pop empty stack的意思|示意

In this case, ensuring that pop throws an exception if push hasn't been called (or logically, if the stack is empty) means that the stack has a state.

在这种情况下，如果push没有被调用(或者从逻辑上讲，栈为空)的情况下确保pop抛出一个异常，则意味着栈有一个状态。

If you called pop again, then foo should be returned and the stack should be empty (assuming there wasn't anything in it before you added the two objects).

如果再次调用pop，那么应该返回foo，然后栈为空(假设在添加这两个对象之前栈中没有对象)。

Now I can code the behavior for the pop method, which ensures that if the stack is logically empty, an exception will be thrown.

现在我可以为pop方法编写行为，即确保当栈在逻辑上为空时，抛出一个异常。