Find the Day of week of a specific date when specified the first day of week

Question

I am implementing a method similar to the VB Weekday function that can give the weekday number given any Day-of-Week as the start for the week.

For the example of "2010-02-16" (which is on a Tuesday), and a first-day-of-week of Sunday (1), I expect the value 3 as outcome.

Answer 1

This can be done with the help of Calendar class and little calculation.

1. Parse the Date
2. Get Calendar Instance and initialize it with parsed Date
3. Get the day of week number at that time
4. Calculate day of week w.r.t. startDay by doing the calculation pointed out by @Markus

Below method demonstrates that:

public int Weekday(String dateString, int startDay) {
    Date date = null;

    try {
        SimpleDateFormat desiredFormat = new SimpleDateFormat("yyyy-MM-dd");
        date = desiredFormat.parse(dateString);
    } catch (ParseException e) {
        e.printStackTrace();
        return -1;
    }

    Calendar c = Calendar.getInstance();
    c.setTime(date);
    if (startDay == 0) {
        return c.get(Calendar.DAY_OF_WEEK);
    }
    else {
        return (((c.get(Calendar.DAY_OF_WEEK) - startDay + 7) % 7) + 1);
    }
}

Update: Thanks to comment by @Markus

Answer 2

i have implemented below menitoned method in java using jodaTime which will return the difference.

public int generateDifference(String dtStr, int dayOfWeek) {
    DateTimeFormatter formatter = DateTimeFormat.forPattern("yyyy-MM-dd");
    DateTime dt = formatter.parseDateTime(dtStr);
    int day = dt.getDayOfWeek() + 1;
    if (dayOfWeek < day) {
        return 8 - Math.abs(dayOfWeek - day);
    }
    return 2 + Math.abs(dayOfWeek - day);
}

Answer 3

Below accepts the date provided as string, and formatted in the ISO-8601 extended local date format (e.g. 2010-02-16). The day of Week is using the DayOfWeek Enum. Note however that this depends on Java 8 (standard libraries, not Joda-Time). Use of LocalDate is important to avoid TimeZone and Time issues (partial days).

UPDATE: also provided a static method mimicking the VB version with an integer parameter for the date. All except 0 (System default based on NLS) are accepted.

UPDATE #2: Had to modify previous into previousOrSame otherwise we would get 8 instead of 1 as result. The result is expected to be in the range 1-7.

import java.time.DayOfWeek;
import java.time.LocalDate;
import static java.time.DayOfWeek.*;
import static java.time.temporal.ChronoUnit.*;
import static java.time.temporal.TemporalAdjusters.*;

public class Main {
  public static void main(String[] args) {
    // Thanks to @Markus for the testcase
    System.out.println(getVbWeekday("2010-02-16", 1));
    System.out.println(getVbWeekday("2015-05-03", 1));
    System.out.println(getVbWeekday("2015-05-04", 1));
    System.out.println(getVbWeekday("2015-05-05", 1));
    System.out.println(getVbWeekday("2015-05-06", 1));
    System.out.println(getVbWeekday("2015-05-07", 1));
    System.out.println(getVbWeekday("2015-05-08", 1));
    System.out.println(getVbWeekday("2015-05-09", 1));
    System.out.println(getVbWeekday("2015-05-10", 1));
  }

  public static int getWeekdayNumber(String dateAsString,DayOfWeek firstDayOfWeek) {
    LocalDate d = LocalDate.parse(dateAsString);
     // add 1 because between is exclusive end date.
    return (int)DAYS.between(d.with(previousOrSame(firstDayOfWeek)),d)+1;
  }

  public static int getVbWeekday(String dateAsString,int firstDayOfWeek) {
    return getWeekdayNumber(dateAsString,SATURDAY.plus(firstDayOfWeek));
  }
}

Answer 4

Summarizing the answers of JoD. and Asif Mujteba, that should be something like this:

import java.time.*;
import java.util.*;

public class SampleClass {

     public static void main(String []args) throws Exception {
        System.out.println(Weekday("2010-02-16", 1));
        System.out.println(Weekday("2015-05-03", 1));
        System.out.println(Weekday("2015-05-04", 1));
        System.out.println(Weekday("2015-05-05", 1));
        System.out.println(Weekday("2015-05-06", 1));
        System.out.println(Weekday("2015-05-07", 1));
        System.out.println(Weekday("2015-05-08", 1));
        System.out.println(Weekday("2015-05-09", 1));
        System.out.println(Weekday("2015-05-10", 1));
     }

    public static int Weekday(String dateString, int startDay) throws Exception {
        LocalDate date = LocalDate.parse(dateString);
        return (((date.getDayOfWeek().ordinal() - startDay + 2) % 7) + 1;
    }
}

This will return:

3
1
2
3
4
5
6
7
1

And thats is the same as DateAndTime.Weekday returns ...