String的学问

String "a" + "b"+ "c";

到底生成了几个对象？这个问题在网上问了无数次了。

我也看的头昏眼花的，直接测试了一下。

 

 写道

public class Test {
public static void main(String[] args) {
String mango = "mango";
String s = "abc" + mango + "def";
System.out.println(s);
}
}

 

 

Test.java直接放在D盘下

 

cmd 命令行下的结果：

 

D:\>javac Test.java

 

D:\>java Test

abcmangodef

 

D:\>javap -c Test

Compiled from "Test.java"

public class Test extends java.lang.Object{

public Test();

  Code:

   0:   aload_0

   1:   invokespecial   #1; //Method java/lang/Object."<init>":()V

   4:   return

 

public static void main(java.lang.String[]);

  Code:

   0:   ldc     #2; //String mango

   2:   astore_1

   3:   new     #3; //class java/lang/StringBuilder

   6:   dup

   7:   invokespecial   #4; //Method java/lang/StringBuilder."<init>":()V

   10:  ldc     #5; //String abc

   12:  invokevirtual   #6; //Method java/lang/StringBuilder.append:(Ljava/lang/

String;)Ljava/lang/StringBuilder;

   15:  aload_1

   16:  invokevirtual   #6; //Method java/lang/StringBuilder.append:(Ljava/lang/

String;)Ljava/lang/StringBuilder;

   19:  ldc     #7; //String def

   21:  invokevirtual   #6; //Method java/lang/StringBuilder.append:(Ljava/lang/

String;)Ljava/lang/StringBuilder;

   24:  invokevirtual   #8; //Method java/lang/StringBuilder.toString:()Ljava/la

ng/String;

   27:  astore_2

   28:  getstatic       #9; //Field java/lang/System.out:Ljava/io/PrintStream;

   31:  aload_2

   32:  invokevirtual   #10; //Method java/io/PrintStream.println:(Ljava/lang/St

ring;)V

   35:  return

 

}

 

 

D:\>

 

 

从这里可以看出编译器创建了一个StringBuilder对象，用以构造最终的String，并为每个字符串调用一次StringBuilder类的append()

 

到底编译器为String优化到了什么程度呢？写道

public class Test {
public String test(String[] fields){
String res = "";
for(int i = 0; i< fields.length;i++)
res+=fields[i];
return res;
}
}

 

 

 

C:\Documents and Settings\Administrator>D:

 

D:\>javac Test.java

 

D:\>javap -c Test

Compiled from "Test.java"

public class Test extends java.lang.Object{

public Test();

  Code:

   0:   aload_0

   1:   invokespecial   #1; //Method java/lang/Object."<init>":()V

   4:   return

 

public java.lang.String test(java.lang.String[]);

  Code:

   0:   ldc     #2; //String

   2:   astore_2

   3:   iconst_0

   4:   istore_3

   5:   iload_3

   6:   aload_1

   7:   arraylength

   8:   if_icmpge       38

   11:  new     #3; //class java/lang/StringBuilder

   14:  dup

   15:  invokespecial   #4; //Method java/lang/StringBuilder."<init>":()V

   18:  aload_2

   19:  invokevirtual   #5; //Method java/lang/StringBuilder.append:(Ljava/lang/

String;)Ljava/lang/StringBuilder;

   22:  aload_1

   23:  iload_3

   24:  aaload

   25:  invokevirtual   #5; //Method java/lang/StringBuilder.append:(Ljava/lang/

String;)Ljava/lang/StringBuilder;

   28:  invokevirtual   #6; //Method java/lang/StringBuilder.toString:()Ljava/la

ng/String;

   31:  astore_2

   32:  iinc    3, 1

   35:  goto    5

   38:  aload_2

   39:  areturn

 

}

 

 

D:\>

无论如何String pool 里面放的就是最终的字符串？

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