DateTimeFormatter of YYYY-ww resolves differently on mac and ubuntu-CodePudding

System.out.println(
    DateTimeFormatter.ofPattern("YYYY-ww").withZone(ZoneOffset.UTC).format(Instant.parse("2022-05-10T00:00:00.00Z"))
);
System.out.println(
    DateTimeFormatter.ofPattern("YYYY-ww").withZone(ZoneOffset.UTC).format(Instant.parse("2022-05-17T00:00:00.00Z"))
);

Why this pattern YYYY-ww resolves differently on Ubuntu and Mac:

Ubuntu: (default locale en_US, my computer)

2022-20
2022-21

Mac: (default locale en_GB)

2022-19
2022-20

EDIT

System.out.println(
    DateTimeFormatter.ofPattern("YYYY-ww").withLocale(Locale.UK).withZone(ZoneOffset.UTC).format(Instant.parse("2022-05-10T00:00:00.00Z"))
);
System.out.println(
    DateTimeFormatter.ofPattern("YYYY-ww").withLocale(Locale.UK).withZone(ZoneOffset.UTC).format(Instant.parse("2022-05-17T00:00:00.00Z"))
);

returns:

2022-19
2022-20

Still, the question is why pattern ww is Locale specific? I don't see that in documentation for https://docs.oracle.com/javase/8/docs/api/java/time/temporal/WeekFields.html or https://docs.oracle.com/javase/8/docs/api/java/time/format/DateTimeFormatter.html

CodePudding user response：

In the US locale, the first week of the year can have one to seven days, whereas in the UK, the first week of the year needs to have at least 4 days.

WeekFields.of(Locale.UK).getMinimalDaysInFirstWeek() // 4
WeekFields.of(Locale.US).getMinimalDaysInFirstWeek() // 1

Also, a US week starts on a Sunday, whereas a UK week starts on a Monday.

This means that for the US, the first week of 2022 is the single Saturday of 2022-01-01, and 2022-01-02 is the start of the second week. On the other hand, for the UK, the first week of 2022 starts from 2022-01-03, since the first two days of the year don't form a week. And this is where that extra week for the US machine came about.

When creating a DateTimeFormatter using ofPattern, the default formatting locale of the machine is used:

The formatter will use the default FORMAT locale. This can be changed using withLocale(Locale) on the returned formatter

Hence the difference.

I suppose you want ISO weeks here, and not want anything to do with locales. One way to do this is to construct a DateTimeFormatter using IsoFields:

var dtf = new DateTimeFormatterBuilder()
  .  appendValue(IsoFields.WEEK_BASED_YEAR, 4)
    .appendLiteral('-')
    .appendValue(IsoFields.WEEK_OF_WEEK_BASED_YEAR, 2)
    .toFormatter().withZone(ZoneOffset.UTC);
System.out.println(
    dtf.format(Instant.parse("2022-05-10T00:00:00.00Z"))
);
System.out.println(
    dtf.format(Instant.parse("2022-05-17T00:00:00.00Z"))
);