Github user gemmellr commented on the pull request: https://github.com/apache/qpid-proton/pull/48#issuecomment-127622789 > I'm not sure how this would work, because it's easy (and I think necessary) to mix and create a tangle > of handlers of different descent in a tree (or even DAG) of handlers with Handler#add(Handler). > > So an Event instance will get dispatched to many handlers, and necessarily only some will be of an assignable type. I'm not sure how to express that with generics. Yeah, the rest of my reply was getting at that. It just seemed a little icky that everyone who implements a handler should need the instanceof check and call unhandled othewise. Not saying I have an answer :) > The core can afford the luxury of declaring 'everybody extend me' but that is not viable for a library that sits on top of proton. Not sure I understood this bit. If I was writing a library on top of Proton I'd possibly want to declare my own 'everybody extend me' point if things outwith that library were expected to extend it (to try and insulate somewhat from changes like this in Proton).
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