当mockito当时返回间谍不工作？

Question

来自文档：Important gotcha on spying real objects!

List list = new LinkedList();
List spy = spy(list);

//Impossible: real method is called so spy.get(0) throws IndexOutOfBoundsException (the list is yet empty)
when(spy.get(0)).thenReturn("foo");

//You have to use doReturn() for stubbing
doReturn("foo").when(spy).get(0);

那么，when thenReturn何时无效{？1}}？

same question here