如何生成16位流水号

/**
     * @Description: 获取long类型16流水号
     * @return serialNo long类型16流水号
     * @date 2017年07月20日 下午03:55:21
     */
    public static long getTimeSerialNo(){
        long serialNo;
        Random r = new Random();
        Calendar c = Calendar.getInstance();
        int year = c.get(Calendar.YEAR)-2000;
        int month = c.get(Calendar.MONTH) + 1;
        int day = c.get(Calendar.DAY_OF_MONTH);
        int hour = c.get(Calendar.HOUR_OF_DAY);
        int minute = c.get(Calendar.MINUTE);
        int second = c.get(Calendar.SECOND);
        int ms = c.get(Calendar.MILLISECOND);
        serialNo = (long)(year * 10000 + month * 100 + day);
        serialNo = serialNo * 1000000 + hour * 10000 + minute * 100 + second;
        serialNo = ( serialNo * 1000 + ms ) * 1000 + r.nextInt(1000);
        return serialNo;
    }
    
    /**
     * @Description: 获取String类型16流水号
     * @return  String类型16流水号
     * @date 2017年07月20日 下午03:55:21
     */
    public static String getTimeSerialNoStr(){
        StringBuilder serialNo=new StringBuilder();
        Random r = new Random();
        Calendar c = Calendar.getInstance();
        int year = c.get(Calendar.YEAR)-2000;
        int month = c.get(Calendar.MONTH) + 1;
        int day = c.get(Calendar.DAY_OF_MONTH);
        int hour = c.get(Calendar.HOUR_OF_DAY);
        int minute = c.get(Calendar.MINUTE);
        int second = c.get(Calendar.SECOND);
        int ms = c.get(Calendar.MILLISECOND);
        serialNo.append(year).append(month).append(day)
        .append(hour).append(minute).append(second)
        .append(ms).append(r.nextInt(1000));
        return serialNo.toString();
    }