private Map<Class<? extends Event>, Map<String, RegisteredListener>> getListenersByEvent()
    {
        if (this.listenersByEvent == null) {
            initializeListeners();
        }

        return this.listenersByEvent;
    }
private Map < Class < ? extends Event > , Map < String , RegisteredListener > > getListenersByEvent ( ) { if ( this . listenersByEvent == null ) { initializeListeners ( ) ; } return this . listenersByEvent ; }
@return the registered listeners indexed on Event classes so that it's fast to find all the listeners registered
for a given event, so that {@link #notify} calls execute fast and in a fixed amount a time.