code: protected boolean isEmpty () {
      if (TraceComponent.isAnyTracingEnabled() && tc.isEntryEnabled()) SibTr.entry(this, tc, "isEmpty");
  
      boolean rc;
  
      synchronized (barrier) {
        rc = queue.isEmpty();
      }
  
      if (TraceComponent.isAnyTracingEnabled() && tc.isEntryEnabled()) SibTr.exit(this, tc, "isEmpty", rc);
      return rc;
    }
docs: Return true if the queue is empty