package com.tool.code; 
import java.util.Random; 
public class RandomNum { 
 private static char ch[] = {
 \'0\', \'1\', \'2\', \'3\', \'4\', \'5\', \'6\', \'7\', \'8\', 
 \'9\', \'A\', \'B\', \'C\', \'D\', \'E\', \'F\', \'G\', \'H\', \'I\', \'J\', \'K\', \'L\', 
 \'M\', \'N\', \'O\', \'P\', \'Q\', \'R\', \'S\', \'T\', \'U\', \'V\', \'W\', \'X\', \'Y\', 
 \'Z\', \'a\', \'b\', \'c\', \'d\', \'e\', \'f\', \'g\', \'h\', \'i\', \'j\', \'k\', \'l\', 
 \'m\', \'n\', \'o\', \'p\', \'q\', \'r\', \'s\', \'t\', \'u\', \'v\', \'w\', \'x\', \'y\', 
 \'z\', \'0\', \'1\' };
 //最后又重复两个0和1，因为需要凑足数组长度为64 
private static Random random = new Random(); 
//生成指定长度的随机字符串 
public static String createRandomString(int length) { 
 if (length > 0) { 
  int index = 0; 
  char[] temp = new char[length]; 
  int num = random.nextInt(); 
  for (int i = 0; i < length % 5; i++) { 
   temp[index++] = ch[num & 63];
   //取后面六位，记得对应的二进制是以补码形式存在的。 
   num >>= 6;//63的二进制为:111111 
   // 为什么要右移6位？因为数组里面一共有64个有效字符。
   //为什么要除5取余？因为一个int型要用4个字节表示，也就是32位。
  } 
  for (int i = 0; i < length / 5; i++) { 
   num = random.nextInt(); 
   for (int j = 0; j < 5; j++) { 
    temp[index++] = ch[num & 63]; 
    num >>= 6; 
   } 
  } 
  return new String(temp, 0, length); 
 } else if (length == 0) { 
  return \"\"; 
 } else { 
  throw new IllegalArgumentException(); 
 } 
} 
//根据指定个数，测试随机字符串函数的重复率 
public static double rateOfRepeat(int number){ 
 int repeat=0; 
 String[] str=new String[number]; 
 for(int i=0;i<number;i++){//生成指定个数的字符串 
  str[i]=RandomNum.createRandomString(10); 
 } 
 for(int i=0;i<number;i++){//查找是否有相同的字符串 
  for(int j=i+1;j<number-1;j++){ 
   if(str[i].equals(str[j])) 
    repeat++; 
  } 
 } 
 return ((double)repeat)/number; 
} 
public static void main(String[] args) { 
 System.out.println(RandomNum.createRandomString(16)+\"-\"+RandomNum.createRandomString(16)); 
 double rate=RandomNum.rateOfRepeat(10000);//测试10000次的重复率 
 System.out.println(\"重复率:\"+rate); 
}
}