Weird behaviour of Calendar and DateFormat

Question

I encountered really strange behaviour when constructing a Calendar object and then formating it in a particular style.

Let the code do the talking:

public class Test 
{
    public static void main(String[] args) 
    {
        SimpleDateFormat frmt = new SimpleDateFormat();
        frmt.applyPattern("yyyy-MM-dd");
        GregorianCalendar date = new GregorianCalendar(2012,1,1);
        System.out.println(frmt.format(date.getTime()));
    }
}

The output is:

2012-02-01

The expected output is of course:

2012-01-01

What am I doing wrong?

Thanks for the answers, but I really do not know why are months zero based? — Nejc, Nov 07 '12 at 11:00
@Nejc the "why" has been answered here: http://stackoverflow.com/questions/344380/why-is-january-month-0-in-java-calendar — assylias, Nov 07 '12 at 11:00
@assylias Thanks! But I really feel it as counter-intuitive. — Nejc, Nov 07 '12 at 11:02
@Nejc It is hence my recommendation to use named constants - that way there is no ambiguity. — assylias, Nov 07 '12 at 11:03
@assylias I am working on an interactive app, so the user inputs the year, month and day. I thought I would use the numerical representation to construct the date. Maybe I will create a new input method. — Nejc, Nov 07 '12 at 11:08
@Nejc You can still use `input - 1` if you get a numerical input. — assylias, Nov 07 '12 at 11:19
Ofcourse! But I am considering to have an String input, which is formated as yyyy-MM-dd, and then use the formatter to parse string and construct a Calendar object. — Nejc, Nov 07 '12 at 11:23
You make this sound like it's such a big deal. java's been like this since 1995. Figure it out, like so many people before you have. Counter-intuitive? Not in C-world. — duffymo, Nov 07 '12 at 11:36
Maybe I make it sound, like it is a big deal. It isn't to me. But nevertheless, I figured it out and now I know. At least I should expect something like this from java, because it mixes everything together (0-based and 1-based structures). — Nejc, Nov 07 '12 at 12:56
I don't know why you brought C in the debate... I still think this is counter intuitieve (because in our world, january is the 1st month of the year, hence number 1). Because java is not as low-level as C, it would be simple to stick with the intuitive thing. Like this, it is also intuitive to me, that array indices start at 0. — Nejc, Nov 07 '12 at 12:59

score 3 · Accepted Answer · answered Nov 07 '12 at 10:57

3

Months start at 0, so new GregorianCalendar(2012,1,1); is the 1st of February. For 1st of January, you would use

new GregorianCalendar(2012,0,1);
// or even better 
new GregorianCalendar(2012,Calendar.JANUARY,1);

answered Nov 07 '12 at 10:57

assylias

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Subhrajyoti Majumder · Answer 2 · 2012-11-07T11:04:03.110

2

Month start with 0 in GregorianCalendar.

public static final int JANUARY = 0;

It is declared in Clanedar class. and for February it is 1. So update code -

GregorianCalendar date = new GregorianCalendar(2012,Calendar.JANUARY,1);

or

GregorianCalendar date = new GregorianCalendar(2012,0,1);

edited Nov 07 '12 at 11:04

answered Nov 07 '12 at 10:57

Subhrajyoti Majumder

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score 1 · Answer 3 · answered Nov 07 '12 at 10:57

1

Month is 0-based. January == 0. From Calendar#MONTH javadoc:

Field number for get and set indicating the month. This is a calendar-specific value. The first month of the year is JANUARY which is 0; the last depends on the number of months in a year.

answered Nov 07 '12 at 10:57

Aleksandr M

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score 0 · Answer 4 · answered Nov 07 '12 at 10:58

0

Try Joda time if you dont want Months that are indexed at 0

answered Nov 07 '12 at 10:58

andy boot

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score 0 · Answer 5 · answered Nov 07 '12 at 11:00

0

It is documented behavior:

month - the value used to set the MONTH calendar field in the calendar. Month value is 0-based. e.g., 0 for January.

It is advisable to use the constants in Calendar, eg [Calendar.JANUARY][2] when using this constructor.

answered Nov 07 '12 at 11:00

Mark Rotteveel

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Weird behaviour of Calendar and DateFormat

5 Answers5