檢視 SCJP 1.6版考題 133 的原始碼

 Given:
 10. public class ClassA{
 11. public void methodA(){
 12.   ClassB classB = new ClassB();
 13.   classB.getValue();
 14. }
 15. }
 And:
 20. class ClassB{
 21.   public ClassC classC;
 22.
 23.   public String getValue(){
 24.   return classC.getValue();
 25.   }
 26. }
  
 
 And:
 30. class ClassC{
 31. public String value;
 32.
 33. public String getValue(){
 34.   value = "ClassB";
 35.   return value;
 36. }
 37. }
 And given:
 ClassA a = new ClassA();
 a.methodA();
 
  
 What is the result?
  A. Compilation fails.
  B. ClassC is displayed.
  C. The code runs with no output.
  D. An exception is thrown at runtime .

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

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

<span style="font-size:medium;"> 解說:
 classC.getValue() 要叫用類別C中的物件方法getValue，
 classC現值為null，未指向一個類別C的物件，
 以null參考來叫用一個物件方法會得到一個nullpointException例外。

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{{SCJP 1.6版考題講解}}