Ikk：新頁面: 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...

2013-03-30T07:21:50Z

新頁面: 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...

新頁面

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.

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<span style="font-size:medium;">解答</span>

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<span style="font-size: medium;">Ans: C </span>

<span style="font-size:medium;"> 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…
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{{SCJP 1.6版考題講解}}

SCJP 1.6版考題 051 - 修訂歷史

Ikk：新頁面: 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...