Clojure: How to Expand Varargs into List for Java Method

Question

I am working on a clojure function which accepts a variable number of arguments, and passes those to a java method call

(defn foo [var1 var2 & vars]
    (let [bar (.foo var1)]
        (.gaz bar var2 vars)))

This works when vars is empty, but when I put something in vars, I get

java.lang.ClassCastException: Cannot cast clojure.lang.ArraySeq to [Ljava.lang.Object;

It seems like the vars arent being spliced / interpolated into a vararg list to the method call. How can I do this? I want to use this as both

(foo a b)

and

(foo a b c d e f)

And have it calling

(.gaz bar b c d e f) ;; <- note the list is embedded

what do you mean by "expand into a list" - convert? separate out of a sequence into positional arguments? — noisesmith
– noisesmith, Commented Sep 21, 2014 at 3:35

noisesmith · Accepted Answer · 2014-09-21 03:43:10Z

4

note this this returns exactly the type your error message is asking for:

user> (into-array Object [1 2 3])
#<Object[] [Ljava.lang.Object;@571bc284>

From context, I assume .gaz is likely varargs. If so, all you need is the following:

(.gaz bar var2 (into-array Object vars))

answered Sep 21, 2014 at 3:43

noisesmith

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2 Comments

David Williams Over a year ago

Thanks, works like a charm. I'm not sure I fully understand why it works but I will read up on into-array

noisesmith Over a year ago

The argument needed for varargs is expected to be a Class that a normal Clojure persistent collection does not implement. By converting to a more primitive, mutable Class, we satisfy the jvm's requirements.

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