if (tryLock()) {
        try {
          maybeDrainReferenceQueues();
        } finally {
          unlock();
        }
      }
    }

    @GuardedBy("this")
    void drainKeyReferenceQueue(ReferenceQueue<K> keyReferenceQueue) {
      Reference<? extends K> ref;
      int i = 0;
      while ((ref = keyReferenceQueue.poll()) != null) {
        @SuppressWarnings("unchecked")

     */
    @GuardedBy("this")
    void drainReferenceQueues() {
      if (map.usesKeyReferences()) {
        drainKeyReferenceQueue();
      }
      if (map.usesValueReferences()) {
        drainValueReferenceQueue();
      }
    }

    @GuardedBy("this")
    void drainKeyReferenceQueue() {
      Reference<? extends K> ref;
      int i = 0;
      while ((ref = keyReferenceQueue.poll()) != null) {