工作中java常用的时间工具类：Calendar

/**
 * 根据传参求出对应月份的最后一天
 */
@Test
public void setAndGetRangeEndTime() {
    int index = 1;
    //int range=2;
    SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    Calendar calendar = Calendar.getInstance();
    calendar.setTime(new Date());
    //昨天
    calendar.set(Calendar.DATE, calendar.get(Calendar.DATE) - 1);
    calendar.set(Calendar.HOUR_OF_DAY, 23);
    calendar.set(Calendar.MINUTE, 59);
    calendar.set(Calendar.SECOND, 59);
    //月份
    calendar.set(Calendar.MONTH, calendar.get(Calendar.MONTH) - index);
    //月份最大天数
    int maxDay = calendar.getActualMaximum(Calendar.DAY_OF_MONTH);
    calendar.set(Calendar.DAY_OF_MONTH, maxDay);
    String format = sdf.format(calendar.getTime());
    System.out.println("结果：" + format);
}


/**
 * 补全开始到结束时间的工具方法
 */
public List<String> testDateUtils(String startTime, String endTime) throws Exception {

    startTime = "2021-02-03";
    endTime = "2021-02-06";

    if (StringUtils.isEmpty(startTime) || StringUtils.isEmpty(endTime)) {
        throw new Exception("开始日期和截止日期都不能为空！");
    }
    //compareTo 当两者相等时为0，当前者小于后者时返回-1，大于时返回1。具体查看源码。
    if (startTime.compareTo(endTime) > 0) {
        throw new Exception("开始日期必须小于等于截止日期！");
    }

    SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd");
    Date startDate = simpleDateFormat.parse(startTime);
    Date endDate = simpleDateFormat.parse(endTime);

    Calendar startCal = Calendar.getInstance();
    Calendar endCal = Calendar.getInstance();
    startCal.setTime(startDate);
    endCal.setTime(endDate);

    List<String> days = new ArrayList<>();
    //当开始时间大于结束时间时则返回
    while (startCal.compareTo(endCal) <= 0) {
        days.add(simpleDateFormat.format(startCal.getTime()));
        //Date本身加减日期过于麻烦，使用Calendar操纵日期；开始时间+1；
        startCal.add(Calendar.DATE, 1);

    }
    return days;

}