Question

I’m trying to understand how java.util.Calendar.get(java.util.Calendar.WEEK_OF_YEAR) works, but it seems that I’m missing some points.

String time = "1998-12-31"; // year month day
java.util.Calendar date = java.util.Calendar.getInstance();
date.setTime((new java.text.SimpleDateFormat("yyyy-MM-dd")).parse(time));
System.err.println("Week of year = " + date.get(java.util.Calendar.WEEK_OF_YEAR));
// Week of year = 1 Why ???

Why date.get(java.util.Calendar.WEEK_OF_YEAR) returns 1 for the last week of the year?

Moreover, WEEK_OF_YEAR for "1998-01-01" is 1 and for "1998-12-23" it is 52.
Does anybody have an explanation for this behavior?

Answer 1

From java.util.Calendar javadoc:

First Week

Calendar defines a locale-specific seven day week using two
parameters: the first day of the week and the minimal days in first
week (from 1 to 7). These numbers are taken from the locale resource
data when a Calendar is constructed. They may also be specified
explicitly through the methods for setting their values.

When setting or getting the WEEK_OF_MONTH or WEEK_OF_YEAR fields,
Calendar must determine the first week of the month or year as a
reference point. The first week of a month or year is defined as the
earliest seven day period beginning on getFirstDayOfWeek() and
containing at least getMinimalDaysInFirstWeek() days of that month or
year. Weeks numbered …, -1, 0 precede the first week; weeks numbered
2, 3,… follow it. Note that the normalized numbering returned by
get() may be different. For example, a specific Calendar subclass may
designate the week before week 1 of a year as week n of the previous
year.

So it’s locale-specific. In your case, if the week contains days from new year, it is counted as week 1 from the new year.

You can change this behavior by using Calendar#setMinimalDaysInFirstWeek(int).