Using Date and Calendar – At your own risk!

You can find lots of issues discussed on the ‘net about the problems of the Calendar/Date classes in Java. Here are a few that recently reared their ugly head in our code:

1. This one is actually not in either of these but in trying to output our Date. Beware of the mm instead of MM when using SimpleDateFormat. Yep, one of my team members used the following code:

SimpleDateFormat sdf = new SimpleDateFormat(“yyyymmdd”);

Surprisingly everyone in our tests had a birthday of the first of the month! I think that the right solution would be that mm is wrong and mi is minutes and mo is month. This would solve the confusion.

2. Did you know that there is a difference whether you use Calendar.HOUR or Calendar.HOUR_OF_DAY? Ok, i admit it this one is documented in the javdocs, but i would still think that it is a bit of surprise that in the afternoon, if you do

cal.set(Calendar.HOUR,0);

that you will get 12:00 PM!

3. Why does toString() of Calendar not put out the date in some format?

4. Beware that DAY_OF_WEEK is not zero based while MONTH is.