int _countDown= 6;
object _lock;
void ReadyMessageReceivedForTheFirstTimeFromAConnection() {
    lock(_lock) {
      --_countDown; //
       if (_countDown==0) Start();
    }
}

(let [latch (CountDownLatch. 6)]
  (on-receive-message this (fn [_] (.countDown latch)))
  (.await latch)