Difference between two maps

Question

I need to very efficiently compare two maps in Clojure/Java, and return the difference as determined by Java\'s .equals(..), with nil/null equivalent to \"not present\".

Answer 1

What about...

(defn map-diff [m1 m2]
  ;; m1: hashmap
  ;; m2: hashmap
  ;; => the difference between them
  (reduce merge
          (map #(hash-map % (- (or (% m1) 0) (or (% m2) 0)))
               (keys (merge m1 m2)))))