clojure gen-class varargs constructor

Question

in the :constructors map and subsequent -init definitions, how do I represent a varargs constructor (assuming the superclass has multiple constructors of which one is varargs) ?

score 2 · Accepted Answer · answered Feb 07 '12 at 14:37

Since varargs are essentially syntax sugar for Object arrays, you could just use "[Ljava.lang.Object;" as the type of constructor's parameter.

Here's some sample code:

(ns t.vtest
  (:gen-class
   :implements   [clojure.lang.IDeref]
   :init init
   :state state
   :constructors {["[Ljava.lang.Object;"] []}))
   ;;                                      ^-----------------------
   ;; You should put "[Ljava.lang.Object;" for superclass varargs constructor here
   ;; I left it blank for the sake of working example 

(defn -init
  [args]
  (println "first element of args" (aget args 0) "total elements" (alength args))
  [[] (into [] args)])

(defn -deref
  [this]
  (.state this))

and that's how it looks in REPL

user=> @(t.vtest. (into-array Object ["A" "B" 1 2]))
first element of args A total elements 4
["A" "B" 1 2]

It doesn't solve problem stated in question. Rather than defining a vararg method it generates method accepting array. This is not the same. The difference is that the vararg is flagged by special ACC_VARARGS opscode in generated byte-code which clojure don't support at the moment. — Lambder, Sep 11 '13 at 09:43

score 1 · Answer 2 · answered Sep 11 '13 at 09:48

Since clojure don't support it at the moment you need to patch it with: https://groups.google.com/forum/#!topic/clojure/HMpMavh0WxA.

And use it with new meta tag:

(ns t.vtest 
  (:gen-class 
   :implements   [clojure.lang.IDeref]
   :init init
   :state state
   :constructors {^:varargs ["[Ljava.lang.Object;"] []} 
  ))

clojure gen-class varargs constructor

2 Answers2