I need to get week of year for a given date. Using SimpleDateFormat produces wrong results for the last week of the year.
Example:
This produces week 52 correctly:
SimpleDateFormat w = new SimpleDateFormat("w");
Calendar c = Calendar.getInstance();
c.set(2021, 11, 25);
System.out.println("Week: ${w.format(c.getTime())}");
produces: Week: 52
But the next day is already considered as 1st week of next year?
SimpleDateFormat w = new SimpleDateFormat("w");
Calendar c = Calendar.getInstance();
c.set(2021, 11, 26);
System.out.println("Week: ${w.format(c.getTime())}");
produces: Week: 1
This only happens in Java 7 and not in Java 8 and above!
CodePudding user response：
Don't use Calendar. It's obsolete, and, more to the point, incredibly bad API.
There's a list as long as my leg as to what's wrong with it. The specific one of about 200 things that is relevant here is that, boneheadedly, its month value is 0 indexed. So, '12, 3'? That's the 3rd of Undecimber, or whatever you'd like to call the 13th month. That, or the calendar doesn't do 13th month in which case it leniently just assumes you meant to say 3rd of jan 2022 . Either way, that's week 1.
So why is The 2nd of undecimber (or if you prefer, via rollover, 2nd jan 2022) "Week 52"?
Week numbering is weird, but they have to be. A week starts on monday (or sunday, for my silly standards loving USian brethren), and can't start on any other day. That means unless Jan 1st so happens to fall on a monday, there's going to be weirdness; either days in 2021 counting as being in 'week 1 of 2022', or days in 2022 counting as 'week 52 of 2021'. In fact, from time to time there'll have to be a week 53. After all, 52*7 is 364, but there are 365.2475 days in a year, so unless you just want to make some days poof out of existence, every so often a week 53 has to exist for it all to add up.
Use java.time instead.
LocalDate ld = LocalDate.of(2021, 12, 3);
WeekFields wf = WeekFields.of(Locale.ENGLISH);
int weekNumber = ld.get(wf.weekOfWeekBasedYear());
java.time does many things fantastically, and one of the things is does great is that it tends not to hide complex things. For example, 'when does a week start' is just not answerable, not unless you tell me where on the planet you're asking this question. Hence, 'which week is it' is also not actually an answerable question until you tell me exactly which week-counting system we're using, and there is no standard that is universally accepted enough. Hence, you have to go through the rigamarole of making a separate WeekFields instance to capture that info. We do it here based on locale.
CodePudding user response：
Actually this is not specific to Calendar as this would also show Week 1 if ran on the 29th of December for example:
System.out.println("Week: ${new SimpleDateFormat("w").format(new Date())}");
But it is specific to Java 7. It was fixed in Java 8.
And I found the explanation for this here (as @rzwitserloot also explained):
https://docs.oracle.com/javase/8/docs/api/java/util/GregorianCalendar.html
A week year is in sync with a WEEK_OF_YEAR cycle. All weeks between the first and last weeks (inclusive) have the same week year value. Therefore, the first and last days of a week year may have different calendar year values.
For example, January 1, 1998 is a Thursday. If getFirstDayOfWeek() is MONDAY and getMinimalDaysInFirstWeek() is 4 (ISO 8601 standard compatible setting), then week 1 of 1998 starts on December 29, 1997, and ends on January 4, 1998. The week year is 1998 for the last three days of calendar year 1997. If, however, getFirstDayOfWeek() is SUNDAY, then week 1 of 1998 starts on January 4, 1998, and ends on January 10, 1998; the first three days of 1998 then are part of week 53 of 1997 and their week year is 1997.
This is really interesting..