@Override
  public Iterator<E> iterator() {
    throw new UnsupportedOperationException();
  }
Creates a PriorityQueue with the specified capacity that orders its elements according to the
specified comparator.
@param capacity   the capacity of this queue
@param comparator the comparator that will be used to order this priority queue
hbase-server/src/main/java/org/apache/hadoop/hbase/util/BoundedPriorityBlockingQueue.java