[求助]有答案都不明白什么意思。。。

We have the following organization of classes.

class Parent { }
class DerivedOne extends Parent { }
class DerivedTwo extends Parent { }

Which of the following statements is correct for the following expression?

Parent p = new Parent();
DerivedOne d1 = new DerivedOne();
DerivedTwo d2 = new DerivedTwo();
d1 = (DerivedOne)d2;

Illegal both compile and runtime
Legal at compile time, but fails at runtime
Legal at compile and runtime
None of the above
给出的答案是A，他的解释如下：
解释是
A is correct. You cannot assign an object to a sibling(no parent-child relation) reference, even with casting.
还请高手指教下。。。。

[此贴子已经被作者于2006-8-11 17:11:24编辑过]

父类是动物，子类分别为猫和狗，不能把猫转换成狗，狗也不能转换为猫
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哦　　明白了　谢　　
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