Shock! Java date formatting pit!

preface

Yyyy and YYYY

Sample code

    @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));
    }

Output results

2019-12-31 转 YYYY/MM/dd 格式: 2020/12/31
2020-01-01 转 YYYY/MM/dd 格式: 2020/01/01
2019-12-31 转 yyyy/MM/dd 格式: 2019/12/31
2020-01-01 转 yyyy/MM/dd 格式: 2020/01/01

How did yyyy do it

DD and DD

Sample code

  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,D,pat,str);
    }
    public static void main(String[] args){
        tryit(2020,01,20,"MM/DD/YYYY");
        tryit(2020,21,"DD/MM/YYYY");
        tryit(2020,22,"YYYY-MM-DD");
        tryit(2020,03,17,18,19,"YYYY-MM-DD");
    }

Output results

Y=2020 M=01 D=20 formatted with "MM/DD/YYYY" -> 01/20/2020
Y=2020 M=01 D=21 formatted with "DD/MM/YYYY" -> 21/01/2020
Y=2020 M=01 D=22 formatted with "YYYY-MM-DD" -> 2020-01-22
Y=2020 M=03 D=17 formatted with "MM/DD/YYYY" -> 03/77/2020
Y=2020 M=03 D=18 formatted with "DD/MM/YYYY" -> 78/03/2020
Y=2020 M=03 D=19 formatted with "YYYY-MM-DD" -> 2020-03-79

conclusion