Ikk：新頁面: 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")...

2013-03-30T07:48:49Z

新頁面: 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")...

新頁面

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”
</span>
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{{SCJP 1.6版考題講解}}

SCJP 1.6版考題 041 - 修訂歷史

Ikk：新頁面: 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")...