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?
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.
