//随机验证码，有数字、字符

         //生成随机数，然后再截取，还要限定随机数的范围

         String zimu = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz1234567890" ;

         Random rm = new Random();    //生成随机数

         int a = rm.nextInt(61);        //0到61之间的随机数赋值给a

         int b = rm.nextInt(61);

         int c = rm.nextInt(61);

         int d = rm.nextInt(61);

         char zf = zimu.charAt(a);    //将数组中索引为a的字符赋给zf

         char zd = zimu.charAt(b);

         char zc = zimu.charAt(c);

         char zq = zimu.charAt(d);

         System.out.println("请输入验证码： "+ zc +" " + zf +" " + zq +" " + zd);