code: public <T> Request<T> add(Request<T> request) {
          // Tag the request as belonging to this queue and add it to the set of current requests.
          request.setRequestQueue(this);
          synchronized (mCurrentRequests) {
              mCurrentRequests.add(request);
          }
  
          // Process requests in the order they are added.
          request.setSequence(getSequenceNumber());
          request.addMarker("add-to-queue");
  
          // If the request is uncacheable, skip the cache queue and go straight to the network.
          if (!request.shouldCache()) {
              mNetworkQueue.add(request);
              return request;
          }
  
          // Insert request into stage if there's already a request with the same cache key in flight.
          synchronized (mWaitingRequests) {
              String cacheKey = request.getCacheKey();
              if (mWaitingRequests.containsKey(cacheKey)) {
                  // There is already a request in flight. Queue up.
                  Queue<Request<?>> stagedRequests = mWaitingRequests.get(cacheKey);
                  if (stagedRequests == null) {
                      stagedRequests = new LinkedList<Request<?>>();
                  }
                  stagedRequests.add(request);
                  mWaitingRequests.put(cacheKey, stagedRequests);
                  if (VolleyLog.DEBUG) {
                      VolleyLog.v("Request for cacheKey=%s is in flight, putting on hold.", cacheKey);
                  }
              } else {
                  // Insert 'null' queue for this cacheKey, indicating there is now a request in
                  // flight.
                  mWaitingRequests.put(cacheKey, null);
                  mCacheQueue.add(request);
              }
              return request;
          }
      }
docs: Adds a Request to the dispatch queue.
  @param request The request to service
  @return The passed-in request