获取几位随机数

    /**
     * 随机数
     * @param len 几位随机数
     * @return
     */
    public static String getRandom4(int len) {
        String source = "0123456789";
        Random r = new Random();
        StringBuilder rs = new StringBuilder();
        for (int j = 0; j < len; j++) {
            rs.append(source.charAt(r.nextInt(10)));
        }
        return rs.toString();
    }