檢視 SCJP 1.6版考題 236 的原始碼

Given: 
<br>1. &nbsp; class Computation extends Thread{ 
<br>2. 
<br>3. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; private int num; 
<br>4. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; private boolean isComplete; 
<br>5. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; private int result; 
<br>6. 
<br>7. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; public Computation(int num){this.num = num;} 
<br>8. 
<br>9. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; public synchronized void run(){ 
<br>10. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; result = num * 2; 
<br>11. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; isComplete = true; 
<br>12. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; notify();
<br>13. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;} 
<br>14. 
<br>15. &nbsp;public synchronized int getResult(){
<br>16. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;while(!isComplete){ 
<br>17. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; try{ 
<br>18. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;wait(); 
<br>19. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;}catch(InterruptedException e){} 
<br>20. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;}
<br>21. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;return result; <br>22. &nbsp; } 
<br>23.
<br>24. &nbsp; public static void main(String[] args){ 
<br>25. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Computation[] computations = new Computation[4]; 
<br>26. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; for(int i=0; i&lt;computations.length; i++){ 
<br>27. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;computations[i] = new Computation(i); 
<br>28. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;computations[i].start(); 
<br>29. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;} 
<br>30. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; for(Computation c&nbsp;: computations) 
<br>31. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; System.out.print(c.getResult() + " "); 
<br>32. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;} 
<br>33. &nbsp; } 
<br> 
<br>
<br>What is the result? <br> 
<br>A. The code will deadlock. 
<br>B. The code may run with no output. 
<br>C. An exception is thrown at runtime. 
<br>D. The code may run with output "0 6".
<br>E. The code may run with output "2 0 6 4".
<br>F. The code may run with output "0 2 4 6". 
 




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

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

<span style="font-size:medium;"> 解說:
第25~29行建立四個Thread物件，然後分別啟動這四個執行緒物件… 
<br> <br>緊接著30~31行分別讓四個執行緒物件呼叫getResult方法，然後分別進入對各自執行緒物件的等待事件中… 
<br>最後四個執行緒的run方法分別通知各自執行緒離開wait狀態…
<br> 
<br>比較可能的輸出是F，因為各執行緒物件的num是依for迴圈0~3再乘以2得來，輸出依0 2 4 6的機率較高。
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