Random随机生成ID

[size=medium]常用UUID
  public static String RandomStringId(){
      //  UUID uuid = UUID.randomUUID();
         String s = UUID.randomUUID().toString();
         //去掉“-”符号
         return s.replaceAll("-", ""); 
     }


////此处分开
package com.date.test;

import java.util.Random;

public class Hello {

static Random random=null;
private static char[] numberList=null;
public static void main(String[] str ){

System.out.println("randomString "+randomString(5));
}
static
{
random=new Random();
//参数10000是指0-9999的数
int number=random.nextInt(10000);
System.out.println("number "+number);
System.out.println("hello world");
//System.exit(5);
}
private  final static  String randomString(int length)
{
if(length<1){
return null;
}
numberList=("0123456789abcdefghijklmnopqrstuvwxyz" +
        "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ").toCharArray();
char[] chand=new char[length];
for(int i=0;i<chand.length;i++)
{
chand[i]=numberList[random.nextInt(72)];
}
System.out.println(chand);
return new String(chand);
}

}

user.setUserId(String.valueOf(System.currentTimeMillis()));
这样生成ID值，确实简单，但不知道是否永远不会重复？
/*
random=new Random(); 没有static初始化，必须new出来
多写了个静态类， 练习的时候写的，比较简单的随机生成ID
*/

另一种方法,可以独立运用

public class Order {
private static int orderNum = 0001;
public static String getOrderNo(){
long No = 0;
SimpleDateFormat sdf = new SimpleDateFormat("yyyyMMdd");
String nowdate = sdf.format(new Date());
No = Long.parseLong(nowdate)*10000;//这里如果一天订单多的话可以用一万或更大
No+=getNo();
return getUserId()+No;
}
public static int getNo(){//返回当天的订单数+1
orderNum++;
return orderNum;
}
public static String getUserId(){
return "123";
}

public static String RandomStringId(){
UUID uuid = UUID.randomUUID();
return uuid.toString();
}

}

也可以user+随机数+流水号(流水号为全局递增变量)  服务器重启就生成新的随机数