|Keep It Simple, Stupid
With regard to the threading comment, you also test if a variable (list) is null and if it is - create a new ArrayList.
You should synchronise on the list which may mean not accepting null list variables.
As to anonymous classes, its 6 of one and half a dozen of another. They do provide the means of creating nice customisable blocks of code - I think its too easy, sometimes, to get caught up making everything defined types and using patterns (etc) but they are there. Additionally, I was thinking about your switch statement and how you are using integer vars to decide what to do. I was kind of hinting that it may be possible to keep your annonymous front end syntax and make the backend a little more complex and eliminate the switch statement as well as allow you to add more perlisms at a later stage. Note, I don't say it *is* possible, only that it *may be* possible :).
Finally, your code would look even better on 1.5 as static imports will allow (if there are no namespace collisions) for you to ignore the class name ie:
So your looping methods will look more perl-ish still (hurrah!).