SCJP題庫第051題

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.

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…