@Override
  public Object[] toArray() {
    lock.lock();
    try {
      return list.toArray();
    } finally {
      lock.unlock();
    }
  }
Returns an array containing all of the elements in this deque, in
proper sequence (from first to last element).
<br>
<p>The returned array will be "safe" in that no references to it are
maintained by this deque.  (In other words, this method must allocate
a new array).  The caller is thus free to modify the returned array.
</p>
<br>This method acts as bridge between array-based and collection-based
APIs.

@return an array containing all of the elements in this deque
llap-server/src/java/org/apache/hadoop/hive/llap/daemon/impl/PriorityBlockingDeque.java