java生成验证码

package com.hanqi;

import java.util.Random;

public class zuoyesjscyzm {
    public static void main(String[] args)
    {   
        System.out.println("请输入验证码：");
        String a="abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
        Random r = new Random();
        {
            for( double i =0; i<4; i++)
            {
                int s=r.nextInt(62);
                System.out.print(a.substring(s,s+1));
            }
            
            
    
        }
        
    }

}