public void put(E e) {
    if (e == null) {
      throw new NullPointerException();
    }
    lock.writeLock().lock();
    try {
      delegate.add(e);
      if (!delegate.isEmpty()) {
        notEmpty.signal();
      }
    } finally {
      lock.writeLock().unlock();
    }
  }
Inserts the specified element at the tail of the queue, if it's not already present.
@param e the element to add
hbase-server/src/main/java/org/apache/hadoop/hbase/master/normalizer/RegionNormalizerWorkQueue.java