Implement a Java interface method with a variable number of args in Clojure

Question

I know that I implement a Java interface in Clojure using proxy like this:

(def print-element-handler
  (proxy [DefaultHandler] []
    (startElement [uri local qname atts]
      (println (format "Saw element: %s" qname)))))

Note that there are four args, [uri local qname atts], for the four args in the interface method, startElement.

Suppose a method in a Java interface has a variable number of args like this:

List<Task> getTasks(Object... args);

What do I put for the arg list in the corresponding Clojure function?

Jeremy · Accepted Answer · 2012-02-01 22:23:08Z

4

I am not 100% certain, as I am not able to test it out at the moment, but I believe the answer is you just have a single parameter for the array. I know for a fact that when you call a Java method with varags, you have to convert the Clojure collection into an array before passing it in. I imagine it's no different here.

For an example, here is the source for format:

(defn format
  "Formats a string using java.lang.String.format,
   see java.util.Formatter for format string syntax"
  {:tag String
   :added "1.0"}
  [fmt & args]
  (String/format fmt (to-array args)))

answered Feb 1, 2012 at 22:23

Jeremy

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1 Comment

amalloy Over a year ago

Right. So you do something like (reify TheInterface (getTasks [this tasks] (doseq [task tasks] (...)))). The Object[] that tasks really is, at the bytecode level, is just a single parameter. Then in the implementation you treat it like any other seq.

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