Java Calendar.set(Calendar.DAY_OF_WEEK, Calendar.SUNDAY), will it roll backwards, forwards or unknown?

Question

Suppose the following code is executed on the 22nd of August 2009 (a Saturday)

   Calendar c = Calendar.getInstance();    c.set(Calendar.DAY_OF_WEEK, Calendar.SUNDAY); 

c.get(Calendar.DAY_OF_MONTH) will return 23. I'm interested in the conditions is would return 14 (last Sunday, rather than the next Sunday).

Are there any rules associated with the direction Calendar will roll the DAY_OF_MONTH/YEAR when DAY_OF_WEEK is set? If so what are they?

Answer 1

It should always keep the same WEEK_OF_MONTH (http://java.sun.com/j2se/1.4.2/docs/api/java/util/Calendar.html#WEEK_OF_MONTH). From the documentation:

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.

Answer 2

the following formula returns "current" day in a week in range of [0;6]

(d + numberOfDaysInAWeek - firstDayOfWeek) % numberOfDaysInAWeek 

or add 1 if you would like range [1;7]

(d + numberOfDaysInAWeek - firstDayOfWeek) % numberOfDaysInAWeek + 1 

d is what Calendar.get(Calendar.DAY_OF_WEEK) returns

to get first day of a week, subtract formula's result from current date. The following code does it:

final int currentDayOfWeek = (calendar.get(Calendar.DAY_OF_WEEK) + 7 - cal.getFirstDayOfWeek()) % 7; cal.add(Calendar.DAY_OF_YEAR, -currentDayOfWeek);