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.