public Comparator<? super E> comparator() {
      return this.comparator;
    }
A generic bounded blocking Priority-Queue. The elements of the priority queue are ordered
according to the Comparator provided at queue construction time. If multiple elements have the
same priority this queue orders them in FIFO (first-in-first-out) manner. The head of this queue
is the least element with respect to the specified ordering. If multiple elements are tied for
least value, the head is the first one inserted. The queue retrieval operations poll, remove,
peek, and element access the element at the head of the queue.
hbase-server/src/main/java/org/apache/hadoop/hbase/util/BoundedPriorityBlockingQueue.java