code: public static RequestQueue newRequestQueue(HttpStack stack) {
          File cacheDir = new File("./", DEFAULT_CACHE_DIR);
  
          String userAgent = "volley/0";
  
  
          if (stack == null) {
  
                  stack = new HurlStack();
  
          }
  
          Network network = new BasicNetwork(stack);
  
          RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
          queue.start();
  
          return queue;
      }
docs: Creates a default instance of the worker pool and calls {@link RequestQueue#start()} on it.
  
  @param stack An {@link HttpStack} to use for the network, or null for default.
  @return A started {@link RequestQueue} instance.