效果图

image.png

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.*;

public class text {

    public static void main(String[] args) {

        System.out.println(monthDays("2021-05"));

    }

    private static List<DateInfo> monthDays(String date){
        SimpleDateFormat yymm = new SimpleDateFormat("yyyy-MM");
        SimpleDateFormat yymmdd = new SimpleDateFormat("yyyy-MM-dd");
        Calendar calendar = Calendar.getInstance();
        List<DateInfo> dateInfoList = new ArrayList<>();
        try {
            //年月转成Date 例：2021-05 ，目的获取本月的第一次
            Date dt =yymm.parse(date);
            String ym = date;
            do{
                //当天日期
                date = yymmdd.format(dt);
                if (!date.startsWith(ym)){ //如果不是本月 退出
                    //判断本月1号是否星期天，如果不是星期天，取上个星期天的天数
                    if (!dateInfoList.isEmpty()){
                        DateInfo oneDate = dateInfoList.get(0);
                        if (oneDate.getWeek() != 0){
                            calendar.setTime(yymmdd.parse(oneDate.getDate()));
                            for (int i = oneDate.getWeek() - 1; i >= 0; i--) {
                                calendar.add(calendar.DATE,-1);
                                dt = calendar.getTime();
                                date = yymmdd.format(dt);
                                DateInfo dateInfo = new DateInfo();
                                dateInfo.setDate(date);
                                dateInfo.setWeek(calendar.get(Calendar.DAY_OF_WEEK)-1);
                                dateInfo.setDay(calendar.get(Calendar.DATE));
                                dateInfoList.add(0,dateInfo);
                            }
                        }
                    }
                    return dateInfoList;
                }
                calendar.setTime(dt);

                //获取星期 和 日
                DateInfo dateInfo = new DateInfo();
                dateInfo.setDate(date);
                dateInfo.setWeek(calendar.get(Calendar.DAY_OF_WEEK)-1);
                dateInfo.setDay(calendar.get(Calendar.DATE));
                dateInfoList.add(dateInfo);

                //设置明天日期
                calendar.add(calendar.DATE,1);
                dt = calendar.getTime();
            } while (true);


        } catch (ParseException e) {
            e.printStackTrace();
        }
        return null;
    }

}

class DateInfo{
    private int week;
    private int day;
    private String date;

    public int getWeek() {
        return week;
    }

    public void setWeek(int week) {
        this.week = week;
    }

    public int getDay() {
        return day;
    }

    public void setDay(int day) {
        this.day = day;
    }

    public String getDate() {
        return date;
    }

    public void setDate(String date) {
        this.date = date;
    }

    @Override
    public String toString() {
        return "\nDateInfo{" +
                "week=" + week +
                ", day=" + day +
                ", date='" + date + '\'' +
                '}';
    }
}

打印数据

[
DateInfo{week=0, day=25, date='2021-04-25'}, 
DateInfo{week=1, day=26, date='2021-04-26'}, 
DateInfo{week=2, day=27, date='2021-04-27'}, 
DateInfo{week=3, day=28, date='2021-04-28'}, 
DateInfo{week=4, day=29, date='2021-04-29'}, 
DateInfo{week=5, day=30, date='2021-04-30'}, 
DateInfo{week=6, day=1, date='2021-05-01'}, 
DateInfo{week=0, day=2, date='2021-05-02'}, 
DateInfo{week=1, day=3, date='2021-05-03'}, 
DateInfo{week=2, day=4, date='2021-05-04'}, 
DateInfo{week=3, day=5, date='2021-05-05'}, 
DateInfo{week=4, day=6, date='2021-05-06'}, 
DateInfo{week=5, day=7, date='2021-05-07'}, 
DateInfo{week=6, day=8, date='2021-05-08'}, 
DateInfo{week=0, day=9, date='2021-05-09'}, 
DateInfo{week=1, day=10, date='2021-05-10'}, 
DateInfo{week=2, day=11, date='2021-05-11'}, 
DateInfo{week=3, day=12, date='2021-05-12'}, 
DateInfo{week=4, day=13, date='2021-05-13'}, 
DateInfo{week=5, day=14, date='2021-05-14'}, 
DateInfo{week=6, day=15, date='2021-05-15'}, 
DateInfo{week=0, day=16, date='2021-05-16'}, 
DateInfo{week=1, day=17, date='2021-05-17'}, 
DateInfo{week=2, day=18, date='2021-05-18'}, 
DateInfo{week=3, day=19, date='2021-05-19'}, 
DateInfo{week=4, day=20, date='2021-05-20'}, 
DateInfo{week=5, day=21, date='2021-05-21'}, 
DateInfo{week=6, day=22, date='2021-05-22'}, 
DateInfo{week=0, day=23, date='2021-05-23'}, 
DateInfo{week=1, day=24, date='2021-05-24'}, 
DateInfo{week=2, day=25, date='2021-05-25'}, 
DateInfo{week=3, day=26, date='2021-05-26'}, 
DateInfo{week=4, day=27, date='2021-05-27'}, 
DateInfo{week=5, day=28, date='2021-05-28'}, 
DateInfo{week=6, day=29, date='2021-05-29'}, 
DateInfo{week=0, day=30, date='2021-05-30'}, 
DateInfo{week=1, day=31, date='2021-05-31'}]