Github user bozzzzo commented on the pull request: https://github.com/apache/qpid-proton/pull/48#issuecomment-127597879 > Could we use generics to enforce the correct type of handler is supplied for dispatch and avoid some instanceof checks? (e.g BuiltinEventTypeImpl.java) 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. 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.
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