synchronized (this) {
        if (!executed) {
          RunnableExecutorPair newTail = new RunnableExecutorPair(runnable, executor);
          if (head == null) {
            head = newTail;
            tail = newTail;
          } else {
            tail.next = newTail;
            tail = newTail;
          }
          return;
        }
      }
      executeListener(runnable, executor);