Java日期处理类Calendar使用

由于工作需要对数据库读出的日期做一些特殊处理，比如获取当前周周一，当前月第一天之类，只借助Date类显然无法简单的完成需求，这种时候需要使用util包下的Calendar类。
Calendar类的帮助文档：https://www.yiibai.com/java/util/java_util_calendar.html
然后自己写了一个util，记录一下。

import java.util.ArrayList;
import java.util.Calendar;
import java.util.Date;
import java.util.List;
import java.util.Set;
import java.util.TreeSet;

import com.mastercom.comm.DateUtil;

/**
 * @author DELL
 *
 */
public class TimeUtil {

    /**
     * @param dt
     * @return
     * 获取本月所有日期
     */
    public static List<Date> monthToDates(Date dt)
    {
        List<Date> dts = new ArrayList<Date>();
        Calendar ca = Calendar.getInstance();
        ca.setTime(dt);
        ca.set(ca.get(Calendar.YEAR),ca.get(Calendar.MONTH)+1, 0);
        int dayNumOfMonth = ca.get(Calendar.DAY_OF_MONTH);
        ca.set(Calendar.DAY_OF_MONTH, 1);// 从一号开始
        for (int i = 0; i < dayNumOfMonth; i++, ca.add(Calendar.DATE, 1)) {
            toZero(ca);
            Date d = ca.getTime();
            dts.add(d);
        }
        return dts;
    }


    /**
     * @param dt
     * @return
     * 获取本月第一天
     */
    public static Date getFstDayOfMonth(Date dt) {
        Calendar ca = Calendar.getInstance();
        ca.setTime(dt);
        ca.set(Calendar.DAY_OF_MONTH, 1);
        toZero(ca);
        return ca.getTime();
    }
    /**
     * @param dt
     * @return
     * 获取本月最后一天
     */
    public static Date getLstDayOfMonth(Date dt) {
        Calendar ca = Calendar.getInstance();
        ca.setTime(dt);
        ca.add(Calendar.MONTH, 1);
        ca.set(Calendar.DAY_OF_MONTH, 0);
        toZero(ca);
        return ca.getTime();
    }

    /**
     * @param dt
     * @return
     * 获取本周所有的日期,按时间先后排序
     */
    public static List<Date> mondayToDates(Date dt)
    {
        List<Date> dts = new ArrayList<Date>();
        Calendar ca = Calendar.getInstance();
        ca.setTime(dt);
        int weekNum = 7;
        ca.setTime(getFstDayOfWeek(dt));// 从周一开始
        for (int i = 0; i < weekNum; i++, ca.add(Calendar.DATE, 1)) {
            toZero(ca);
            Date d = ca.getTime();
            dts.add(d);
        }
        return dts;
    }

    /**
     * @param date
     * @return
     * 获取后一天
     */
    public static Date getNextDay(Date date) {
        return getDayAfterToday(date,1);
    }

    /**
     * @param date
     * @return
     * 获取n天后,为负数则表示几天前
     */
    public static Date getDayAfterToday(Date date,int n) {
        Calendar ca = Calendar.getInstance();
        ca.setTime(date);
        ca.add(Calendar.DAY_OF_YEAR, n);
        toZero(ca);
        return ca.getTime();
    }

    /**
     * @param dt
     * @return
     * 获取本周第一天
     */
    public static Date getFstDayOfWeek(Date dt) {
        Calendar ca = Calendar.getInstance();
        ca.setTime(dt);
        ca.setFirstDayOfWeek(Calendar.MONDAY); //设置周一为第一天
        ca.setMinimalDaysInFirstWeek(7);
        ca.set(Calendar.DAY_OF_WEEK, Calendar.MONDAY);
        toZero(ca);
        return ca.getTime();
    }

    /**
     * @param days
     * @return
     * 获取每一天的周一并排序
     * 除去当前周
     */
    public static List<Date> DaysToMondays(List<Date> days){
        Set<Date> mondays = new TreeSet<Date>();
        for(Date dt : days) {
            if(isNowWeek(dt)) continue;
            mondays.add(getFstDayOfWeek(dt));
        }
        return new ArrayList<>(mondays);
    }

    /**
     * @param days
     * @return
     * 获取每月第一天并排序
     * 除去当前月
     */
    public static List<Date> DaysToMonths(List<Date> days){
        Set<Date> mondays = new TreeSet<Date>();
        Calendar ca = Calendar.getInstance();
        for(Date dt : days) {
            if(isNowMonth(dt)) continue;
            ca.setTime(dt);
            ca.set(Calendar.DAY_OF_MONTH, 1);
            toZero(ca);
            mondays.add(ca.getTime());
        }
        return new ArrayList<>(mondays);
    }

    /**
     * @param ca
     * @return
     * 将时间调到0时0分0秒
     */
    public static Calendar toZero(Calendar ca) {
        ca.set(Calendar.HOUR_OF_DAY, 0);
        ca.set(Calendar.MINUTE, 0);
        ca.set(Calendar.SECOND, 0);
        ca.set(Calendar.MILLISECOND, 0);
        return ca;
    }

    /**
     * @param date
     * @return
     * 判断是否是当前月
     */
    public static boolean isNowMonth(Date date) {
        Date fst = getFstDayOfMonth(new Date());
        Date fstOfDate = getFstDayOfMonth(date);
        if(fst.equals(fstOfDate)) return true;
        return false;
    }

    /**
     * @param date
     * @return
     * 判断是否是当前周
     */
    public static boolean isNowWeek(Date date) {
        Date fst = getFstDayOfWeek(new Date());
        Date fstOfDate = getFstDayOfWeek(date);
        if(fst.equals(fstOfDate)) return true;
        return false;
    }
    public static void main(String[] args){
        List<Date> days = new ArrayList<>();
        Date dt = new Date();
        days.add(dt);
//      days.add(TimeUtil.getDayAfterToday(dt, 11));
//      days.add(TimeUtil.getDayAfterToday(dt, -11));
//      days.add(TimeUtil.getDayAfterToday(dt, 111));
//      days.add(TimeUtil.getDayAfterToday(dt, -311));
//      days.add(TimeUtil.getDayAfterToday(dt, 17));
        days.add(DateUtil.getDate(2018, 5, 20));
//      System.out.println(TimeUtil.DaysToMondays(days));
        System.out.println(TimeUtil.mondayToDates(DateUtil.getDate(2018, 5, 20)));
//      System.out.println(isNowMonth(new Date()));
//      System.out.println(isNowWeek(new Date()));

    }
}