I’ve recently hit on a nasty bug in Java. It sits in
Date class and shows up only at noon!
I’m going to demonstrate it using Groovy shell, but you can reproduce it in plain Java environment too.
So far so good.
I chose a specific formatter on line 1 to make the bug even more evident. With this formatter lines 5, 8, and 11 must be identical on my machine, and they are in this example. The assertion on line 13 also proves the equality.
Now let’s change the example date to one hour earlier
Lines 2, 5, and 8 are not identical any more. The String representation of the date is 12 hours off.
To make sure the problem is in
toString and not in
parse, let’s format the date using formatter
Looks good. The problem is in
toString indeed. Or is it?
Let’s parse 13 o’clock date and 12 o’clock date. The difference between them should be 1 hour. In reality
Wow, the problem is actually in
parse. Then how come the
format returned the correct value? That’s still a mystery to me.
I’m actually quite surprised that this bug survived through JDK 1.7.0_09, and neither Sun nor Oracle hasn’t fixed it yet.