随机生成的字符串工具类

package com.gx.utils;

import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.Random;
import java.util.UUID;

public class RandomUtils {
    
    private static SimpleDateFormat format1=new SimpleDateFormat("yyyyMMddHHmmssSSS");
    
    private static SimpleDateFormat sdf2=new SimpleDateFormat("yyyy-MM-dd");
    
    private static Random random = new Random();
    
    /**
     * 使用时间+四位随机数
     * @param oldName  a.sd.df.ds.jpg
     */
    public static String createRandomFileNameUseTime(String oldName) {
        //得到文件名的最后一点的下标
        int lastDotIndex=oldName.lastIndexOf(".");
        //从lastDotIndex截取到oldName.length
        String suffix = oldName.substring(lastDotIndex,oldName.length());
        //生成时间字符串
        String timeStr = format1.format(new Date());
        //生成四位随机数
        int random4Num = random.nextInt(9000)+1000;
        return timeStr+random4Num+suffix;
    }
    public static void main(String[] args) {
        String createRandomFileNameUseTime = createRandomFileNameUseTime("a.sd.df.ds.jpg");
        String createRandomFileNameUseUuid = createRandomFileNameUseUuid("a.sd.df.ds.jpg");
        String createRandomFileNameUseGNTime = createRandomFileNameUseGNTime("a.sd.df.ds.jpg");
        System.out.println(createRandomFileNameUseTime);
        System.out.println(createRandomFileNameUseUuid);
        System.out.println(createRandomFileNameUseGNTime);
    }
    
    /**
     * 使用uuid
     */
    public static String createRandomFileNameUseUuid(String oldName) {
        //得到文件名的最后一点的下标
        int lastDotIndex=oldName.lastIndexOf(".");
        //从lastDotIndex截取到oldName.length
        String suffix = oldName.substring(lastDotIndex,oldName.length());
        //生成uuid字符串
        String uuid = UUID.randomUUID().toString().replace("-", "");
        return uuid+suffix;
    }
    
    /**
     * 使用格林毫秒+四位随机数
     */
    public static String createRandomFileNameUseGNTime(String oldName) {
        //得到文件名的最后一点的下标
        int lastDotIndex=oldName.lastIndexOf(".");
        //从lastDotIndex截取到oldName.length
        String suffix = oldName.substring(lastDotIndex,oldName.length());
        //生成时间字符串
        long time = System.currentTimeMillis();
        //生成四位随机数
        int random4Num = random.nextInt(9000)+1000;
        return time+random4Num+suffix;
    }
    /**
     * 得到当前上期下的日期  去生成文件夹名
     * @return
     */
    public static String getCurrentDateToStr() {
        return sdf2.format(new Date());
    }
    
}