一直以来糊里糊涂的使用Calendar类
近期学习了下基本用法
calendar.add(Calendar.DAY_OF_MONTH, +1);
根据第二个int值的正负代表天数加一或减一

package com.cy.manage.util;

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import java.util.Locale;

/**
 * author:gkq
 * date:2020/9/25
 */
public class TimeUtil {
    /**
     * 获取某一天的下一天
     *
     * @param time
     * @return
     */
    public static String getNextDate(String time) {
        Calendar calendar = Calendar.getInstance();
        calendar.setTime(toData(time));
        calendar.add(Calendar.DAY_OF_MONTH, +1);
        Date date = calendar.getTime();
        SimpleDateFormat sdr = new SimpleDateFormat("yyyy-MM-dd",
                Locale.CHINA);
        return sdr.format(date);
    }

    /**
     * 获取某一天的上一天
     *
     * @param time
     * @return
     */
    public static String getLastDate(String time) {
        Calendar calendar = Calendar.getInstance();
        calendar.setTime(toData(time));
        calendar.add(Calendar.DAY_OF_MONTH, -1);
        Date date = calendar.getTime();
        SimpleDateFormat sdr = new SimpleDateFormat("yyyy-MM-dd",
                Locale.CHINA);
        return sdr.format(date);
    }

    /**
     * 获取当前日期
     *
     * @return
     */
    public static String getToday() {
        Calendar calendar = Calendar.getInstance();
        Date date = calendar.getTime();
        SimpleDateFormat sdr = new SimpleDateFormat("yyyy-MM-dd",
                Locale.CHINA);
        return sdr.format(date);
    }

    /**
     * 获取某一月的上一月
     * @param time
     * @return
     */
    public static String getLastMonth(String time) {
        String date;
        Calendar cal = Calendar.getInstance();
        cal.setTime(toData(time));
        SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd",
                Locale.CHINA);
        cal.add(Calendar.MONTH, -1);
        date = df.format(cal.getTime());
        return date;
    }

    /**
     * 获取某一月的下一月
     * @param time
     * @return
     */
    public static String getNextMonth(String time) {
        String date;
        Calendar cal = Calendar.getInstance();
        cal.setTime(toData(time));
        SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd",
                Locale.CHINA);
        cal.add(Calendar.MONTH, 1);
        date = df.format(cal.getTime());
        return date;
    }

    /**
     * 获取当前月的第一天
     *
     * @return
     */
    public static String getMonthFirstDay() {
        String firstDay;
        Calendar cal = Calendar.getInstance();
        SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd",
                Locale.CHINA);
        cal.set(Calendar.DAY_OF_MONTH, cal.getActualMinimum(Calendar.DAY_OF_MONTH));
        firstDay = df.format(cal.getTime());
        return firstDay;
    }

    /**
     * 获取某月的最后一天
     *
     * @return
     */
    public static String getMonthLastDay(String time) {
        String date;
        Calendar cal = Calendar.getInstance();
        cal.setTime(toData(time));
        int firstDay = cal.getActualMaximum(Calendar.DAY_OF_MONTH);
        //设置日历中月份的最小天数
        cal.set(Calendar.DAY_OF_MONTH, firstDay);
        //格式化日期
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
        date = sdf.format(cal.getTime());
        return date;
    }

    /**
     * 获取当前周的周一
     * @return
     */
    public static String getWeekFirstDay() {
        String firstDay;
        Calendar curStartCal = Calendar.getInstance();
        SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd",
                Locale.CHINA);
        Calendar cal = (Calendar) curStartCal.clone();
        cal.setFirstDayOfWeek(Calendar.MONDAY);
        cal.set(Calendar.DAY_OF_WEEK, Calendar.MONDAY);
        firstDay = df.format(cal.getTime());
        return firstDay;
    }

    /**
     * 配合getWeekFirstDay获得周末
     * @return
     */
    public static String getWeekLastDay(String time) {
        String date;
        Calendar cal = Calendar.getInstance();
        cal.setTime(toData(time));
        SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd",
                Locale.CHINA);
        cal.add(Calendar.DATE, 6);
        date = df.format(cal.getTime());
        return date;
    }

    /**
     * 配合getWeekFirstDay获得上周
     * @return
     */
    public static String getLastWeekDate(String time) {
        Calendar calendar = Calendar.getInstance();
        calendar.setTime(toData(time));
        calendar.add(Calendar.DAY_OF_MONTH, -7);
        Date date = calendar.getTime();
        SimpleDateFormat sdr = new SimpleDateFormat("yyyy-MM-dd",
                Locale.CHINA);
        return sdr.format(date);
    }

    /**
     * 配合getWeekFirstDay获得下周
     * @return
     */
    public static String getNextWeekDate(String time) {
        Calendar calendar = Calendar.getInstance();
        calendar.setTime(toData(time));
        calendar.add(Calendar.DAY_OF_MONTH, 7);
        Date date = calendar.getTime();
        SimpleDateFormat sdr = new SimpleDateFormat("yyyy-MM-dd",
                Locale.CHINA);
        return sdr.format(date);
    }


    private static Date toData(String time) {
        SimpleDateFormat sdr = new SimpleDateFormat("yyyy-MM-dd",
                Locale.CHINA);
        Date date = null;
        try {
            date = sdr.parse(time);
        } catch (ParseException e) {
            e.printStackTrace();
        }

        return date;
    }

}