Roll method doesn't work properly

Question

Hi there,
I ran into a problem with the roll method of GregorianCalendar class. Suppose the date is April 6, 00:00:00 2002, the following code will go throught the time changes:
GregorianCalendar cal = new GregorianCalender();
for (int i = 0; i < 24; i++) {
cal.roll(Calenader.HOUR_OF_DAY, 1);
April 6 01:00:00 2002
April 6 02:00:00 2002
April 6 22:00:00 2002
April 6 23:00:00 2002
April 6 00:00:00 2002
But if the current date is April 7, 2002 (daylight saving time change will occur at 2:00 am this day), the above code will result:
April 7 01:00:00 2002
April 7 02:00:00 2002
April 7 22:00:00 2002
April 7 23:00:00 2002
April 6 23:00:00 2002
April 6 00:00:00 2002
Can anyone tell me how to fix this problem or how to avoid this problem.
Thanks a lot.

Answer 1

It doesn't work quite that way for me, my first two lines are these:
April 6 01:00:00 PST 2002
April 6 03:00:00 PDT 2002
You'll notice that I printed the time zone out as well. My last three lines are these:
Sun Apr 07 23:00:00 PDT 2002
Sat Apr 06 23:00:00 PST 2002
Sat Apr 06 00:00:00 PST 2002
and I think there is an argument to be made that these are correct: applying roll() to Sun Apr 07 23:00:00 PDT 2002 should yield Sun Apr 07 00:00:00 PDT 2002; but since daylight saving time was not in effect at that moment, this is equivalent to Sat Apr 06 23:00:00 PST 2002, which is the actual result. On the other hand, here's a quote from the API documentation for Calendar:
"Roll rule. Larger fields are unchanged after the call."
which is an argument that they are incorrect: after the call, the hour should change but not the day. So I'd report it as a bug and see what answer you get.
How to fix the problem? What's the purpose of what you are doing?