日期格式引起的问题

public class SunKbTest {
	public static void main(String[] args) {
		Calendar calendar=Calendar.getInstance();
		calendar.set(2019, Calendar.AUGUST,31);
		Date strDate=calendar.getTime();
		SimpleDateFormat format1=new SimpleDateFormat("yyyy/MM/dd");
		System.out.println("2019-08-31 to yyyy/MM/dd:"+format1.format(strDate));
		SimpleDateFormat format2=new SimpleDateFormat("YYYY/MM/dd");
		System.out.println("2019-08-31 to YYYY/MM/dd:"+format2.format(strDate));
		calendar.set(2019, Calendar.DECEMBER,31);
		Date strDate1=calendar.getTime();
		SimpleDateFormat format3=new SimpleDateFormat("yyyy-MM-dd");
		System.out.println("2019-12-31 to yyyy-MM-dd:"+format3.format(strDate1));
		SimpleDateFormat format4=new SimpleDateFormat("YYYY-MM-dd");
		System.out.println("2019-12-31 to YYYY-MM-dd:"+format4.format(strDate1));
		
	}
}

结果:

2019-08-31 to yyyy/MM/dd:2019/08/31
2019-08-31 to YYYY/MM/dd:2019/08/31
2019-12-31 to yyyy-MM-dd:2019-12-31
2019-12-31 to YYYY-MM-dd:2020-12-31

yyyy获取的是当天所属的年份
YYYY获取的是当周所在的年份