Get starting date of week(configurable starting day of week)

Question

I have current date, and a constant which tells from which day the week starts. I want to get the start date of the week based on that constant. If I hardcode the first day

Answer 1

From the java calendar API http://docs.oracle.com/javase/6/docs/api/java/util/Calendar.html#getFirstDayOfWeek()

public int getFirstDayOfWeek()
Gets what the first day of the week is; e.g., SUNDAY in the U.S., MONDAY in France.
Returns:
the first day of the week.
See Also:

Answer 2

I used the following method:

/** 1 = Sunday, 2 = Monday, 3 = Tuesday, 4 = Wednesday, 5 = Thursday,
* 6 = Friday, 7 = Saturday
*/
public static Date getFirstDayOfWeekDate(int firstDay)
{
    // Calculate the date of the first day of the week

    // First get the today's date
    Calendar c = new GregorianCalendar();

    // Now set the day of week to the first day of week
    while (c.get(Calendar.DAY_OF_WEEK) != firstDay)
    {
        c.add(Calendar.DAY_OF_MONTH, -1);
    }

    return c.getTime();
}

Answer 3

DateTimeFormatter format = DateTimeFormatter.ofPattern("MM/dd/yyyy");
    LocalDate now = LocalDate.now();
String  startDate = now.with(TemporalAdjusters.previous(DayOfWeek.SUNDAY)).format(format);

Answer 4

tl;dr

LocalDate.now( ZoneId.of( "America/Montreal" ) )
         .with( TemporalAdjusters.previousOrSame( DayOfWeek.SUNDAY ) )  // Specify your desired `DayOfWeek` as start-of-week.
         .atStartOfDay( ZoneId.of( "America/Montreal" ) )

See this code run live at IdeOne.com.

zdt: 2017-07-09T00:00-04:00[America/Montreal] | day-of-week: SUNDAY

Avoid legacy classes

You are using the troublesome old date-time classes that are now legacy, supplanted by the java.time classes.

DayOfWeek

Rather than use mere integer numbers to represent day-of-week in your code, use the DayOfWeek enum built into Java. This gains you type-safety, ensures valid values, and makes your code more self-documenting.

DayOfWeek weekStart = DayOfWeek.SUNDAY ;  // Pass whatever `DayOfWeek` object you want.

TemporalAdjuster & LocalDate

The TemporalAdjuster interface enables ways to manipulate a date to get another date. Find some implementations in TemporalAdjusters class (note plural).

ZoneId z = ZoneId.of( "America/Montreal" ) ;
LocalDate today = LocalDate.now( z ) ;
LocalDate start = today.with( TemporalAdjusters.previousOrSame( weekStart ) ) ;  

ZonedDateTime

To get an exact moment, ask the LocalDate for its first moment of the day. That moment depends on a time zone, as the date varies around the globe for any given moment.

ZonedDateTime zdt = start.atStartOfDay( z ) ;

Instant

If you want to view that some moment as in UTC, extract an Instant object.

Instant instant = zdt.toInstant() ;  // Same moment, different wall-clock time.