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From: Tassilo Horn <tsdh@gnu.org>
Newsgroups: comp.lang.java.programmer
Subject: Parsing calendar weeks to LocalDates with DateTimeFormatter
Date: Wed, 07 Sep 2016 08:58:48 +0200
Organization: University Koblenz-Landau Campus Koblenz
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Hi all,

I have a

  DateTimeFormatter FORMATTER_KW = DateTimeFormatter.ofPattern("ww.YYYY");

for formatting LocalDates as calendar weeks.  That works fine, however I
can't use that to parse calendar week strings to LocalDates, e.g.,

  FORMATTER_KW.parse("44.2016", LocalDate::from);

throws

  java.time.DateTimeException: Unable to obtain LocalDate from TemporalAccessor:
    {WeekBasedYear[WeekFields[MONDAY,4]]=2016,
    WeekOfWeekBasedYear[WeekFields[MONDAY,4]]=50},ISO of type java.time.format.Parsed
	at java.time.LocalDate.from(LocalDate.java:368)
	at java.time.format.Parsed.query(Parsed.java:226)
	at java.time.format.DateTimeFormatter.parse(DateTimeFormatter.java:1851)
	... 63 more

Sounds like it is complaining that the date is underspecified which it
obviously is.  So I'd like such a calendar week to resolve to the last
work day in that week.  (The calendar weeks are estimated deliveries.)

After a bit of doc reading and experimenting, I came up with this
solution which seems to work ok but looks a bit complicated for such a
common task such as week-date conversion.  So is there anything better?

         // When parsing calendar weeks like "22.2016", we want to get the last work day in that week.  We
         // assume that everyone in every locale has a 5 days work week (don't know how to do better).
         return FORMATTER_KW.parse(editorText, temporal ->
         {
            WeekFields weekFields = WeekFields.of(Locale.getDefault());
            DayOfWeek firstDayInWeek = weekFields.getFirstDayOfWeek();
            DayOfWeek lastWorkDay = firstDayInWeek.minus(4);
            long weekOfWeekBasedYear = temporal.getLong(weekFields.weekOfWeekBasedYear());
            long year = temporal.getLong(weekFields.weekBasedYear());
            LocalDate date = LocalDate.of(0, 1, 1);
            date = date.plusYears(year);
            date = date.plusWeeks(weekOfWeekBasedYear);
            date = date.with(lastWorkDay);
            return date;
         });

Bye,
Tassilo