程式碼：

package jc_wis;

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

/**
 * 測試類
 * @author wang-xiaoming
 *
 */
public class TestDate {
    public static void main(String[] args) {
        System.
 
out.println("2020-12 : " + getDaysByMonth("2020-12"));
        System.out.println("2021-01 : " + getDaysByMonth("2021-01"));
        System.out.println("2021-02 : " + getDaysByMonth("2021-02"));
    }
    
    public static final String FORMAT_SHORTER = "yyyy-MM";
    
    /**
     * 日期格式化   
     * @param date
     * @param pattern
     * @return
     
 
*/
    public static Date parse(String date, String pattern) {        
        SimpleDateFormat sdf = new SimpleDateFormat(pattern);        
            try {            
                return sdf.parse(date);        
            } catch (ParseException e) {           
                e.printStackTrace();            
                
 
return null;        
            }    
        }
    
    /**
     * 返回指定某一年某一月的所有日期號
     * @param date
     * @return
     */
    public static List<Integer> getDaysByMonth(String date){
        Calendar c = Calendar.getInstance();     
        c.setTime(parse(date, FORMAT_SHORTER));      
        int year = c.get(Calendar.YEAR); 
        int month = c.get(Calendar.MONTH);
        List<Integer> list = null;
        if(month >= 0 && month <= 11){
            list = new ArrayList<>(31);
            c = Calendar.getInstance();
            c.set(year, month, 1);
            int lastDay = c.getActualMaximum(Calendar.DAY_OF_MONTH);
            for (int i = 0; i <= lastDay; i++) {
                list.add(i);
            }
        }
        return list;
    }
    
}

結果：

2020-12 : [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31]
2021-01 : [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31]
2021-02 : [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28]