code: private boolean enqueueLocked(Reference<? extends T> r) {
          // Verify the reference has not already been enqueued.
          if (r.queueNext != null) {
              return false;
          }
  
          /* J2ObjC removed.
          if (r instanceof Cleaner) {
              // If this reference is a Cleaner, then simply invoke the clean method instead
              // of enqueueing it in the queue. Cleaners are associated with dummy queues that
              // are never polled and objects are never enqueued on them.
              Cleaner cl = (sun.misc.Cleaner) r;
              cl.clean();
  
              // Update queueNext to indicate that the reference has been
              // enqueued, but is now removed from the queue.
              r.queueNext = sQueueNextUnenqueued;
              return true;
          }
          */
  
          if (tail == null) {
              head = r;
          } else {
              tail.queueNext = r;
          }
          tail = r;
          tail.queueNext = SENTINEL;
          return true;
      }
docs: Enqueue the given reference onto this queue.
  The caller is responsible for ensuring the lock is held on this queue,
  and for calling notifyAll on this queue after the reference has been
  enqueued. Returns true if the reference was enqueued successfully,
  false if the reference had already been enqueued.
  @GuardedBy("lock")