深入JVM(2): 由深入JVM(1)想到的一个面试题

面试题：i++和i--哪个快些？

这个不知道是哪位朋友跟我说的一个面试题，当时我听到这个题目的时候，我也不知所措，或许是对i++和i--的底层实现不知道，也或许没有关注过这个问题.

今天就在这里做个测试：

1.测试环境：电脑配置： 系统配置： jdk版本：jdk1.6.0_20

2.测试代码：

package Sort;



public class Increament {



    /**

     * @param args

     * @throws InterruptedException 

     */

    public static void main(String[] args) throws InterruptedException {

        decrement();

        increment();

    }

    

    

    public static void decrement() {

        long start2 = System.currentTimeMillis();

        for(long i = 100000000L; i > 0; i--) {

        }

        long end2 = System.currentTimeMillis() - start2;

        System.out.println(end2);

    }

    

    public static void increment() {

        long start = System.currentTimeMillis();

        for(long i = 1; i < 100000000L; i ++) {

        }

        long end1 = System.currentTimeMillis() - start;

        System.out.println(end1);

    }

}

3.字节码

D:\jdk1.6.0_20\bin>javap -c Increament

Compiled from "Increament.java"

public class Increament extends java.lang.Object{

public Increament();

  Code:

   0:   aload_0

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

   4:   return



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

  Code:

   0:   invokestatic    #2; //Method decrement:()V

   3:   invokestatic    #3; //Method increment:()V

   6:   return



public static void decrement();

  Code:

   0:   invokestatic    #4; //Method java/lang/System.currentTimeMillis:()J

   3:   lstore_0

   4:   ldc2_w  #5; //long 100000000l

   7:   lstore_2

   8:   lload_2

   9:   lconst_0

   10:  lcmp

   11:  ifle    21

   14:  lload_2

   15:  lconst_1

   16:  lsub

   17:  lstore_2

   18:  goto    8

   21:  invokestatic    #4; //Method java/lang/System.currentTimeMillis:()J

   24:  lload_0

   25:  lsub

   26:  lstore_2

   27:  getstatic       #7; //Field java/lang/System.out:Ljava/io/PrintStream;

   30:  lload_2

   31:  invokevirtual   #8; //Method java/io/PrintStream.println:(J)V

   34:  return



public static void increment();

  Code:

   0:   invokestatic    #4; //Method java/lang/System.currentTimeMillis:()J

   3:   lstore_0

   4:   lconst_1

   5:   lstore_2

   6:   lload_2

   7:   ldc2_w  #5; //long 100000000l

   10:  lcmp

   11:  ifge    21

   14:  lload_2

   15:  lconst_1

   16:  ladd

   17:  lstore_2

   18:  goto    6

   21:  invokestatic    #4; //Method java/lang/System.currentTimeMillis:()J

   24:  lload_0

   25:  lsub

   26:  lstore_2

   27:  getstatic       #7; //Field java/lang/System.out:Ljava/io/PrintStream;

   30:  lload_2

   31:  invokevirtual   #8; //Method java/io/PrintStream.println:(J)V

   34:  return

 

这里为什么要把i置为100亿，因为在这个数字下面他们之间的区别才明显:27765ms，38516ms；经多次测试i++略耗时些。

其实还是不知道原来是什么....