Clojure Programming Example 2-1. Implementation of a mutable integer in "Clojure"

StatefulIntegerExample.clj

;;; Clojure Programming Chapter 2. Functional Programming 
;;;   Example 2-1. Implementation of a mutable integer in "Clojure"
(defprotocol IStatefulInteger
  (setInt [this new-state])
  (intValue [this]))

(deftype StatefulInteger [^{:volatile-mutable true} state]
  IStatefulInteger
  (setInt [this new-state] (set! state new-state))
  (intValue [this] state)
  Object
  (hashCode [this] state)
  (equals [this obj]
    (and (instance? (class this) obj)
         (= state (.intValue obj)))))

;;;
;;; P.55 example
;;;
user=> (def five (StatefulInteger. 5))
;#'user/five
user=> (def six (StatefulInteger. 6))
;#'user/six
user=> (.intValue five)
;5
user=> (.intValue six)
;6
user=> (= five six)
;false
user=> (.setInt five 6)
;6
user=> (= five six)
;true
user=>