InvocationCallback queuePoll() {
      InvocationCallback element;
      synchronized (this) {
         if (tail != head) {
            element = elements[head & mask];
            head++;
         } else {
            element = null;
            frozen = true;
         }
      }
      return element;
   }
InvocationCallback queuePoll ( ) { InvocationCallback element ; synchronized ( this ) { if ( tail != head ) { element = elements [ head & mask ] ; head ++ ; } else { element = null ; frozen = true ; } } return element ; }
Remove one handler from the deque, or freeze the deque if there are no more elements.

@return The next handler, or {@code null} if the deque is empty.