java練習 隨機生成4位驗證碼

        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("驗證碼："+b);