Java calendar start of the year

Question

I've experienced unpleasant problems with Java Calendar. I want to get first day of some week as Date, it is working well until the week is in between years. How to get around it?

Calendar cal = Calendar.getInstance();
cal.set(2017,0,1);
cal.setFirstDayOfWeek(Calendar.MONDAY);
Calendar first = (Calendar) cal.clone();
first.add(Calendar.DAY_OF_WEEK,
first.getFirstDayOfWeek() - first.get(Calendar.DAY_OF_WEEK));
Calendar last = (Calendar) first.clone();
last.add(Calendar.DAY_OF_YEAR, 6);
System.out.println(first.getTime());
System.out.println(last.getTime());

Output

Mon Jan 02 22:30:26 GMT 2017

Sun Jan 08 22:30:26 GMT 2017

The output should be

Mon Dec 26 22:30:26 GMT 2016

Sun Jan 01 22:30:26 GMT 2017

Answer 1

Don't use Calendar, it's known to be a very bad API to use (see this answer). Use the time api instead (if you're not on java8, use the ThreeTen backport).

With that, your code would look like this:

LocalDate ld = LocalDate.of(2017,1,1);
ld = ld.with(ChronoField.DAY_OF_WEEK, 1);
System.out.println(ld);
System.out.println(ld.plusDays(6));

If you do need to end up with a Date, use Date.from(ld.atStartOfDay().atZone(ZoneId.of("UTC")).toInstant()) to convert it (assuming UTC, easy enough to replace by the zone you need)

Answer 2

tl;dr

The accepted Answer by Hendrickx is correct. Here is an alternative that I think is a bit more readable.

LocalDate.of( 2017 , Month.JANUARY , 1 )
    .with( TemporalAdjusters.previous( DayOfWeek.MONDAY ) )

TemporalAdjuster

The TemporalAdjuster interface provides classes that manipulate java.time objects. Implementations can be found in the TemporalAdjusters class. The previousOrSame adjuster finds the previous occurrence of a day-of-week or sticks with the same date if it already is that day-of-week.

LocalDate target = LocalDate.of( 2017 , Month.JANUARY , 1 );
LocalDate ld = target.with( TemporalAdjusters.previousOrSame( DayOfWeek.MONDAY ) );

target.toString(): 2017-01-01 | ld.toString(): 2016-12-26

If you always want the previous Monday, even if the date is already a Monday, then use the adjuster previous.

LocalDate ld = target.with( TemporalAdjusters.previous( DayOfWeek.MONDAY ) );

To get the end of the week, use the nextOrSame adjuster.

LocalDate nextOrSameSunday = target.with( TemporalAdjusters.nextOrSame( DayOfWeek.SUNDAY ) );

You can see the day-of-week in a string.

ZoneId z = ZoneId.of( "America/Montreal" );
DateTimeFormatter f = DateTimeFormatter.ofLocalizedDate( FormatStyle.FULL ).withLocale( Locale.CANADA );
System.out.println( "target: " + target.format( f ) + " | ld: " + ld.format( f ) );

target: Sunday, January 1, 2017 | ld: Monday, December 26, 2016

See this code live in IdeOne.com.

Also notice the use of enums, DayOfWeek and Month. See Tutorials.

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About java.time

The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date, Calendar, & SimpleDateFormat.

The Joda-Time project, now in maintenance mode, advises migration to the java.time classes.

To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.

Where to obtain the java.time classes?

• Java SE 8 and SE 9 and later
  • Built-in.
  • Part of the standard Java API with a bundled implementation.
  • Java 9 adds some minor features and fixes.
• Java SE 6 and SE 7
  • Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport.
• Android
  • The ThreeTenABP project adapts ThreeTen-Backport (mentioned above) for Android specifically.
  • See How to use ThreeTenABP….

The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval, YearWeek, YearQuarter, and more.