SCJP 1.6版考題 051

1. public interface A{
2.   public void doSomething(String thing);
3. }
1. public class AImpl implements A{
2.   public void doSomething(String msg){}
3. }
1. public class B{
2.   public A doit(){
3.     //more code here
4.   }
5.
6.   public String execute(){
7.     //more code here
8.   }
9. }
  
1. public class C extends B{
2.   public AImpl doit(){
3.     //more code here
4.   }
5.
6.   public Object execute(){
7.     //more code here
8.   }
9. }
 

Which statement is true about the classes and interfaces?
  A. Compilation will succeed for all classes and interfaces.
  B. Compilation of class C will fail because of an error in line 2.
  C. Compilation of class C will fail because of an error in line 6.
  D. Compilation of class AImpl will fail because of an error in line 2.

解答

---

Ans: C

解說:

Overriding 規則：方法名、參數、回傳皆要和父類別中的方法一樣，此例類別C中的execute方法其傳回值是Object，
與父類別B中的execute中的傳回值String不同，類別C中的execute允許傳回非String類別物件，
因為以最大的Object限定傳回值型態，而Object是所有類別的父類別，也就是說類別C的execute有可能傳回非String類別型態，違背覆載的規則。 

String is an Object. 但Object未必一定是String…