Been working on a large project using by-reference classes (OpenG LabVOOP templates). Got into situations where I wished LabVOOP had native support for interfaces to reduce coupling between the different classes without having to create intermediate classes all the time; an approach one can only take so far since LabVOOP doesn't support multiple inheritance (which, btw, is fine by me: just gimme interfaces).

I prefer getting Scala style traits and mixin class composition instead of Java style interfaces. Practically traits are similar to interfaces but they can have private data of their own and method implementations as well. In mixin class composition the traits are included into some class in very much the same way as in multiple inheritance, however avoiding most of the problems of multiple inheritance.