java生成隨機數

    /**
     * 生成20位的隨機數,用系統當前時間的毫秒數(13)+七位隨機數
     * @return
     */
    public String generateStr() {
        StringBuffer var = new StringBuffer(String.valueOf(System.currentTimeMillis()));
        int len = var.length();
        if (len < 20) len = 20 - len;
        Random random = new Random();
        String allChar = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
        for (int i = 0; i < len; i++) {
            var.append(allChar.charAt(random.nextInt(allChar.length())));
        }
        return var.toString();
    }