code: public void clear() {
          final ReentrantLock lock = this.lock;
          lock.lock();
          try {
              q.clear();
          } finally {
              lock.unlock();
          }
      }
docs: Atomically removes all of the elements from this delay queue.
  The queue will be empty after this call returns.
  Elements with an unexpired delay are not waited for; they are
  simply discarded from the queue.