[ajss]

(defn [o n] (.write o n)) will have to look up the .write method by doing reflection on o & n at runtime, which is really slow.

(defn [^WriteInterface o ^long n] (.write o n)) can be much faster because the compiler knows the type of o & n.

[abc_lisper]

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.