public void putFirst(E e) {
    if (e == null) {
      throw new NullPointerException();
    }
    putAllFirst(Collections.singleton(e));
  }
Inserts the specified element at the head of the queue.
@param e the element to add
hbase-server/src/main/java/org/apache/hadoop/hbase/master/normalizer/RegionNormalizerWorkQueue.java