Scala Array[String] to Java String varargs

Question

I have some Scala code that needs to call a Java 7 method that is defined as follows:

public void addListener(InputListener listener, String... mappingNames) {
    <details omitted here for brevity>
}

Here is my Scala code that call addListener:

inputManager.addListener(myListener, getActionInputs())

// Then the getActionInputs method:
def getActionInputs() : Array[String] = {
  Array("Red", "Fruit", "Cow")
}

This yields a compiler error:

Type mistmatch, expected: String, actual: Array[String]

I tried importing the Java/Scala conversions classes to convert my Scala Array[String] to a Java array, but have not been successful. Any ideas as to what the fix is?

Yuval Itzchakov · Accepted Answer · 2017-03-20T15:30:56.477

9

You need to add :_* to transform the Array[String] to varargs:

Java:

public class C {
    public void addListener(String... mappingNames) {
        System.out.println(Arrays.toString(mappingNames));
    }
}

Scala:

def main(args: Array[String]): Unit = {
  val c = new C
  c.addListener(Array("a", "b", "c") :_*)
}

Yields:

[a, b, c]

edited Mar 20 '17 at 15:30

answered Mar 20 '17 at 15:19

Yuval Itzchakov

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Isn't that only for Scala varargs? – Jasper-M Mar 20 '17 at 15:27
It appears you're right. I don't think that always used to be the case. Is this new since 2.12? – Jasper-M Mar 20 '17 at 15:31
@Jasper-M I ran this on 2.11.8 – Yuval Itzchakov Mar 20 '17 at 15:32
Yes, it seems to have always been like that. Now I wonder why I thought it wasn't... – Jasper-M Mar 20 '17 at 15:34
@Jasper-M The oldest I found is a forum question from 2010 with Scala 2.8.0 claiming this works. – Yuval Itzchakov Mar 20 '17 at 15:36
Thanks @YuvalItzchakov (+1), does this magical `:_*` construct have a name? Would like to read up on it... – smeeb Mar 20 '17 at 16:13
@smeeb The specification names this [repeated parameters](http://www.scala-lang.org/files/archive/spec/2.12/04-basic-declarations-and-definitions.html#repeated-parameters) (*) and this goes along side [type ascription](http://stackoverflow.com/questions/2087250/what-is-the-purpose-of-type-ascriptions-in-scala) (:_). – Yuval Itzchakov Mar 20 '17 at 16:18

Scala Array[String] to Java String varargs

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