docs: @Override
      public void addListener(final WorkerListener listener, final WorkerEvent... events) {
          if (listener != null) {
              for (final WorkerEvent event : events) {
                  final ConcurrentSet<WorkerListener> listeners = this.eventListenerMap.get(event);
                  if (listeners != null) {
                      listeners.add(listener);
                  }
              }
          }
      }