I think you meant to say, (defn func[o n] (.write ^WriteInterface o ^long n)) This is needed only when calling Java methods, because Java allows overloading. C

I think you meant to say, (defn func[o n] (.write ^WriteInterface o ^long n)) This is needed only when calling Java methods, because Java allows overloading. Clojure only does arity-overloading, not type overloading, and hence doesn't need the type info.