使用java.util.Calendar获取Java下一周的Java

Question

I'd like to have a function which returns the next week and year given a week and year. It looks something like:

public static int[] getNextWeek(int year, int week) {
    Calendar c = Calendar.getInstance(); // I'm Locale.US and TimeZone "America/New_York"
    c.clear();
    c.set(Calendar.YEAR, year);
    c.set(Calendar.WEEK_OF_YEAR, week);

    c.add(Calendar.WEEK_OF_YEAR, 1);

    return new int[] {c.get(Calendar.YEAR), c.get(Calendar.WEEK_OF_YEAR)}
}

This sometimes does not work around year boundaries. It seems to depend on what day you invoke it and for what parameters you use! For example, if I invoke it with year 2012 and week 52 then I expect the result to be year 2013 and week 1. If you invoke it today (Tuesday July 17 2012) it works. If you set the day of week to yesterday it does not; and oddly results in year 2012 week 1. Weird. What is going on? It appears to relate to the day of the week because it doesn't work if invoked with SUNDAY or MONDAY, which are the last two days of 2012! If I set the day of the week to the last day of the week the function seems to work; before calling Calendar.add() I do:

    // Must set to last day of week; initially set to first day due to API
    c.set(Calendar.DAY_OF_WEEK, c.getFirstDayOfWeek()); // SUNDAY
    c.add(Calendar.DAY_OF_WEEK, 6); // Must roll forward not backwards

There doesn't seem to be any weirdness like this if I create a getPreviousWeek method. Is this a java.util.Calendar bug? Next time I guess I'll use Joda time!

Answer 1

Have you considered the fact that ISO8601 week algorithm considers the first week of the year to be the one that has at least 4 days fall within that year?

So if the Jan 1 is on thurdsay, that week is actually considered week 52 of the previous year, not week one of the current year.

You might want to at least consider joda-time for this kind of calculation, as it has proper handling of the ISO standard.

    LocalDate ld = new LocalDate();
    ld = ld.withWeekOfWeekyear(29);
    ld = ld.withWeekyear(2012);

    System.out.println(ld.getWeekOfWeekyear());
    System.out.println(ld.getWeekyear());

    // 29
    // 2012


    System.out.println(ld.plusWeeks(1).getWeekOfWeekyear());
    System.out.println(ld.plusWeeks(1).getWeekyear());
    // 30
    // 2012

And this will work across year boundaries.

Answer 2

Just do an explicit check to see if there's a rollover.

return new int[] {c.get(Calendar.WEEK_OF_YEAR) == 1 ? c.get(Calendar.YEAR) + 1 :
    c.get(Calendar.YEAR), c.get(Calendar.WEEK_OF_YEAR)};

As far as I can tell, the reason why WEEK_OF_YEAR doesn't add correctly is because it's not an actual property of Calendar like SECOND , MINUTE , MONTH , DAY , or YEAR . It's a derived property like WEEK_OF_MONTH . One way to get around this is to use c.add(Calendar.DAY, 7) to add exactly 7 days instead of incrementing WEEK_OF_YEAR .

Answer 3

At this point I'm going to assume this really is a bug. I've filled a bug report with Oracle here:

http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=7197761