java常用函數收錄集

1. 獲取當前月份的所有日期

    /**
     * get all date of specified month(date format)
     * 
     * @param date
     */
    public static void printDays(Date date) {
        Calendar calendar = Calendar.getInstance();
        calendar.setTime(date);

        // get total days of month
        int daysOfMonth = calendar.getActualMaximum(Calendar.DAY_OF_MONTH);

        // set 1st day of month,like 2019-03-01
        calendar.set(Calendar.DAY_OF_MONTH, 1);

        for (int i = 0; i < daysOfMonth; i++, calendar.add(Calendar.DATE, 1)) {
            System.out.println(calendar.getTime());
        }
    }

2. 生成指定日期段的每一天日期：

    public static List<Date> listDays(Date startDate, Date endDate) {
        List dateList = new ArrayList();
        dateList.add(startDate);

        Calendar startCalendar = Calendar.getInstance();
        startCalendar.setTime(startDate);
        Calendar endCalendar = Calendar.getInstance();
        endCalendar.setTime(endDate);

        while (endDate.after(startCalendar.getTime())) {
            startCalendar.add(Calendar.DAY_OF_MONTH, 1);
            dateList.add(startCalendar.getTime());
        }
        return dateList;

    }

注：如果是指定時段的每個月，只需要將步進值的單位改爲月份即可（Calendar.DAY_OF_MONTH -> Calendar.MONTH），根據需要進行格式化即可。

3. 合併多個Map對象

merge()函數可以很方便的進行Map對象的合併。下面是將map1和map2合併到一個新的Map對象midMap中：

// 從map1複製出一個新對象，用於存放合併結果
Map<String, Integer> map = new HashMap<>(map1);
// 合併，key不變，value直接相加
map2.forEach((key, value) -> map.merge(key, value, (otherKey, otherValue) -> value + otherValue));

4. List轉Map

Map<String, Integer> dateCountsMap = dateAndCounts.stream().collect(Collectors.toMap(DateAndCount::getDate, DateAndCount::getCount));