"SCJP 1.6版考題 002" 修訂間的差異

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(新頁面: Given:<br>1.    public class TestSeven extends Thread{<br>2.             private static int x;<br>3.             p...)
 
 
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Given:<br>1. &nbsp; &nbsp;public class TestSeven extends Thread{<br>2. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; private static int x;<br>3. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; public synchronized void doThings(){<br>4. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; int current = x;<br>5. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; current++;<br>6. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; x = current;<br>7. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }<br>8. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;public void run(){<br>9. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; doThings();<br>10. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;}<br>11. &nbsp;}
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1. public class TestString1{
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2.   public static void main(String[] args){
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3.     String str = "420";
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4.     str += 42;
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5.     System.out.print(str);
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6.   }
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7. }
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What is the output?
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    A. 42
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    B. 420
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    C. 462
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    D. 42042
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    E. Compilation fails.
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    F. An exception is thrown at runtime.
  
  
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Which statement is true?<br>A. Compilation fails.<br>B. An exception is thrown at runtime.<br>C. Synchronizing the run() method would make the class thread-safe.<br>D. The data in variable "x" are protected from concurrent access problems.<br>E. Declaring the doThings() method as static would make the class thread-safe.<br>F. Wrapping the statements within doThings() in a synchronized(new Object()){} block would make the class thread-safe.
 
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<span style="font-size:medium;">解答</span>
 
<span style="font-size:medium;">解答</span>
  
 
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<span style="font-size: medium;">Ans: E   </span>
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<span style="font-size: medium;">Ans: D   </span>
  
 
<span style="font-size:medium;"> 解說:
 
<span style="font-size:medium;"> 解說:
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E是說若宣告doThings方法為靜態的方法將使得該類別為"執行緒安全"。
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&nbsp; &nbsp;&nbsp; &nbsp; &nbsp;&nbsp;thread-safe是說不管執行緒如何執行，都不會破壞掉類別資料完整性。
 
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&nbsp; &nbsp;&nbsp; &nbsp;&nbsp; &nbsp;若方法是物件的成員，則多個執行緒就可以執行多個版本的doThings方法，好像多人一起操作一個共同的整數資料x(因為x是靜態的/類別的，只有一個)，
 
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&nbsp; &nbsp;&nbsp; &nbsp;&nbsp; &nbsp;若是把doThings方法變成類別，那麼也就只有一個版本的doThings方法，加上此方法又是同步的，同一時間只有此方法可以對x進行操作，不會因為多人方法的存取x而造成x的破壞。
 
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{{SCJP 1.6版考題講解}}
 
{{SCJP 1.6版考題講解}}

於 2013年3月30日 (六) 09:05 的最新修訂

1. public class TestString1{
2.   public static void main(String[] args){
3.     String str = "420";
4.     str += 42;
5.     System.out.print(str);
6.   }
7. }


What is the output?
   A. 42
   B. 420
   C. 462
   D. 42042
   E. Compilation fails. 
   F. An exception is thrown at runtime.


解答

Ans: D

解說: 無

SCJP 1.6版考題講解清單
基本程式語言
物件類別與介面
繼承、覆載等
物件集合、泛型
垃圾收集
例外與斷言
多緒行緒
同步機制
其他(序別化、I/O等)
取自 "https://fgchen.com/wiki/index.php?title=SCJP_1.6版考題_002&oldid=1427"