Java never fails to astound in its verbosity, even today, even with much better utility functions for things like lists (flatMap/collect are new to me). But, now that I've taken the time to read through it, I see the actual flatten function isn't actually all that different an implementation from the Perl and Python variants other folks have suggested. But, Java sure does make you work for that list data structure.

I know it's verbose because of type definitions, and because there's no bare functions in Java, but it sure does balloon up small programs with a lot of boilerplate, and tends to hide the point of the program behind stuff that doesn't look like the purpose of the program.