从日历周信息创建LocalDate

Question

Given:

• A Year: 2019
• A Calender Week Number: 1 (== 1st Week of a year)
• A DayOfWeek: SUNDAY

Needed:

A transformation f of those Infos into a LocalDate Object so that

assertEquals(LocalDate.of(2019,1,6), f(2019,1,SUNDAY))

What I tried

I did not find a way with java.time.* to create a Date from an Info like "The Sunday of the first calender week in 2019". I found that the old java.util.Calendar class had a setWeekDate() function that could be usefull. But the following code caused an Exception:

    ...
    Calendar c = Calendar.getInstance();
    c.setWeekDate(2019, 1, Calendar.MONDAY);
    c.setTimeZone(TimeZone.getDefault());

    return LocalDate.from(c.toInstant());

---

java.time.DateTimeException: Unable to obtain LocalDate from TemporalAccessor: 2018-01-08T20:03:55.602Z of type java.time.Instant
at java.time.LocalDate.from(LocalDate.java:379)
at ...

Answer 1

Check ChronoField or IsoFields for a suitable way to get week for the calendar system that you want to use. LocalDate object that is used as "base" value should have a week value of the expected year.

int year = 2019;
int weekNumber = 1;
LocalDate localDate = LocalDate.of(year, 2, 1)
        .with(IsoFields.WEEK_OF_WEEK_BASED_YEAR, weekNumber)
        .with(ChronoField.DAY_OF_WEEK, DayOfWeek.SUNDAY.getValue());

Selecting first date is meaningful. Using LocalDate.of(year, 1, 1) would give a day that can belong to last week of the previous year, same goes for LocalDate.now() but only for given period of time.

LocalDate.of(2017, 1, 1)
    .with(IsoFields.WEEK_OF_WEEK_BASED_YEAR, 26)
    .with(ChronoField.DAY_OF_WEEK, DayOfWeek.SUNDAY.getValue());
// 2016-07-03

LocalDate.of(2017, 1, 1).get(IsoFields.WEEK_OF_WEEK_BASED_YEAR);
// 52

So jumping from week 52 to week 26 would give date of 2016-07-03 because date is calculated by subtracking time between weeks 52 and 26.

Also see answer here for more discussion about withYear : Unexpected date calculation result