How to get first day of a given week number in Java

Question

Let me explain myself. By knowing the week number and the year of a date:

Date curr = new Date();
Calendar cal = Calendar.getInstance();
cal.setTime(curr);
i         


        

Answer 1

Try this:

public static Calendar setWeekStart(Calendar calendar) {
  while (calendar.get(Calendar.DAY_OF_WEEK) != Calendar.MONDAY) {
    calendar.add(Calendar.DATE, -1);
  }
  setDayStart(calendar); // method which sets H:M:S:ms to 0
  return calendar;
}

Answer 2

Yet another way...

    GregorianCalendar cal = new GregorianCalendar();
    cal.clearTime();
    Integer correction = 1-cal.get(GregorianCalendar.DAY_OF_WEEK)
    cal.add(Calendar.DATE, correction);

cal is now the first day of the week

1-cal.get(GregorianCalendar.DAY_OF_WEEK)

evaluates to 1-1 for Sunday (first day of week in my Locale) and 1-2 for Monday, so this will give you the correction needed to rewind the clock back to Sunday

Answer 3

I haven't done much Date stuff in java but a solution might be:

cal.set(Calendar.DAY_OF_YEAR, cal.get(Calendar.DAY_OF_YEAR) - cal.get(Calendar.DAY_OF_WEEK));

Logic:

Get the day of the week and substract it from the current date (might need -1, depending on wether you need monday to be first day of the week or sunday)

Answer 4

Try the Gregorian Calendar algorithm:

public int getFirstDay(int m, int year)
    {
        int k=1;
        int c, y, w, M=0;
        if(m>2 && m<=12) M=m-2;
        else if(m>0 && M<=2)
        {
            M=m+10;
            year-=1;
        }
        c=year/100;
        y=year%100;
        w=(int)((k+(Math.floor(2.6*M - 0.2))-2*c+y+(Math.floor(y/4))+(Math.floor(c/4)))%7);//a fantastic formula           
        if(w<0) w+=7;
        return w;//thus the day of the week is obtained!
    }

Answer 5

Be cautious with those, calendar.get(Calendar.WEEK_OF_YEAR) returns 1 if it is end of December and already a week that ends in the next year.

Using

//            cal2.set(Calendar.WEEK_OF_YEAR, cal.get(Calendar.WEEK_OF_YEAR));
//            cal2.set(Calendar.DAY_OF_WEEK, Calendar.MONDAY);

sets the cal2 date to e.g. end of 2009 on 29/12/2010 !!

I used this:

cal2.set(Calendar.DAY_OF_YEAR, cal.get(Calendar.DAY_OF_YEAR)); //to round at the start of day
cal2.set(Calendar.YEAR, cal.get(Calendar.YEAR));
cal2.set(Calendar.DAY_OF_WEEK, Calendar.MONDAY); //to round at the start of week

I also make sure that weeks in my calendars, no matter what locale they are in, are starting on Mondays:

cal.setFirstDayOfWeek(Calendar.MONDAY);
cal2.setFirstDayOfWeek(Calendar.MONDAY);

Answer 6

Those fields does not return the values. Those are constants which identifies the fields in the Calendar object which you can get/set/add. To achieve what you want, you first need to get a Calendar, clear it and set the known values. It will automatically set the date to first day of that week.

// We know week number and year.
int week = 3;
int year = 2010;

// Get calendar, clear it and set week number and year.
Calendar calendar = Calendar.getInstance();
calendar.clear();
calendar.set(Calendar.WEEK_OF_YEAR, week);
calendar.set(Calendar.YEAR, year);

// Now get the first day of week.
Date date = calendar.getTime();

Please learn to read the javadocs to learn how to use classes/methods/fields and do not try to poke random in your IDE ;)

That said, the java.util.Date and java.util.Calendar are epic failures. If you can, consider switching to Joda Time.