SCJP題庫第041題

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.

Ans:B

解說:

Integer x = 400; è (auto boxing) Integer x = new Integer(400);

Integer y = x; à (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 à Integer (400)

x++ à 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”