getting Monday date from given week and year using java.time package

Issue

I want to get Monday date from given week and year using Java 8 package java.time. But at some point I am facing issue as it's not returning proper date.

private LocalDate getDateFromWeekAndYear(final String week,final String year){
    LocalDate date = LocalDate.now();
    date = date.with(WeekFields.ISO.dayOfWeek(), 1);
    date = date.with(WeekFields.ISO.weekOfWeekBasedYear(), Long.parseLong(week));
    date = date.with(WeekFields.ISO.weekBasedYear(), Long.parseLong(year));

    return date;
}

For example:

• If I pass week=1 and year=2013 then date is : 2012-12-31.
• But if I pass week=53 and year=2015 then date is : 2014-12-29. I expected 2014-12-28.

Is there any logical mistake I am making or some other issue ?

Solution

This is astonishingly more difficult than the partially invalid expectations of OP and most answers show.

First to say: It is very important to define the correct order of week-based-manipulations. The OP has first applied day-manipulation, then year-based manipulation. The correct approach is in reverse! I will show the right helper method implementation:

public static void main(String... args) {
    System.out.println(
      getDateFromWeekAndYear("53", "2015")); // 2015-12-28, NOT 2014-12-28
    System.out.println(
      getDateFromWeekAndYear("53", "2015").get(WeekFields.ISO.weekOfWeekBasedYear())); // 53
    System.out.println(
      getDateFromWeekAndYear("53", "2014")); // 2014-12-29
    System.out.println(
      getDateFromWeekAndYear("53", "2014").get(WeekFields.ISO.weekOfWeekBasedYear())); // 1
}

private static LocalDate getDateFromWeekAndYear(final String week,final String year) {
    int y = Integer.parseInt(year);
    LocalDate date = LocalDate.of(y, 7, 1); // safer than choosing current date
    // date = date.with(WeekFields.ISO.weekBasedYear(), y); // no longer necessary
    date = date.with(WeekFields.ISO.weekOfWeekBasedYear(), Long.parseLong(week));
    date = date.with(WeekFields.ISO.dayOfWeek(), 1);

    return date;
}

If you don't respect this specific order then you will indeed get sometimes a 2014-date for the input 2015-W53 (depending on the current date).

Second problem: I have also avoided to start with current date in order to be not near start or end of calendar year (calendar year != week-based-year) and instead chosen midth of year as starting point.

The third problem is lenient handling of week 53 in (week-based)-year 2014. It does not exist because 2014 had only 52 weeks!!! A strict algorithm should recognize and reject such an input. Therefore I advise against using YearWeek.of(2014, 53) (in the external library Threeten-Extra) resulting in the first week of 2015, see also its javadoc. Better than such lenient handling would have been

YearWeek yw = YearWeek.of(2014, 52);
if (yw.is53WeekYear()) {
  yw = YearWeek.of(2014, 53);
}

or using this code from my own time library Time4J (whose class CalendarWeek has extra i18n-features and extra week arithmetic in comparison with YearWeek):

CalendarWeek.of(2014, 53); // throws an exception
System.out.println(CalendarWeek.of(2014, 1).withLastWeekOfYear()); // 2014-W52

Only using java.time-package:

Using such external libraries would at least have helped to solve the first problem in a transparent way. If you are not willing to add an extra dependency then you can do this to handle week 53 if invalid:

If the expression WeekFields.ISO.weekOfWeekBasedYear() applied on the result of your helper method yields the value 1 then you know that week 53 was invalid. And then you can decide if you want to accept lenient handling or to throw an exception. But silent adjusting such an invalid input is IMHO bad design.

Answered By - Meno Hochschild