java练习 随机生成4位验证码

        String str="abcdefghigklmnopqrstuvwxyzABCDEFGHIGKLMNOPQRSTUVWXYZ0123456789";
        Random r=new Random();
        String arr[]=new String [4];
        String b="";
        for(int i=0;i<4;i++)
        {
            int n=r.nextInt(62);

            arr[i]=str.substring(n,n+1);
            b+=arr[i];

        }

        System.out.print("验证码："+b);