ManageListeners template

ManageListeners.java

public void addListener(Listener listener) {
  if (listeners == null) {
    listeners = new ArrayList<>();
  }
  listeners.add(listener);
}

public void removeListener(Listener listener) {
  if (listeners != null) {
    int position = listeners.indexOf(listener);
    if (position != -1) {
      listeners.remove(position);
    }
  }
}

public void clearListener() {
  if (listeners != null) {
    listeners.clear();
    listeners = null;
  }
}