import java.text.SimpleDateFormat;
import java.util.Calendar;

public class test {
	
	private static String startDay = "2020-02-20";

	public static void main(String[] args) {
	    Calendar calendar = Calendar.getInstance();
	    calendar.set(Integer.parseInt(startDay.substring(0,4)), Integer.parseInt(startDay.substring(5,7)) - 1, 1);
            String firstDayOfMonth = new SimpleDateFormat( "yyyy-MM-dd ").format(calendar.getTime());
            System.out.println("第一天："+firstDayOfMonth);
        
            calendar.set(Integer.parseInt(startDay.substring(0,4)), Integer.parseInt(startDay.substring(5,7)), 1);//这里先设置要获取月份的下月的第一天
            calendar.add(Calendar.DATE, -1);//这里将日期值减去一天，从而获取到要求的月份最后一天
            String lastDayOfMonth = new SimpleDateFormat( "yyyy-MM-dd ").format(calendar.getTime());
            System.out.println("最后一天："+lastDayOfMonth);
	}
}

参考博客：https://blog.csdn.net/Xiehou_java/article/details/87937749
感谢大佬！！！

.end