如何获得一年中某一周的年份？

Question

I know that Calendar.WEEK_OF_YEAR could been used to obtain the "week of year" for a given date, but how do I get the corresponding YEAR?.

I know that the definition of Week of Year is dependent on Locale . And I am mostly interested in a DIN 1355-1 / ISO 8601 / German solution (but I do not want to make it a fix implementation) .

So it could happen that the 1 January belongs to:

• the first week of year (1) of the new year: for example 2015-01-01 is in week of year 1 of 2015
• the last week of year (53) of the year before: for example 2016-01-01 is in week of year 53 of year 2015

My question is how to get this year?*

Date date =
Calender calendar = new GregorianCalendar(Locale.GERMAN)
calendar.setTime(date);

int weekOfYear = calendar.get(Calendar.WEEK_OF_YEAR);
int year       = ????                                             <------------

System.out.println(date + " is in " + weekOfYear + " of " + year);

* the solution is not: date.getYear , date.getFirstDayOfWeek.getYear or date.getLastDayOfWeek.getYear

Answer 1

java.time

You should be using the java.time framework built into Java 8 and later. Avoid the old and notoriously troublesome java.util.Date/.Calendar classes.

These new java.time classes include LocalDate for date-only value without time-of-day or time zone. Note that you must specify a time zone to determine 'today' as the date is not simultaneously the same around the world.

ZoneId zoneId = ZoneId.of ( "America/Montreal" );
ZonedDateTime now = ZonedDateTime.now ( zoneId );

The IsoFields class provides info according to the ISO 8601 standard including the week-of-year for a week-based year.

int calendarYear = now.getYear();
int weekNumber = now.get ( IsoFields.WEEK_OF_WEEK_BASED_YEAR );
int weekYear = now.get ( IsoFields.WEEK_BASED_YEAR );  // Answer to Question

Near the beginning/ending of a year, the week-based-year may be ±1 different than the calendar-year. For example, notice the difference between the Gregorian and ISO 8601 calendars for the end of 2015: Weeks 52 & 1 become 52 & 53.

Answer 2

Java 8 or later?

Refer to Basil Bourque's answer to use Java's java.time API.

Otherwise (think twice.) see my original answer to use the obsolete Java API as asked in this question.

---

Pre Java 8?

calendar.getWeekYear(); returns 2015 for your given example.

See API documentation for GregorianCalendar#getWeekYear() as well as week year .

For reference

Calendar calendar = new GregorianCalendar(Locale.GERMAN);
calendar.set(2016, Calendar.JANUARY, 1, 0, 0, 0);
Date date = calendar.getTime();

int weekOfYear = calendar.get(Calendar.WEEK_OF_YEAR);
int year       = calendar.getWeekYear();

System.out.println(date + " is in " + weekOfYear + " of " + year);

returns

Fri Jan 01 00:00:00 CET 2016 is in 53 of 2015