SCJP 1.6版考題 058

10. interface Foo{}
11. class Alpha implements Foo{}
12. class Beta extends Alpha{}
13. class Delta extends Beta{
14.   public static void main(String[] args){
15.     Beta x = new Beta();
16.     //insert code here
17.   }
18. }


Which code, inserted at line 16, will cause a java.lang.ClassCastException?
  A. Alpha a = x;
  B. Foo f = (Delta)x;
  C. Foo f = (Alpha)x;
  D. Beta b = (Beta)(Alpha)x;

解答

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Ans: B

解說: x物件是Beta型態，不能向下轉型為其子類別Delta型態。在執行時會發生java.lang.ClassCastException