How do I obtain the number of days within a given month using Joda-Time?

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)

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();
}

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 :)

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

Here is another simple function:

int daysOfMonth(DateTime dt) {
    int month = dt.getMonthOfYear();
    int month2 = month;
    int days = dt.getDay();
    DateTime dt2 = dt;
    while (month == month2 ) {
       days++;
       dt2.addDays(1);
       month2 = dt2.getMonthOfYear();
    }
    return (days - 1);
}