需求描述：

根据两个日期，计算两日期之前所有日期
根据两个月份，计算出两个月份之间所有月份

解决方案：

package com.ems.openflow.utils;

import java.text.DecimalFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Calendar;
import java.util.Date;
import java.util.List;

public class CountDate {

    private static transient int gregorianCutoverYear = 1582;

    /** 闰年中每月天数 */
    private static final int[] DAYS_P_MONTH_LY= {31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};

    /** 非闰年中每月天数 */
    private static final int[] DAYS_P_MONTH_CY= {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};

    /** 代表数组里的年、月、日 */
    private static final int Y = 0, M = 1, D = 2;

    /**
     * 将代表日期的字符串分割为代表年月日的整形数组 
     * @param date
     * @return
     */
    public static int[] splitYMD(String date){
        date = date.replace("-", "");
        int[] ymd = {0, 0, 0};
        ymd[Y] = Integer.parseInt(date.substring(0, 4));
        ymd[M] = Integer.parseInt(date.substring(4, 6));
        ymd[D] = Integer.parseInt(date.substring(6, 8));
        return ymd;
    }

    /**
     * 检查传入的参数代表的年份是否为闰年 
     * @param year
     * @return
     */
    public static boolean isLeapYear(int year) {
        return year >= gregorianCutoverYear ?
                ((year%4 == 0) && ((year%100 != 0) || (year%400 == 0))) : (year%4 == 0);
    }

    /**
     * 日期加1天 
     * @param year
     * @param month
     * @param day
     * @return
     */
    private static int[] addOneDay(int year, int month, int day){
        if(isLeapYear( year )){
            day++;
            if( day > DAYS_P_MONTH_LY[month -1 ] ){
                month++;
                if(month > 12){
                    year++;
                    month = 1;
                }
                day = 1;
            }
        }else{
            day++;
            if( day > DAYS_P_MONTH_CY[month -1 ] ){
                month++;
                if(month > 12){
                    year++;
                    month = 1;
                }
                day = 1;
            }
        }
        int[] ymd = {year, month, day};
        return ymd;
    }

    /**
     * 将不足两位的月份或日期补足为两位 
     * @param decimal
     * @return
     */
    public static String formatMonthDay(int decimal){
        DecimalFormat df = new DecimalFormat("00");
        return df.format( decimal );
    }

    /**
     * 将不足四位的年份补足为四位 
     * @param decimal
     * @return
     */
    public static String formatYear(int decimal){
        DecimalFormat df = new DecimalFormat("0000");
        return df.format( decimal );
    }

    /**
     * 计算两个日期之间相隔的天数 
     * @param begin
     * @param end
     * @return
     * @throws ParseException
     */
    public static long countDay(String begin,String end){
        SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd");
        Date beginDate , endDate;
        long day = 0;
        try {
            beginDate= format.parse(begin);
            endDate=  format.parse(end);
            day=(endDate.getTime()-beginDate.getTime())/(24*60*60*1000);
        } catch (ParseException e) {
            e.printStackTrace();
        }
        return day;
    }

    /**
     * 以循环的方式计算日期 
     * @param beginDate endDate 
     * @param days
     * @return
     */
    public static List<String> getEveryday(String beginDate , String endDate){
        long days = countDay(beginDate, endDate);
        int[] ymd = splitYMD( beginDate );
        List<String> everyDays = new ArrayList<String>();
        everyDays.add(beginDate);
        for(int i = 0; i < days; i++){
            ymd = addOneDay(ymd[Y], ymd[M], ymd[D]);
            everyDays.add(formatYear(ymd[Y])+"-"+formatMonthDay(ymd[M])+"-"+formatMonthDay(ymd[D]));
        }
        return everyDays;
    }

    /**
     * 计算日期内所有月份
     * @param beginDate
     * @param endDate
     * @return
     */
    public static List<String> getEveryMonth(String beginDate , String endDate) throws ParseException {
        
        Date start = new SimpleDateFormat("yyyy-MM").parse(beginDate);
        Date end = new SimpleDateFormat("yyyy-MM").parse(endDate);

        Calendar calendar = Calendar.getInstance();
        calendar.setTime(start);
        // 获取开始年份和开始月份
        int startYear = calendar.get(Calendar.YEAR);
        int startMonth = calendar.get(Calendar.MONTH);
        // 获取结束年份和结束月份
        calendar.setTime(end);
        int endYear = calendar.get(Calendar.YEAR);
        int endMonth = calendar.get(Calendar.MONTH);
        //
        List<String> list = new ArrayList<String>();
        for (int i = startYear; i <= endYear; i++) {
            String date = "";
            if (startYear == endYear) {
                for (int j = startMonth; j <= endMonth; j++) {
                    if (j < 9) {
                        date = i + "-0" + (j + 1);
                    } else {
                        date = i + "-" + (j + 1);
                    }
                    list.add(date);
                }

            } else {
                if (i == startYear) {
                    for (int j = startMonth; j < 12; j++) {
                        if (j < 9) {
                            date = i + "-0" + (j + 1);
                        } else {
                            date = i + "-" + (j + 1);
                        }
                        list.add(date);
                    }
                } else if (i == endYear) {
                    for (int j = 0; j <= endMonth; j++) {
                        if (j < 9) {
                            date = i + "-0" + (j + 1);
                        } else {
                            date = i + "-" + (j + 1);
                        }
                        list.add(date);
                    }
                } else {
                    for (int j = 0; j < 12; j++) {
                        if (j < 9) {
                            date = i + "-0" + (j + 1);
                        } else {
                            date = i + "-" + (j + 1);
                        }
                        list.add(date);
                    }
                }
            }
        }
        return list;
        
    }

    /** 获取一个月的最后一天 */
    public static String getLastDay(String date) throws ParseException {
        SimpleDateFormat d = new SimpleDateFormat("yyyy-MM");
        Date da = null;
        da = d.parse(date);
        SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd");
        Calendar calendar = Calendar.getInstance();
        calendar.setTime(da);
        int lastDays = calendar.getActualMaximum(Calendar.DAY_OF_MONTH);
        calendar.set(Calendar.DAY_OF_MONTH, lastDays);
        return df.format(calendar.getTime());
    }

    /** 获取一个月的第一天 */
    public static String getFirstDay(String date) throws ParseException {
        SimpleDateFormat d = new SimpleDateFormat("yyyy-MM");
        Date da = null;
        da = d.parse(date);
        SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd");
        Calendar calendar = Calendar.getInstance();
        calendar.setTime(da);
        //calendar.add(Calendar.MONTH, -1);
        calendar.set(Calendar.DAY_OF_MONTH, 1);
        String time = df.format(calendar.getTime());
        return time;
    }

    public static void main(String[] args) throws ParseException {
        List<String> list1 = CountDate.getEveryday("2021-07-01", "2021-09-02");
        for (String result : list1) {
            System.out.println(result);
        }
        List<String> list2 = CountDate.getEveryMonth("2021-02", "2021-09");
        for (String result : list2) {
            System.out.println(result);
        }
    }

}