For instance:
Calendar c = Calendar.getInstance();
DateFormat sdf = new SimpleDateFormat("dd/MM/yyyy");
c.setTime( sdf.parse("31/12/2010"));
out.println( c.get( Calendar.WEEK_OF_YEAR ) );
Prints 1
Same happens with Joda time.
:)
The definition of Week of Year is Locale
dependent.
How it is defined in US is discused in the other posts. For example in Germany (DIN 1355-1 / ISO 8601): the first Week* of Year is the first week with 4 or more days in the new year.
*first day of week is Monday and last day of week is Sunday
And Java’s Calendar
pays attention to the locale. For example:
public static void main(String[] args) throws ParseException {
DateFormat sdf = new SimpleDateFormat("dd/MM/yyyy");
Date lastDec2010 = sdf.parse("31/12/2010");
Calendar calUs = Calendar.getInstance(Locale.US);
calUs.setTime(lastDec2010);
Calendar calDe = Calendar.getInstance(Locale.GERMAN);
calDe.setTime(lastDec2010);
System.out.println( "us: " + calUs.get( Calendar.WEEK_OF_YEAR ) );
System.out.println( "de: " + calDe.get( Calendar.WEEK_OF_YEAR ) );
}
prints:
us: 1
de: 52
ADDED For the US (and I can think of that it is the same for Mexico) the 1. Week of Year is the week where the 1. January belongs to. -- So if 1. Januar is a Saturday, then the Friday before (31. Dec) belongs the same week, and in this case this day belongs to the 1. Week of Year 2011.