Java中日期格式化YYYY-DD的操作bug

@Test
 public void testWeekBasedYear() {
 Calendar calendar = Calendar.getInstance();
 // 2019-12-31
 calendar.set(2019, Calendar.DECEMBER, 31);
 Date strDate1 = calendar.getTime();
 // 2020-01-01
 calendar.set(2020, Calendar.JANUARY, 1);
 Date strDate2 = calendar.getTime();
 // 大写 YYYY
 SimpleDateFormat formatYYYY = new SimpleDateFormat("YYYY/MM/dd");
 System.out.println("2019-12-31 转 YYYY/MM/dd 格式: " + formatYYYY.format(strDate1));
 System.out.println("2020-01-01 转 YYYY/MM/dd 格式: " + formatYYYY.format(strDate2));
 // 小写 YYYY
 SimpleDateFormat formatyyyy = new SimpleDateFormat("yyyy/MM/dd");
 System.out.println("2019-12-31 转 yyyy/MM/dd 格式: " + formatyyyy.format(strDate1));
 System.out.println("2020-01-01 转 yyyy/MM/dd 格式: " + formatyyyy.format(strDate2));
 }

 private static void tryit(int Y, int M, int D, String pat) {
 DateTimeFormatter fmt = DateTimeFormatter.ofPattern(pat);
 LocalDate  dat = LocalDate.of(Y,M,D);
 String  str = fmt.format(dat);
 System.out.printf("Y=%04d M=%02d D=%02d " +
  "formatted with " +
  "\"%s\" -> %s\n",Y,M,D,pat,str);
 }
 public static void main(String[] args){
 tryit(2020,01,20,"MM/DD/YYYY");
 tryit(2020,01,21,"DD/MM/YYYY");
 tryit(2020,01,22,"YYYY-MM-DD");
 tryit(2020,03,17,"MM/DD/YYYY");
 tryit(2020,03,18,"DD/MM/YYYY");
 tryit(2020,03,19,"YYYY-MM-DD");
 }