Calendar get year returns wrong result

I want to create a calendar object and set it to a certain year and a week in that year. 

Calendar calendar = Calendar.getInstance();
calendar.set(Calendar.WEEK_OF_YEAR, weekOfYear);  // 1
calendar.set(Calendar.YEAR, year); // 2016
setWeekChecked(calendar);

This is the toString of the calendar object as I pass it to the setWeekChecked method:

java.util.GregorianCalendar[time=?,areFieldsSet=false,lenient=true,zone=America/New_York,firstDayOfWeek=1,minimalDaysInFirstWeek=1,ERA=1,YEAR=2016,MONTH=0,WEEK_OF_YEAR=1,WEEK_OF_MONTH=2,DAY_OF_MONTH=7,DAY_OF_YEAR=7,DAY_OF_WEEK=5,DAY_OF_WEEK_IN_MONTH=1,AM_PM=0,HOUR=5,HOUR_OF_DAY=5,MINUTE=25,SECOND=43,MILLISECOND=219,ZONE_OFFSET=-18000000,DST_OFFSET=0]

In the setWeekChecked method:

public void setWeekChecked(final Calendar cal) {
    final int targetWeek = cal.get(Calendar.WEEK_OF_YEAR); // Returns 1
    final int targetYear = cal.get(Calendar.YEAR); // Returns 2015??
}

This is the toString of the calendar object now:

java.util.GregorianCalendar[time=1451557543219,areFieldsSet=true,lenient=true,zone=America/New_York,firstDayOfWeek=1,minimalDaysInFirstWeek=1,ERA=1,YEAR=2015,MONTH=11,WEEK_OF_YEAR=1,WEEK_OF_MONTH=5,DAY_OF_MONTH=31,DAY_OF_YEAR=365,DAY_OF_WEEK=5,DAY_OF_WEEK_IN_MONTH=5,AM_PM=0,HOUR=5,HOUR_OF_DAY=5,MINUTE=25,SECOND=43,MILLISECOND=219,ZONE_OFFSET=-18000000,DST_OFFSET=0]

What am I doing wrong?

Jon Skeet
people
quotationmark

I suspect that the calendar is trying to use the current day-of-week (it's Thursday today) in the first week of 2016.

Now, looking at your calendar settings, you've got firstDayOfWeek=1 (so weeks run Sunday to Saturday) and minimalDaysInFirstWeek=1 (so the first week of the year is the one that includes January 1st).

That means that the first week of 2016 in your calendar ran from Decemember 27th 2015 to January 2nd 2016. Therefore Thursday in the first week was December 31st - which is exactly what the calendar you've shown us says.

Fundamentally, calendar arithmetic with "week of year" is tricky because:

  • There are lots of different culture-specific ways of looking at them
  • Typically requirements don't specify which of those you're actually interested in

I'd strongly recommend using Joda Time if at all possible to make your date/time-handling code clearer to start with, but you'll still need to work out exactly what you mean by "set it to a certain year and a week in that year". Note that Joda Time separates the concepts of "week-year" (used with week-of-week-year and day-of-week) from "year" (used with month and day-of-month) which helps greatly. You need to be aware that for a given date, the week-year and year may be different.

people

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