How to skip weekends while adding days to LocalDate in Java 8?

Question

Other answers here refer to Joda API. I want to do it using java.time.

Suppose today's date is 26th Nov 2015-Thursday, when I add 2 business days to it, I want the result as Monday 30th Nov 2015.

I am working on my own implementation but it would be great if something already exists!

EDIT:

Is there a way to do it apart from looping over?

I was trying to derive a function like:

Y = f(X1,X2) where
Y is actual number of days to add,
X1 is number of business days to add, 
X2 is day of the week (1-Monday to 7-Sunday)

Then given X1 and X2 (derived from day of week of the date), we can find Y and then use plusDays() method of LocalDate.

I have not been able to derive it so far, its not consistent. Can anyone confirm that looping over until desired number of workdays are added is the only way?

Answer 1

The following method adds days one by one, skipping weekends, for positive values of workdays:

public LocalDate add(LocalDate date, int workdays) {
    if (workdays < 1) {
        return date;
    }

    LocalDate result = date;
    int addedDays = 0;
    while (addedDays < workdays) {
        result = result.plusDays(1);
        if (!(result.getDayOfWeek() == DayOfWeek.SATURDAY ||
              result.getDayOfWeek() == DayOfWeek.SUNDAY)) {
            ++addedDays;
        }
    }

    return result;
}

---

After some fiddling around, I came up with an algorithm to calculate the number of workdays to add or subtract.

/**
 * @param dayOfWeek
 *            The day of week of the start day. The values are numbered
 *            following the ISO-8601 standard, from 1 (Monday) to 7
 *            (Sunday).
 * @param businessDays
 *            The number of business days to count from the day of week. A
 *            negative number will count days in the past.
 * 
 * @return The absolute (positive) number of days including weekends.
 */
public long getAllDays(int dayOfWeek, long businessDays) {
    long result = 0;
    if (businessDays != 0) {
        boolean isStartOnWorkday = dayOfWeek < 6;
        long absBusinessDays = Math.abs(businessDays);

        if (isStartOnWorkday) {
            // if negative businessDays: count backwards by shifting weekday
            int shiftedWorkday = businessDays > 0 ? dayOfWeek : 6 - dayOfWeek;
            result = absBusinessDays + (absBusinessDays + shiftedWorkday - 1) / 5 * 2;
        } else { // start on weekend
            // if negative businessDays: count backwards by shifting weekday
            int shiftedWeekend = businessDays > 0 ? dayOfWeek : 13 - dayOfWeek;
            result = absBusinessDays + (absBusinessDays - 1) / 5 * 2 + (7 - shiftedWeekend);
        }
    }
    return result;
}

Usage Example:

LocalDate startDate = LocalDate.of(2015, 11, 26);
int businessDays = 2;
LocalDate endDate = startDate.plusDays(getAllDays(startDate.getDayOfWeek().getValue(), businessDays));

System.out.println(startDate + (businessDays > 0 ? " plus " : " minus ") + Math.abs(businessDays)
        + " business days: " + endDate);

businessDays = -6;
endDate = startDate.minusDays(getAllDays(startDate.getDayOfWeek().getValue(), businessDays));

System.out.println(startDate + (businessDays > 0 ? " plus " : " minus ") + Math.abs(businessDays)
        + " business days: " + endDate);

Example Output:

2015-11-26 plus 2 business days: 2015-11-30

2015-11-26 minus 6 business days: 2015-11-18