我看不到你增加大小变量的任何地方。

你在 isEmpty中依赖它方法，但它没有在 insertFirst 方法中递增。



希望这有帮助，

Fredrik

I can't see any place where you're increasing the size variable.
You're relying on it in the isEmpty method but it's not incremented in the insertFirst method.

Hope this helps,
Fredrik

removeLast必须如下：

removeLast must be like:

if (!isEmpty()) {
  Node<T> last = trailer.getPrev();
  T o = last.getElement();
  Node<T> secondtolast = last.getPrev();
  trailer.setPrev(secondtolast);
  secondtolast.setNext(trailer);
  size--;
  return o;
  } else {
    throw new DequeEmptyException("Deque is empty!");
  }