Java 5 requires Iterable to be able to use a collection in the new Java 5 for-loop. Easier, I think, than doing something to implement iterable would be to provide a method that returns the collection being used, wrapped in an Unmodifiable wrapper. A Java 5 collection would probably not return an enumerator, as that's a Java 1 collection method, but that could be left in place and deprecated, for backwards compatibility.