主函数

public static int dateDiff(char flag, Calendar calSrc, Calendar calDes) {

		long millisDiff = getMillis(calSrc) - getMillis(calDes);

		if (flag == 'y') {
            return (calSrc.get(Calendar.YEAR) - calDes.get(Calendar.YEAR));
		}

		if (flag == 'd') {
			return (int) (millisDiff / DAY_IN_MILLIS);
		}

		if (flag == 'h') {
			return (int) (millisDiff / HOUR_IN_MILLIS);
		}

		if (flag == 'm') {
			return (int) (millisDiff / MINUTE_IN_MILLIS);
		}

		if (flag == 's') {
			return (int) (millisDiff / SECOND_IN_MILLIS);
		}

		return 0;
}

调用：

	public static void main(String[] args) {
		Date beginDate = DateUtils.str2Date("2023-06-01 12:00:00", DateUtils.getSDFormat("yyyy-MM-dd HH:mm:ss"));
		Date endDate = DateUtils.str2Date("2023-06-01 14:00:00", DateUtils.getSDFormat("yyyy-MM-dd HH:mm:ss"));
		Calendar calendar1 = Calendar.getInstance();
		Calendar calendar2 = Calendar.getInstance();
		calendar1.setTime(beginDate);
		calendar2.setTime(endDate);
		Integer diff = DateUtils.dateDiff('h', calendar2, calendar1);
		System.out.println("相差小时数：" + diff.toString());

		beginDate = DateUtils.str2Date("2023-06-01 12:00:00", DateUtils.getSDFormat("yyyy-MM-dd HH:mm:ss"));
		endDate = DateUtils.str2Date("2023-06-02 14:00:00", DateUtils.getSDFormat("yyyy-MM-dd HH:mm:ss"));
		calendar1 = Calendar.getInstance();
		calendar2 = Calendar.getInstance();
		calendar1.setTime(beginDate);
		calendar2.setTime(endDate);
		diff = DateUtils.dateDiff('d', calendar2, calendar1);
		System.out.println("相差天数：" + diff.toString());
		
	}

字符转日期函数

	public static Date str2Date(String str, SimpleDateFormat sdf) {
		if (null == str || "".equals(str)) {
			return null;
		}
		Date date = null;
		try {
			date = sdf.parse(str);
			return date;
		} catch (ParseException e) {
			e.printStackTrace();
		}
		return null;
	}

调用结果如下：