Android Calendar problem with day of the week

Question

I'm trying to list all days of the week for current week from Monday to Sunday. For example, today (day of this posting) is September 4th, 2011 and it's Sunday.

I'm starting calendar and setting first day of the week to Monday:

Calendar cal = Calendar.getInstance();
cal.setFirstDayOfWeek(Calendar.MONDAY);

When I check day of the month, I get correct result:

int check = cal.get(Calendar.DAY_OF_MONTH);
// check is equal to 4

But when I set weekday to Monday, it jumps to the next week instead of returning Monday of this week:

cal.set(Calendar.DAY_OF_WEEK, Calendar.MONDAY);
int mon = cal.get(Calendar.DAY_OF_MONTH);
// mon is equal to 5, when expected to be 29 (last Monday of August)

Even setting weekday to Sunday returns next Sunday and not today.

Can someone explain why it works that way and what's the best way to solve this problem?

Answer 1

In fact, when I check my own tests, it seems to work as expected, except when the date is not set again:

Display 4-29:

Calendar cal = Calendar.getInstance();
cal.setFirstDayOfWeek(Calendar.MONDAY);
cal.setMinimalDaysInFirstWeek(4);
cal.set(2011, 8, 4);
int test = cal.get(Calendar.DAY_OF_MONTH);
cal.set(Calendar.DAY_OF_WEEK, Calendar.MONDAY);
int mon = cal.get(Calendar.DAY_OF_MONTH);
bTest.setText("" + test + "-" + mon);

Display 5-5:

Calendar cal = Calendar.getInstance();
cal.setFirstDayOfWeek(Calendar.MONDAY);
cal.setMinimalDaysInFirstWeek(4);
cal.set(2011, 8, 5);
int test = cal.get(Calendar.DAY_OF_MONTH);
cal.set(Calendar.DAY_OF_WEEK, Calendar.MONDAY);
int mon = cal.get(Calendar.DAY_OF_MONTH);
bTest.setText("" + test + "-" + mon);

Display 14-12:

Calendar cal = Calendar.getInstance();
cal.setFirstDayOfWeek(Calendar.MONDAY);
cal.setMinimalDaysInFirstWeek(4);
cal.set(2011, 8, 14);
int test = cal.get(Calendar.DAY_OF_MONTH);
cal.set(Calendar.DAY_OF_WEEK, Calendar.MONDAY);
int mon = cal.get(Calendar.DAY_OF_MONTH);
bTest.setText("" + test + "-" + mon);

So, this doesn't work:

Calendar cal = Calendar.getInstance();
cal.setFirstDayOfWeek(Calendar.MONDAY);
cal.setMinimalDaysInFirstWeek(4);
//cal.set(cal.get(Calendar.YEAR), cal.get(Calendar.MONTH), cal.get(Calendar.DAY_OF_MONTH));
int test = cal.get(Calendar.DAY_OF_MONTH);
cal.set(Calendar.DAY_OF_WEEK, Calendar.MONDAY);
int mon = cal.get(Calendar.DAY_OF_MONTH);
bTest.setText("" + test + "-" + mon); // Display 4-5

and this works:

Calendar cal = Calendar.getInstance();
cal.setFirstDayOfWeek(Calendar.MONDAY);
cal.setMinimalDaysInFirstWeek(4);

// Workaround
cal.set(cal.get(Calendar.YEAR), cal.get(Calendar.MONTH), cal.get(Calendar.DAY_OF_MONTH));

int test = cal.get(Calendar.DAY_OF_MONTH);
cal.set(Calendar.DAY_OF_WEEK, Calendar.MONDAY);
int mon = cal.get(Calendar.DAY_OF_MONTH);
bTest.setText("" + test + "-" + mon); // Display 4-29

and this works too:

Calendar cal = Calendar.getInstance();
cal.setFirstDayOfWeek(Calendar.MONDAY);
cal.setMinimalDaysInFirstWeek(4);

// Workaround
cal.set(cal.get(Calendar.YEAR), cal.get(Calendar.MONTH), cal.get(Calendar.DAY_OF_MONTH));
cal.get(Calendar.DAY_OF_MONTH);

cal.set(Calendar.DAY_OF_WEEK, Calendar.MONDAY);
int mon = cal.get(Calendar.DAY_OF_MONTH);
bTest.setText("" + mon); // Display 29

But this one doesn't:

Calendar cal = Calendar.getInstance();
cal.setFirstDayOfWeek(Calendar.MONDAY);
cal.setMinimalDaysInFirstWeek(4);

// Workaround
cal.set(cal.get(Calendar.YEAR), cal.get(Calendar.MONTH), cal.get(Calendar.DAY_OF_MONTH));
cal.set(Calendar.DAY_OF_WEEK, Calendar.MONDAY);
int mon = cal.get(Calendar.DAY_OF_MONTH);
bTest.setText("" + mon); // Display 5

Answer 2

tl;dr

Use LocalDate, never Calendar.

LocalDate.now( ZoneId.of( "Africa/Tunis" ) )
    .with( TemporalAdjusters.previousOrSame( DayOfWeek.MONDAY ) )
    .plusDays( i ) )

java.time

Avoid the troublesome old legacy class Calendar as it is now legacy, supplanted by the java.time classes (specifically ZonedDateTime).

For date-only values without time-of-day, use LocalDate rather than a date+time type like Calendar or ZonedDateTime. The LocalDate class represents a date-only value without time-of-day and without time zone.

A time zone is crucial in determining a date. For any given moment, the date varies around the globe by zone. For example, a few minutes after midnight in Paris France is a new day while still “yesterday” in Montréal Québec.

If no time zone is specified, the JVM implicitly applies its current default time zone. That default may change at any moment, so your results may vary. Better to specify your desired/expected time zone explicitly as an argument.

Specify a proper time zone name in the format of continent/region, such as America/Montreal, Africa/Casablanca, or Pacific/Auckland. Never use the 3-4 letter abbreviation such as EST or IST as they are not true time zones, not standardized, and not even unique(!).

ZoneId z = ZoneId.of( "America/Montreal" ) ;  
LocalDate today = LocalDate.now( z ) ;

If you want to use the JVM’s current default time zone, ask for it and pass as an argument. If omitted, the JVM’s current default is applied implicitly. Better to be explicit, as the default may be changed at any moment during runtime by any code in any thread of any app within the JVM.

ZoneId z = ZoneId.systemDefault() ;  // Get JVM’s current default time zone.

Or specify a date. You may set the month by a number, with sane numbering 1-12 for January-December.

LocalDate ld = LocalDate.of( 1986 , 2 , 23 ) ;  // Years use sane direct numbering (1986 means year 1986). Months use sane numbering, 1-12 for January-December.

Or, better, use the Month enum objects pre-defined, one for each month of the year. Tip: Use these Month objects throughout your codebase rather than a mere integer number to make your code more self-documenting, ensure valid values, and provide type-safety.

LocalDate ld = LocalDate.of( 1986 , Month.FEBRUARY , 23 ) ;

DayOfWeek

For day-of-week, use the DayOfWeek enum class. It offers seven instances, one for each day of the week, Monday-Sunday.

DayOfWeek dow = ld.getDayOfWeek() ;  // Get an enum representing the day-of-week of this date, such as `DayOfWeek.MONDAY`.

Adjusting date

We need find the previous Monday, or stick with today’s date if already a Monday. To move to such dates, use the TemporalAdjusters.previousOrSame implementation of TemporalAdjuster.

TemporalAdjuster ta = TemporalAdjusters.previousOrSame( DayOfWeek.MONDAY ) ;
LocalDate previousOrSameMonday = ld.with( ta ) ;

Collect your desired dates by incrementing with a call to LocalDate.plusDays(). Notice how java.time uses immutable objects. We get a fresh object based on the values of the original rather than mutating (altering) the original.

// Hard-coded `7` is the seven days in a week.
List< LocalDate > dates = new ArrayList<>( 7 ) ;
for( int i = 0 , i < 7 , i ++ ) {
    LocalDate localDate = previousOrSameMonday.plusDays( i ) ;
    dates.add( localDate ) ;
}

---

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.

You may exchange java.time objects directly with your database. Use a JDBC driver compliant with JDBC 4.2 or later. No need for strings, no need for java.sql.* classes.

Where to obtain the java.time classes?

• Java SE 8, Java 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 Java SE 7
  • Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport.
• Android
  • Later versions of Android bundle implementations of the java.time classes.
  • For earlier Android (<26), the ThreeTenABP project adapts ThreeTen-Backport (mentioned above). 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.

---

UPDATE: The Joda-Time project is now in maintenance mode, with the team advising migration to the java.time classes. This section left for history.

Joda-Time

what's the best way to solve this problem?

The best way is to avoid using the notoriously troublesome java.util.Date & .Calendar classes, and instead use the Joda-Time library. Joda-Time works in Android.

Joda-Time offers the LocalDate class for date-only values without any time or time zone.

Example Code

Here is some example code using Joda-Time 2.3.

LocalDate localDate = new LocalDate( 2011, DateTimeConstants.SEPTEMBER, 4 );
LocalDate firstDateOfWeek = localDate.withDayOfWeek( DateTimeConstants.MONDAY );
for ( int i = 0; i < 7; i++ ) {
    LocalDate someDateOfWeek = firstDateOfWeek.plusDays( i );
    System.out.println( "someDateOfWeek: " + someDateOfWeek + "  le jour de la semaine: " + someDateOfWeek.dayOfWeek().getAsText( Locale.CANADA_FRENCH ) );
}

When run…

someDateOfWeek: 2011-08-29  le jour de la semaine: lundi
someDateOfWeek: 2011-08-30  le jour de la semaine: mardi
someDateOfWeek: 2011-08-31  le jour de la semaine: mercredi
someDateOfWeek: 2011-09-01  le jour de la semaine: jeudi
someDateOfWeek: 2011-09-02  le jour de la semaine: vendredi
someDateOfWeek: 2011-09-03  le jour de la semaine: samedi
someDateOfWeek: 2011-09-04  le jour de la semaine: dimanche

Week Number

Bonus tip: If you want the week number as defined by the ISO 8601 standard, call the weekOfWeekYear method. Like this:

int weekNumber = firstDateOfWeek.getWeekOfWeekyear();

Your date is in week 35.