java中字符串加号的实现

例子展示

public class Test{
public static void main(String[] args){
System.out.println(new String("hello world")+new String("lp"));
}
}

输出结果hello world lp

实现原理

java中的字符串的相加其实是编译器帮我们转化的，编译器会将字符串相加编译成使用StringBuilder拼接相加的字符串。

我们看下使用javap -verbose Test 得到的java字节码。

Code:
stack=5, locals=1, args_size=1
0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
3: new           #3                  // class java/lang/StringBuilder
6: dup
7: invokespecial #4                  // Method java/lang/StringBuilder."<init>":()V
10: new           #5                  // class java/lang/String
13: dup
14: ldc           #6                  // String hello world
16: invokespecial #7                  // Method java/lang/String."<init>":(Ljava/lang/String;)V
19: invokevirtual #8                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
22: new           #5                  // class java/lang/String
25: dup
26: ldc           #9                  // String lp
28: invokespecial #7                  // Method java/lang/String."<init>":(Ljava/lang/String;)V
31: invokevirtual #8                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
34: invokevirtual #10                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
37: invokevirtual #11                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
40: return

我们看到34和37分别是使用了stringbuilder.append(“hello world”)和stringbuilder.append(“lp”)