) {
              listenersMap = new HashMap<String, Collection<InstallEventListener>>();
          }
          Collection<InstallEventListener> listeners = listenersMap.get(notificationType);
          if (listeners == null) {
              listeners = new ArrayList<InstallEventListener>(1);
              listenersMap.put(notificationType, listeners);
          }
          listeners.add(listener);
      }
docs: Adds an install event listener to the listenersMap with a specified notification type
  
  @param listener InstallEventListener to add
  @param notificationType Notification type of listener