Invoking Java varargs Methods with Scala

A quick little example of invoking a Java varags method from Scala. Suppose you’ve code like the following:

object MyApplication {
  private lazy val javaApi = new JavaApi

  def main(args: Array[String]) {
    val numbers = Array(1, 2, 3)
    javaApi.methodWithVarArgs(numbers)
  }
}

This will generate a nice error message telling you exactly what to do:

.../MyApplication.scala:6: warning: I'm seeing an array passed into a Java vararg.
I assume that the elements of this array should be passed as individual arguments to the vararg.
Therefore I wrap the array in a `: _*', to mark it as a vararg argument.
If that's not what you want, compile this file with option -Xno-varargs-conversion.
    javaApi.methodWithVarArgs(numbers)
                              ^
one warning found
Compile succeeded with 1 warning; see the compiler output for details.

What this is telling you is that you need to help the inferencer by type annotating the argument:

object MyApplication {
  private lazy val javaApi = new JavaApi

  def main(args: Array[String]) {
    val numbers = Array(1, 2, 3)
    javaApi.methodWithVarArgs(numbers: _*)
  }
}

Nice one! Type annotating is something you’ll get used to doing with Scala…