Unpredictable Program Behaviour in Java

Question

I'm pulling my hair out with this and I thought I'd see if the greater Java experience of others might be able to shed some light on the problem. There is a large amount of program code I have written, which is itself within a larger project so I can't simply post it. However, I will outline the problem...

The issue: My code does not return predicatble results from a simulation. Each time the simulation is run, various statistics are recorded. Despite the fact that the code is identical (!), the results produced differ between executions.

Things I am pretty sure are not a problem (although if you think there's a "gotcha" please shout out):

• A random number generator is used but it is seeded with the same value each time.
• The program is single threaded so race conditions should not be an issue.
• Behaviour occurs in both debug mode, standalone jar and normal execution from the IDE (I'm using eclipse).
• Static members are used between objects in lists. The for-each construct is used, but this should execute in the same order each time.
• In the case of sorting the lists just mentioned, Collections.sort() is utilised and so should be stable (again, lists should be ordered in the same order)

Can anyone think of something I might be overlooking here? This seems unfathomable at the moment! Thanks!

Answer 1

Are you sure that you are using the instance of Random that you created in all places? Remember that calls to Math.random() use their own instance of Random.

Answer 2

I'm assuming you've overridden both equals() and hashCode() in your objects (or you've written a custom comparator). If your random number is used in your objects, it's not a criteria in the sort, right?

Also, if you're sorting these items then placing them into a HashMap or a HashSet - these structures will not preserve your order - use TreeSet or TreeMap instead.