code: public void subscribe(Object identifier) {
          final long id = Thread.currentThread().getId();
          queues.put(id, new ConcurrentLinkedQueue<T>());
          if (identifier != null) {
              if (subscriberIdentifiers == null) {
                  subscriberIdentifiers = new ConcurrentHashMap<>();
              }
              subscriberIdentifiers.put(identifier, id);
          }
      }
docs: Subscribes the thread to the queue, providing an identifier object to allow the publisher to recognize and target the subscriber (if not null).
  
  @param identifier The identifier object, can be null