java - Calculate number of business days between two dates

Question

I have requirement to calculate number of business days between two given dates
I have the list of holidays as an Array list provided by the user.
So I can investigate each and every day between the dates and check if its weekday and not federal holiday like the code I provided below (which is working fine)

But this is very expensive, lets say 12 federal holidays and each day I will have to check its not a weekend,
so if I need to count between 5 years it will take 365 * 5 * 12 its 21,000 iterations! its crazy (not even including the calculation for business day)
Is there a better way?

package test;

import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Arrays;

import java.util.Calendar;
import java.util.Date;
import java.util.List;

import org.apache.commons.lang3.time.DateUtils;

public class TestDiff {

    public static void main(String[] args) throws ParseException {
        DateFormat formatter = new SimpleDateFormat("MM/dd/yy");
        // add 4 years as an example
        Date fromDate = formatter.parse("11/06/2017"),toDate = formatter.parse("11/29/2017");// DateUtils.addDays(fromDate,365 * 4);
        int numberOfDaysCount=0;
        int daysBetween  = daysBetween(fromDate,toDate);
        Date caurDate = fromDate;

        for(int i=0;i<=daysBetween ; i++ ) {
            if(isWeekDay(caurDate) && !isFederalHoliday(caurDate) )
                numberOfDaysCount++;
            caurDate = DateUtils.addDays(caurDate,1); // add one day
        }
        System.out.println("number of business days between "+fromDate+" and "+toDate+" is: "+numberOfDaysCount);
    }

   private static boolean isWeekDay(Date caurDate) {
             Calendar c = Calendar.getInstance();
             c.setTime(caurDate);
              int dayOfWeek = c.get(Calendar.DAY_OF_WEEK);
              return dayOfWeek!= Calendar.SATURDAY && dayOfWeek!= Calendar.SUNDAY ;
    }

        private static boolean isFederalHoliday(Date caurDate) throws ParseException {
            DateFormat formatter = new SimpleDateFormat("MM/dd/yy");    //list will come from dao.getFederalHoliday();
                List<Date> federalHolidays =  Arrays.asList(formatter.parse("01/02/2017"),formatter.parse("01/16/2017"),formatter.parse("02/20/2017"),formatter.parse("05/29/2017"),formatter.parse("07/04/2017"),formatter.parse("09/04/2017"),formatter.parse("10/09/2017"),formatter.parse("07/04/2017"),formatter.parse("11/10/2017"),formatter.parse("11/23/2017"),formatter.parse("12/25/2017"));
                for (Date holiday : federalHolidays) {
                    if(DateUtils.isSameDay(caurDate,holiday)) //using Apache commons-lang 
                        return true;
                }
                return false;
    }

        public static int daysBetween(Date d1, Date d2){
             return (int)( (d2.getTime() - d1.getTime()) / (1000 * 60 * 60 * 24));
     }

}

Answer 1

Here's an answer implemented in Java 8 using java.time.*.

public class TestSo47314277 {

  /**
   * A set of federal holidays. Compared to iteration, using a
   * hash-based container provides a faster access for reading
   * element via hash code. Using {@link Set} avoids duplicates.
   * <p>
   * Add more dates if needed.
   */
  private static final Set<LocalDate> HOLIDAYS;

  static {
    List<LocalDate> dates = Arrays.asList(
        LocalDate.of(2017, 1, 2),
        LocalDate.of(2017, 1, 16),
        LocalDate.of(2017, 2, 20),
        LocalDate.of(2017, 5, 29),
        LocalDate.of(2017, 7, 4),
        LocalDate.of(2017, 9, 4),
        LocalDate.of(2017, 10, 9),
        LocalDate.of(2017, 11, 10),
        LocalDate.of(2017, 11, 23),
        LocalDate.of(2017, 12, 25)
    );
    HOLIDAYS = Collections.unmodifiableSet(new HashSet<>(dates));
  }

  public int getBusinessDays(LocalDate startInclusive, LocalDate endExclusive) {
    if (startInclusive.isAfter(endExclusive)) {
      String msg = "Start date " + startInclusive
          + " must be earlier than end date " + endExclusive;
      throw new IllegalArgumentException(msg);
    }
    int businessDays = 0;
    LocalDate d = startInclusive;
    while (d.isBefore(endExclusive)) {
      DayOfWeek dw = d.getDayOfWeek();
      if (!HOLIDAYS.contains(d)
          && dw != DayOfWeek.SATURDAY
          && dw != DayOfWeek.SUNDAY) {
        businessDays++;
      }
      d = d.plusDays(1);
    }
    return businessDays;
  }
}