A good chunk of the debugging skill is just having seen a lot of bugs before, and knowing what to look for based on the symptoms alone. This is an extension of your ability to mentally model the behavior of the code, which is fairly central to any programming activity.

Another part of debugging involves various methods for reducing the size of the haystack, but it's not really realistic to rawdog every bug like that, as that would mean each bug would likely take hours rather than a few minutes.

Any half-experienced Java developer should spot the stink in that code immediately.