# 日期相加
簡易版:

private String addDays(String dateStr,String numberDays) {
    if(StringUtils.isBlank(dateStr) || StringUtils.isBlank(numberDays)) {
        return null;
    }
    int days = Integer.valueOf(numberDays);
    SimpleDateFormat sdf = new SimpleDateFormat("yyyyMMdd");

    try {
        Date d = sdf.parse(dateStr);
        Calendar c = Calendar.getInstance();
        c.setTime(d);
        c.add(Calendar.DATE, days);

        String newTime = sdf.format(c.getTime());
        return newTime;
    }catch(ParseException e) {
        e.printStackTrace();
    }
    return null;
}

進階版:

public static Date addDays(String dateStr,String numberDays) {
    if(StringUtils.isBlank(dateStr) || StringUtils.isBlank(numberDays)) {
        return null;
    }
    int days = Integer.valueOf(numberDays);
    Date d = parseDate(dateStr);
    Calendar c = Calendar.getInstance();
    c.setTime(d);
    c.add(Calendar.DATE, days);
    return c.getTime();
}

public static Date parseDate(String dateString) {
    Date result = null;
    String[] dateSplit;

    try {
        if (null == dateString) {
            // null 不處理
        } else if (dateString.matches("\\d{8}")) {
            result = SIMPLE_FORMAT.parse(dateString);
        } else if (dateString.matches("\\d{4}\\D\\d{1,2}\\D\\d{1,2}")) {
            dateSplit = dateString.split("\\D");
            result = SIMPLE_FORMAT.parse(String.format("%1$04d%2$02d%3$02d", Integer.parseInt(dateSplit[0]),
                    Integer.parseInt(dateSplit[1]), Integer.parseInt(dateSplit[2])));
        }
    } catch (ParseException e) {
        logger.error("解析日期字串失敗! dateString: {}", dateString , e);
    }

    return result;
}

public static String formatDate(Date d, String pattern) {
    SimpleDateFormat df = (SimpleDateFormat) DateFormat.getDateTimeInstance();
    df.setTimeZone(TimeZone.getDefault());
    df.applyPattern(pattern);
    return df.format(d);
}

###### tags: `Java`