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)。
3
Now I can code the behavior for the pop method, which ensures that if the stack is logically empty, an exception will be thrown.