code: protected Queue<AsyncResourceRequest<V>> getRequestQueueForKey(K key) {
          Queue<AsyncResourceRequest<V>> requestQueue = requestQueueMap.get(key);
          if(requestQueue == null) {
              Queue<AsyncResourceRequest<V>> newRequestQueue = new ConcurrentLinkedQueue<AsyncResourceRequest<V>>();
              requestQueue = requestQueueMap.putIfAbsent(key, newRequestQueue);
              if(requestQueue == null) {
                  requestQueue = newRequestQueue;
              }
          }
          return requestQueue;
      }
docs: /*
  Get the queue of work for the given key. If no queue exists, create one.