檢視 SCJP 1.6版考題 105 的原始碼

 Given:
 1. public class A {
 2.
 3. private int counter = 0;
 4
 5. public static int getInstanceCount(){
 6. return counter;
 7. }
 8.
 9. public A(){
 10. counter++;
 11. }
 12.
 13. }
 
  
 And given this code from Class B:
 25. A a1 = new A();
 26. A a2 = new A();
 27. A a3 = new A();
 28. System.out.println(A.getInstanceCount());
 
 
 What is the result?
   A. Compilation of class A fails.
   B. Line 28 prints the value 3 to System.out.
   C. Line 28 prints the value 1 to System.out.
   D. A runtime error occurs when line 25 executes.
   E. Compilation fails because of an error on line 28.
 

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

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

<span style="font-size:medium;"> 解說:
 getInstanceCount是類別A的類別成員方法，在類別方法中要存取非類別成員counter，即物件成員，
 先要建立物件或以一個明確的物件參考存取counter物件變數
 第6行須改為return new A().counter;

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{{SCJP 1.6版考題講解}}