【Java】 之 Date、Calendar

Topic：

* Topic 1： 使用Calendar 对 天数 进行加减
* Topic 2： 使用Calendar 对 天数 进行加减

**Tips：**对格式转换一定要注意格式的正确性。

public void test() {

        String dateStr = "2017-8-31";

        DateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");

        try {

            Date date = sdf.parse(dateStr);
            
            System.out.println("before : " + sdf.format(date));

            Calendar c = Calendar.getInstance();

            c.setTime(date);
            
            //加2天
            c.add(Calendar.DAY_OF_MONTH, 2);

            date = c.getTime();

            System.out.println("after  : " + sdf.format(date));


        } catch (Exception e) {
            e.printStackTrace();
        }

    }

根据两个时间，求得其之间的月份数

/**
     * 适用 当前时间 大于 起始时间
     * @param nowDateStr       当前时间字符串
     * @param originalDateStr  起始时间字符串
     * @return                 期数
     */
    private int getMonthInterval(String nowDateStr, String originalDateStr) {

        DateFormat sdf = new SimpleDateFormat("yyyy-MM");

        try {

            Date originalDate = sdf.parse(originalDateStr);

            Date nowDate= sdf.parse(nowDateStr);

            Calendar nowCalender = Calendar.getInstance();

            Calendar originalCalender = Calendar.getInstance();

            originalCalender.setTime(originalDate);

            nowCalender.setTime(nowDate);

            int originalYear = originalCalender.get(Calendar.YEAR);
            int nowYear = nowCalender.get(Calendar.YEAR);
            int originalMonth = originalCalender.get(Calendar.MONTH);
            int nowMonth = nowCalender.get(Calendar.MONTH);

            if (nowYear < originalYear || (nowYear == originalYear && nowMonth < originalMonth)) {
                return 0;
            }

            int yearInterval = Math.abs(nowYear - originalYear);

            int monthInterval = 0;

            if (nowMonth < originalMonth) {
                yearInterval --;
                monthInterval = nowMonth + 12 - originalMonth;
            } else {
                monthInterval = nowMonth - originalMonth;
            }

            return Math.abs(yearInterval * 12 + monthInterval);

        } catch (Exception e) {
            e.printStackTrace();
        }

        return 0;
    }