java字符串相加的解析

程序如下：

public class mytest {
public static void StringAppend(){
String a = "aa";
String b = "bb";
a = a + b;
}
public static void main(String[] args) {
}
}

编译后，利用javap查看字节码文件：

>javap -c mytest

结果如下：

H:/Project/BaoGang/BaoAudit/build/classes/Audit/Util>javap -c mytest
Compiled from "mytest.java"
public class Audit.Util.mytest extends java.lang.Object{
public Audit.Util.mytest();
Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."<init>":()V
4: return

public static void StringAppend();
Code:
0: ldc #2; //String aa
2: astore_0
3: ldc #3; //String bb
5: astore_1
6: new #4; //class java/lang/StringBuilder
9: dup
10: invokespecial #5; //Method java/lang/StringBuilder."<init>":()V
13: aload_0
14: invokevirtual #6; //Method java/lang/StringBuilder.append:(Ljava/lang/
String;)Ljava/lang/StringBuilder;
17: aload_1
18: invokevirtual #6; //Method java/lang/StringBuilder.append:(Ljava/lang/
String;)Ljava/lang/StringBuilder;
21: invokevirtual #7; //Method java/lang/StringBuilder.toString:()Ljava/la
ng/String;
24: astore_0
25: return

public static void main(java.lang.String[]);
Code:
0: return

}

H:/Project/BaoGang/BaoAudit/build/classes/Audit/Util>

很明显，在第6行new了一个StringBuilder（jdk是1.5版本），然后调用它的append()方法。

记住一点：String是一个常量，一旦初始化，不会再改变。