java - String.valueOf(long a) 和连接 ("" + a) 之间的区别

Question

哪种方法更好，为什么？

如果我写

cmissValue = String.valueOf(callDBDatasource.cMiss());

或者

cmissValue = "" + callDBDatasource.cMiss();

应该采取什么方法？

Answer 1

"" + callDBDatasource.cMiss();

将编译为：

new StringBuilder().append("").append(callDBDatasource.cMiss()).toString();

这将创建一个新对象，因此速度明显变慢。看到这个问题：字符串连接真的那么慢吗？

Answer 2

这在这里很有用（“将数字转换为字符串”部分）：http ://www.odi.ch/prog/design/newbies.php

不久：

    String.valueOf(callDBDatasource.cMiss());

Answer 3

对于那些感兴趣的人，我已经对这两种情况进行了建模并为它们生成了字节码。

这是第一种情况的程序：

import java.util.Random;

public class Test1 {
  public static void main(String[] args) {
    long l = new Random().nextLong();
    String s = String.valueOf(l);
    System.out.println(s);
  }
}

...这是字节码：

0:  new             #2; //class java/util/Random
3:  dup
4:  invokespecial   #3; //Method java/util/Random."<init>":()V
7:  invokevirtual   #4; //Method java/util/Random.nextLong:()J
10: lstore_1
11: lload_1
12: invokestatic    #5; //Method java/lang/String.valueOf:(J)Ljava/lang/String;
15: astore_3
16: getstatic       #6; //Field java/lang/System.out:Ljava/io/PrintStream;
19: aload_3
20: invokevirtual   #7; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
23: return

这是你所期望的。

现在，这是一个模拟第二种情况的程序：

import java.util.Random;

public class Test2 {
  public static void main(String[] args) {
    long l = new Random().nextLong();
    String s = "" + l;
    System.out.println(s);
  }
}

...这是字节码：

0:  new             #2;  //class java/util/Random
3:  dup
4:  invokespecial   #3;  //Method java/util/Random."<init>":()V
7:  invokevirtual   #4;  //Method java/util/Random.nextLong:()J
10: lstore_1
11: new             #5;  //class java/lang/StringBuilder
14: dup
15: invokespecial   #6;  //Method java/lang/StringBuilder."<init>":()V
18: ldc             #7;  //String 
20: invokevirtual   #8;  //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
23: lload_1
24: invokevirtual   #9;  //Method java/lang/StringBuilder.append:(J)Ljava/lang/StringBuilder;
27: invokevirtual   #10; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
30: astore_3
31: getstatic       #11; //Field java/lang/System.out:Ljava/io/PrintStream;
34: aload_3
35: invokevirtual   #12; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
38: return

As you can see, many more bytecodes are produced than the first case. You can also see (at 11 through 27) that a StringBuilder is used to concatenate the values, as mentioned in the accepted answer.