How to use Scala varargs from Java code

Question

There are plenty of articles on calling Java varargs from Scala code, but the only thing I could find the opposite way round was this question: Using scala vararg methods in java, which doesn't have any concrete examples.

I'm trying to use scala.Console from some Java code, for the reason that java.io.Console doesn't work in Eclipse, whereas the Scala one does. But I cannot get the method

def readLine (text: String, args: Any*): String

to work because it seems to be expecting a scala.collection.Seq[Any] for the second argument, and I don't see how to create a Seq in Java. How can I work around this?

Things I've tried:

1) Using null

// Java
String s = scala.Console.readLine("Enter text: ", null);

- get rewarded with a NullPointerException.

2) Replacing the null with scala.collection.Seq.empty(), but javac reports all sorts of errors such as Seq not having an empty method.

3) Using the Nil object in the scala.collection.immutable package object, but the syntax suggested here, which would be scala.collection.immutable.package$Nil$.MODULE$, but that can't be resolved.

Of course I could just use the readLine() method that doesn't take varargs, but that would be too easy.

Answer 1

You can use:

scala.collection.Seq$.MODULE$.empty();

from Java code to create an empty sequence. Otherwise, you can use:

new scala.collection.mutable.ArrayBuffer();

to create an empty array buffer into which you can then add elements and use it as an argument to Scala vararg methods.

Otherwise, if you design a Scala library with vararg methods which you want to use from Java code, then use the varargs annotation. It will generate a Java version of the method which takes an array instead of a Seq.

scala> class A {
     |   @annotation.varargs def foo(x: Int*) { println(x) }
     | }
defined class A

scala> println(classOf[A].getMethods.toList)
List(public void $line1.$read$$iw$$iw$A.foo(scala.collection.Seq), public void $line1.$read$$iw$$iw$A.foo(int[]), ...)

Above, reflection shows that there are 2 versions of method foo generated - one that takes a Seq[Int] and another which takes an int[].