Parsing an ISO 8601 date in Java8 when all fields (including separators, but not including years) are optional

Question

I have a requirement for parsing ISO8601 formatted strings in Java with various levels of accuracy. Some examples of the string I need to parse are:

• 2018

Answer 1

Well that took me longer than I had expected. The only valid parser is:

DateTimeFormatter dtf = new DateTimeFormatterBuilder()
        .appendValue(ChronoField.YEAR, 4)
        .appendPattern("[['-']MM[['-']dd[['T']HH[[':']mm[[':']ss['.'SSS]]]]]]")
        .parseDefaulting(ChronoField.MONTH_OF_YEAR, 1)
        .parseDefaulting(ChronoField.DAY_OF_MONTH, 1)
        .parseDefaulting(ChronoField.HOUR_OF_DAY, 0)
        .parseDefaulting(ChronoField.MINUTE_OF_HOUR, 0)
        .parseDefaulting(ChronoField.SECOND_OF_MINUTE, 0)
        .parseDefaulting(ChronoField.NANO_OF_SECOND, 0)
        .toFormatter();

String[] s = {
        "2018",
        "2018-10",
        "2018-10-15",
        "2018-10-15T12:00",
        "2018-10-15T12:00:30",
        "2018-10-15T12:00:30.123",
        "20181015",
        "201810151200",
        "20181015120030",
        "20181015120030.123",
        "20181015T12:00:30.123"
};
for (String line : s) {
  System.out.println(LocalDateTime.parse(line, dtf));
}

---

The problem is that yyyy creates a ValueParser(minWidth=4, maxWidth=19, SignStyle.PAD_EXEEDS) which parses the date 20181015 as year=20181015 as an example. So we have to restrict the digit width of year to 4.

The documentation states:

Year: The count of letters determines the minimum field width below which padding is used.

But does not specify a maximum width.