How to calculate Date from ISO8601 week number in Java

Question

Is there any way of doing this: How to get dates of a week (I know week number)? for ISO 8601 week number without using any library or calender in Java?

Answer 1

The Java 8 / java.time way

In Java 8, you can use TemporalField combined with LocalDate::with(TemporalField, long) method for getting the week of the week based year and TemporalAdjuster combined with LocalDate::with(TemporalAdjuster) method to jump to required day of week, like this:

final int weekNumber = 34;
LocalDate weekByNumber = date.with(IsoFields.WEEK_OF_WEEK_BASED_YEAR, weekNumber);

final TemporalAdjuster adjuster = TemporalAdjusters.previousOrSame(DayOfWeek.MONDAY);
LocalDate mondayOfWeekByNumber = weekByNumber.with(adjuster);