Question

I am having the following code

    Calendar calendar = Calendar.getInstance();

    calendar.set(2011, 7, 29);      //case 1    
   // calendar.set(2011, 7, 30);      //case 2
   // calendar.set(2011, 7, 31);    //case 3

    System.out.println("===After seting the date== "+calendar.getTime());

    System.out.println("=================================================================");

    calendar.add(Calendar.MONTH, 6);


    System.out.println("===result after adding 6 months==  "+calendar.getTime());

and for the case 2 and case 3 also i am getting the same results. it should overflow to next month and show the new date. but its not happening.

Answer 1

It’s not clear whether you’re using "should" to mean "this is what I want it to do" or to mean "this is what I believe it’s documented to do". The documentation actually backs up the behaviour given:

add(f, delta) adds delta to field f. This is equivalent to calling set(f, get(f) + delta) with two adjustments:

Add rule 1. The value of field f after the call minus the value of field f before the call is delta, modulo any overflow that has occurred in field f. Overflow occurs when a field value exceeds its range and, as a result, the next larger field is incremented or decremented and the field value is adjusted back into its range.

Add rule 2. If a smaller field is expected to be invariant, but it is impossible for it to be equal to its prior value because of changes in its minimum or maximum after field f is changed or other constraints, such as time zone offset changes, then its value is adjusted to be as close as possible to its expected value. A smaller field represents a smaller unit of time. HOUR is a smaller field than DAY_OF_MONTH. No adjustment is made to smaller fields that are not expected to be invariant. The calendar system determines what fields are expected to be invariant.

In addition, unlike set(), add() forces an immediate recomputation of the calendar’s milliseconds and all fields.

Example: Consider a GregorianCalendar originally set to August 31, 1999. Calling add(Calendar.MONTH, 13) sets the calendar to September 30, 2000. Add rule 1 sets the MONTH field to September, since adding 13 months to August gives September of the next year. Since DAY_OF_MONTH cannot be 31 in September in a GregorianCalendar, add rule 2 sets the DAY_OF_MONTH to 30, the closest possible value. Although it is a smaller field, DAY_OF_WEEK is not adjusted by rule 2, since it is expected to change when the month changes in a GregorianCalendar.

The second part of the example is exactly the situation you’re facing: the DAY_OF_MONTH is expected to be invariant, but has to change in order to stay within the right month, so it’s adjusted to the closest possible value (29 in your case).

So it looks like the behaviour is consistent to me – in what way do you believe it’s inconsistent?