java Calendar setFirstDayOfWeek not working

Question

Here is the real calendar now:

   March 2015       
Su Mo Tu We Th Fr Sa  
 1  2  3  4  5  6  7  
 8  9 10 11 12 13 14  
15 16 17 18 19 20 21  
22 23 24 25 26 27 28  
29 30 31

And I get DAY_OF_WEEK of 2015/3/24 like this:

public class TestCalendar {
    public static void main(String[] argvs){
        Calendar cal = Calendar.getInstance();
        cal.setFirstDayOfWeek(Calendar.MONDAY);
        cal.set(2015,Calendar.MARCH,24);
        System.out.println(cal.get(Calendar.DAY_OF_WEEK));
    }
}

Since I have cal.setFirstDayOfWeek to MONDAY the result I expecting is 2, but Whatever day I set to the first day of week(have tried SUNDAY and others) .It kept show me the same result which is 3. So It seemed that firstDayOfWeek won't affect the result.

Have I do something wrong?

EDIT

I just figured and thanks to answers below, that this setFirstDayOfWeek will not affect the result of get(Calendar.DAY_OF_WEEK) nor get(Calendar.WEEK_OF_YEAR)

Then what is this method setFirstDayOfWeek() designed for? I mean How can I told the program that I want 2015/3/29 be the last day of the 12th week instead of treating it as the first day of the 13th week?

If you use `System.out.println(cal.getFirstDayOfWeek());` it will print out `2` which is `Calendar.MONDAY`. If you add `cal.set(Calendar.DAY_OF_WEEK, cal.getFirstDayOfWeek());` and the call `cal.getTime()` it will print `Mon Mar 23 18:41:44 EST 2015` — MadProgrammer, Mar 24 '15 at 07:42
You are setting the date to Tuesday with: `cal.set(2015,Calendar.MARCH,24);` — Zion, Mar 24 '15 at 07:42

score 3 · Accepted Answer · edited May 23 '17 at 11:46

tl;dr

LocalDate.of( 2015 , Month.MARCH , 24 )  // `LocalDate` object for 2015-03-24.
         .getDayOfWeek()                 // DayOfWeek.TUESDAY constant object
         .getValue()                     // 2

Avoid legacy date-time classes

Calendar is a ugly mess, as are its sibling classes. Fortunately these old date-time classes are now legacy, supplanted by the java.time classes.

ISO 8601

If you want Monday as the first day of the week, Sunday the last, numbered 1-7, then use the ISO 8601 calendar used by default in the java.time classes.

`DayOfWeek`

The DayOfWeek enum hold predefined objects for each of those ISO days of the week. You can interrogate for its number if need be, though generally better to pass around objects of this enum rather than mere integers.

`LocalDate`

The LocalDate class represents a date-only value without time-of-day and without time zone.

LocalDate ld = LocalDate.of( 2015 , Month.MARCH , 24 );
DayOfWeek dow = ld.getDayOfWeek();
int value = dow.getValue(); // 1-7 for Monday-Sunday. But often better to use the `DayOfWeek` object rather than a mere integer number.

For working with other definitions of a week where Monday is not day number one, see the WeekFields class.

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.

score 2 · Answer 2 · answered Mar 24 '15 at 07:41

2

cal.get(Calendar.DAY_OF_WEEK) will return you which day it is (SUNDAY, MONDAY, etc...) for the given date. So it will return you TUESDAY and then 3, whatever the first day of week is. This has nothing to do with the setFirstDayOfWeek method.

If you want to compute the number of day since the beginning of the week, you just have to get the first day of the week using getFirstDayOfWeek and do some simple math.

answered Mar 24 '15 at 07:41

benzonico

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Thanks for pointing that out, I think I just misunderstood what this method `setFirstDayofWeek` does, but will it affect the result of `cal.WEEK_OF_YEAR` ? – armnotstrong Mar 24 '15 at 07:46
I would say no, but that would deserve some verification and test. – benzonico Mar 24 '15 at 07:48
Damm, I just tested, and it will not affect the result of `cal.WEEK_OF_YEAR`, then what good is this `setFirstDayOfWeek` for? :-( – armnotstrong Mar 24 '15 at 07:49
See the other answer. It is mainly to handle local differences. – benzonico Mar 24 '15 at 07:49
Hi benzonico, I have just edit the question, do you have any opinion on the additional question? – armnotstrong Mar 24 '15 at 07:56
You should probably open another question describing what you want to achieve rather than editing your question. – benzonico Mar 24 '15 at 08:03
Yes, `setFirstDayOfWeek` WILL affect the value for the `cal.WEEK_OF_YEAR` field. That's what it does. – Dawood ibn Kareem Mar 24 '15 at 08:13

score 0 · Answer 3 · answered Mar 24 '15 at 07:49

setFirstDayOfWeek just tells the Calendar which day is to be considered the first day,i.e., Sunday or Monday or any other day. It will not change the dayOfWeek for any arbitrary date. The javadoc for this method states the following:

public void setFirstDayOfWeek(int value)

Sets what the first day of the week is; e.g., SUNDAY in the U.S., MONDAY in France.

Parameters: value - the given first day of the week.