Problems calling a variadic Java function from Clojure

Question

I'm having a play with the Java NIO.2 API from JDK 7.

In particular, I want to call the method: Paths#get(String first, String... more)

This is a static method which takes in at least one string, and returns a Path object corresponding to it. There's an overloaded form: Paths#get(URI uri)

However, I can't seem to call the top method from Clojure. The nearest I can seem to get is this:

(Paths/get ^String dir-fq (object-array 0))

which fails with:

java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.String;

as you might expect. After all, we're passing in an Object[] to something that's expecting String[].

I've tried removing the (object-array) form - but that just causes Clojure to try to call the get(URI) method - both with and without the type hint.

Passing nil as the second argument to Paths#get(String, String...) causes the right method to be called, but Java 7 then fails with an NPE.

I can't seem to find a way in Clojure to express the type String[] - I'm guessing I either need to do that or provide a hint to the dispatch system.

Any ideas?

amalloy · Accepted Answer · 2011-04-12 16:34:50Z

15

As you noticed, it doesn't want an Object[], it wants a String[]. object-array does exactly what it says: it makes an array of objects. If you want to create an array with some different type, make-array and into-array are your friends. For example here:

(Paths/get "foo" (into-array String ["bar" "baz"]))

The String specifier there is optional in this case: if you leave out the array's desired type, Clojure uses the type of the first object as the array's component type.

answered Apr 12, 2011 at 16:34

amalloy

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2 Comments

user650228 Over a year ago

Cool. Any idea why just passing a single String parameter causes the runtime to attempt to call the Paths#get(URI) method and coerce the String into a URI?

amalloy Over a year ago

If there's only one one-argument get method, this wouldn't surprise me. At the bytecode level, as I think you're aware, the vararg version is just a two-arg version taking String, String[]. If you only pass one object, the compiler might say "well there's only one one-arg version, so I'll call that and hope for the best instead of wasting time on reflection to see if it matches".

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