obtaining ammount of days of a given month with jodatime (Java)

Question

30 days hath September,
   April, June and November,
 All the rest have 31,
   Excepting February alone
(And that has 28 days clear,
   With 29 in each leap year).

Can I obtain this info anagrammatically? (I don't mean the poem, of course)

Thanks!
Manuel

Answer 1

int datsOfMonth(DateTime dt) {
    switch(dt.getMonthOfYear()) {
        case 1: return 31;
        case 2: return isLeap(dt.year()) ? 29 : 28; // you'll have to do isleap on your own, but it's easy, and probably included somewhere
        case 3: return 31;
        case 4: return 30;
        case 5: return 31;
        case 6: return 30;
        case 7: return 31;
        case 8: return 31;
        case 9: return 30;
        case 10: return 31;
        case 11: return 30;
        case 12: return 31;
    }
    throw new IllegalArgumentException();
}

Answer 2

Calendar cal = Calendar.getInstance();
cal.set(Calendar.MONTH, yourMonth);
cal.set(Calendar.YEAR, yourYear);
cal.getActualMaximum(Calendar.DAY_OF_MONTH); // <-- the result!

Answer 3

If you have a DateTime object which represents a value in the month, then it's pretty straightforward. You get the dayOfMonth property from that DateTime object and get the maximum value of the property. Here is a sample function:

public static int daysOfMonth(int year, int month) {
  DateTime dateTime = new DateTime(year, month, 14, 12, 0, 0, 000);
  return dateTime.dayOfMonth().getMaximumValue();
}

Answer 4

Yes, although it's not as pretty as it might be:

import org.joda.time.*;
import org.joda.time.chrono.*;
import org.joda.time.field.*;

public class Test {
    public static void main(String[] args) {
        GregorianChronology calendar = GregorianChronology.getInstance();
        DateTimeField field = calendar.dayOfMonth();

        for (int i = 1; i < 12; i++) {
            LocalDate date = new LocalDate(2010, i, 1, calendar);
            System.out.println(field.getMaximumValue(date));
        }
    }
}

Note that I've hard-coded the assumption that there are 12 months, and that we're interested in 2010. I've explicitly selected the Gregorian chronology though - in other chronologies you'd get different answers, of course. (And the "12 month" loop wouldn't be a valid assumption either...)

I've gone for a LocalDate rather than a DateTime in order to fetch the value, to emphasize (however faintly :) that the value doesn't depend on the time zone.

This is still not as simple as it looks, mind you. I don't know off-hand what happens if use one chronology to construct the LocalDate, but ask for the maximum value of a field in a different chronology. I have some ideas about what might happen, knowing a certain amount about Joda Time, but it's probably not a good idea :)