code: public int getIdleExecutors() {
          int r=0;
          for (Computer c : get_all())
              if((c.isOnline() || c.isConnecting()) && c.isAcceptingTasks())
                  r += c.countIdle();
          return r;
      }
docs: {@code getTotalExecutors()-getBusyExecutors()}, plus executors that are being brought online.