question: private static void addListenersForReal ( final Service service , final Set < Listener > existingListeners , final Listener listener , final Executor executor ) { if ( ! existingListeners . contains ( listener ) ) { existingListeners . add ( listener ) ; service . addListener ( listener , executor ) ; } }
target:  Adds the given listener to the service. If the service already contains the given listener it wont be added again.