So far I'm really missing good examples how to build a sound collection hierarchy.

One thing we could do better than the Java API is to use self-types.

Further, I was playing around with the slightly crazy idea to see every collection as something map-like, with "keys" to access "values". These "keys" could be unit (e.g. for a stack or a stream), an enum for front and back (for a dequeue), integers (for random access lists) or the elements itself (for sets). See https://github.com/neco4j-team/neco4j for a sketch of such a hierarchy, starting with org.neco4j.collect.Coll as top-level interface. This is work in progress, and I'm not convinced that it will lead to something useful.

What do you think, do we need some abstraction for our collections? Or are stand-alone collections good enough?