Java 1.5 Calendar#compareTo() confusion

Question

Ok, I'm officially stumped on this one. I have a GregorianCalendar object that I would like to determine if it is in the past, present, or future. So far, the Calendar#compareTo docs are confusing to me, in that I am getting erratic results. A simple test class will illustrate my problem:

import java.util.Calendar;
import java.util.GregorianCalendar;
public class MyCal
{

        public static void main( String[] args )
        {
                GregorianCalendar datePast = new GregorianCalendar();
                datePast.add(Calendar.MONTH, -6); // subtract 6 months
                GregorianCalendar datePresent = new GregorianCalendar();
                GregorianCalendar dateFuture = new GregorianCalendar();
                datePast.add(Calendar.MONTH, 6); // add 6 months

                System.out.println("compare datePresent to datePast: "+datePresent.compareTo(datePast));
                System.out.println("compare datePresent to datePresent: "+datePresent.compareTo(datePresent));
                System.out.println("compare datePresent to dateFuture: "+datePresent.compareTo(dateFuture));
        }
}

And the output:

compare datePresent to datePast: 1
compare datePresent to datePresent: 0
compare datePresent to dateFuture: 0

My understanding of compareTo is that the last line should be a -1. Can anyone tell me what I'm doing wrong?

Answer 1

You've added 6 months to datePast, not dateFuture. Here's the working code:

import java.util.Calendar;
import java.util.GregorianCalendar;
public class MyCal
{

    public static void main( String[] args )
    {
        GregorianCalendar datePast = new GregorianCalendar();
        datePast.add(Calendar.MONTH, -6); // subtract 6 months
        GregorianCalendar datePresent = new GregorianCalendar();
        GregorianCalendar dateFuture = new GregorianCalendar();
        dateFuture.add(Calendar.MONTH, 6); // add 6 months

        System.out.println("compare datePresent to datePast: "+datePresent.compareTo(datePast));
        System.out.println("compare datePresent to datePresent: "+datePresent.compareTo(datePresent));
        System.out.println("compare datePresent to dateFuture: "+datePresent.compareTo(dateFuture));
    }
}

Your results would have been inconsistent because sometimes dateFuture would be later than datePresent, depending on when the internal clock "ticked".

In addition to all of this, I can't help but give my standard Java date and time advice: use Joda Time instead. It's not responsible for this particular problem, although the fact that many of the types are immutable would have helped. But in general it's a much better library.