code: public int getNumberAvailable() {
          int result = 0;
          for (int i = 0; i < data.size(); i++) {
              if (data.get(i) != null) {
                  result += 1;
              }
          }
          return result;
      }
docs: Get the number of items already received from the server.
  
  @return Nsumber of received items