When you recompile with -Xlnit what are the error messages that come up? Also can you post us the whole class, or at the least where you create and instantiate the ArrayList?
As the post above suggests, this warning does not seem to just plague classes that don't implement Comparable. I've seen this warning plenty of times using things like Vector<Double> etc.
There are all sorts of reasons to get this warning. And you are right,it doesn't just plague classes that don't implement Comparable. In theory it can plague all classes. It's just a matter of figuring out what it's talking about.