code: public static long sumFutures(List<Future<Object>> futures) {
          long total = 0;
  
          for (Future<Object> f : futures) {
              try {
                  total += (Long) f.get();
              } catch (Exception ignore) {
                  System.out.println("### sumFutures got exception!");
              }
          }
  
          return total;
      }
docs: This must NOT run on any IO worker thread