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 IncreamentCompiled from "Increament.java"public class Increament extends java.lang.Object{public Increament();  Code:   0:   aload_0   1:   invokespecial   #1; //Method java/lang/Object."
     
      ":()V   4:   returnpublic static void main(java.lang.String[])   throws java.lang.InterruptedException;  Code:   0:   invokestatic    #2; //Method decrement:()V   3:   invokestatic    #3; //Method increment:()V   6:   returnpublic 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:  returnpublic 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....