第63題

Given:
11. class ClassA{}
12. class ClassB extends ClassA{}
13. class ClassC extends ClassA{}
and:
21. ClassA p0 = new ClassA();
22. ClassB p1 = new ClassB();
23. ClassC p2 = new ClassC();
24. ClassA p3 = new ClassB();
25. ClassA p4 = new ClassC();
Which three are valid? (Choose three.)
A. p0 = p1;
B. p1 = p2,
C. p2 = p4;
D. p2 = (ClassC)p1;
E. p1 = (ClassB)p3;
F. p2 = (ClassC)p4;
答案：AEF
參考：7-9 多型

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A. p0 = p1; //p1 Class B 跟 p0 Class A 有繼承的關係, 父類別po 可以承接 子類別 p1.
E. p1 = (ClassB)p3; // p3 的實體是 Class B,所以可以強迫轉型, p1的實體市 Class B 所以可以承接 實體為class B 的p3
F. p2 = (ClassC)p4; //同上;