檢視 SCJP 1.6版考題 041 的原始碼

 1. public class KungFu{
 2.   public static void main(String[] args){
 3.     Integer x = 400;
 4.     Integer y = x;
 5.     x++;
 6.     StringBuilder sb1 = new StringBuilder("123");
 7.     StringBuilder sb2 = sb1;
 8.     sb1.append("5");
 9.     System.out.println((x == y) + " " + (sb1 == sb2));
 10. }
 11.}
 
 What is the result?
   A. true true
   B. false true
   C. true false
   D. false false
   E. Compilation fails.
   F. 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: B   </span>

<span style="font-size:medium;"> 解說:
 Integer x = 400; -> (auto boxing) Integer x = new Integer(400);
 Integer y = x; -> (auto boxing) Integer y = new Integer (x); 
    //Integer的建構子要代入的是整數值，因此，x被unboxing成整數 ->   Integer (400)
 x++ -> x.add(1);
 x與y應看成二個整數變數，不是物件參考
 String是不可變的字串物件
 
 Ex.
   String s = “ABC”; s.concat(“D”);
   此時會產生一個新字串物件”ABCD”，只是，s還是指向舊的字串物件”ABC”
   StringBuilder和StringBuffer物件其內含的字串是可變的
 
 Ex.
   StringBuilder s = new StringBuilder("ABC");
   s.append("D");
   s所指向的字串物件內容由”ABC”變為”ABCD”
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{{SCJP 1.6版考題講解}}