SCJP題庫第058題

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;

 

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