There is a incorect remove method call on ConcurrentHashSet object in inner class QueueImpl.DelayedAddRedistributor.
Check this line on github

Following is problematic part of code.

public class QueueImpl implements Queue {

  private final Set<ScheduledFuture<?>> futures = new ConcurrentHashSet<>();

  private class DelayedAddRedistributor implements Runnable {
        @Override
        public void run() {
            .....
            futures.remove(this); 
            ..... 
       }
   }
}

futures is ConcurrentHashSet of ScheduledFuture<?>. However, QueueImpl.DelayedAddRedistributor.run() calls futures.remove(this), where this is an instance of QueueImpl.DelayedAddRedistributor class. It is not ScheduledFuture<?>. remove method never removes instance of QueueImpl.DelayedAddRedistributor class, since it stores Scheduled Futures.

QueueImpl$DelayedAddRedistributor is incompatible with expected argument type java.util.concurrent.ScheduledFuture<?>