This seems very good to me compared by the old way of all kinds of weird enqueue methods, the more we rely on the core Java the better right :+1: Might be overkill, but we might be able to use a [ConcurrentSkipListMap](http://docs.oracle.com/javase/6/docs/api/java/util/concurrent/ConcurrentSkipListMap.html) as a threadsafe alternative for a treeset.