Two tips for Java and Scala interoperability

I'm writing more and more Scala these days but don't want to give up on Java libraries I know and love. To this end, I've been writing ScalaTests with Mockito. It's fairly straightforward but two things initially foxed me.

Varargs

The first were varargs. "Both java and scala have varargs" but calling a Java method using varargs from Scala is not obvious.

I had code that looked like:

val x: List[X]       = ...
val ordered: InOrder = Mockito.inOrder(x)

which didn't compile because x was just being treated like an Object rather than an array of them.

The solution looks like this:

val ordered: InOrder = Mockito.inOrder(x:_*)


Conversions between Java and Scala collections

This is solved with a simple import.

import collection.JavaConversions._

Now we can do things like:

Thread.getAllStackTraces.map(x => println(x._1 + ":\n" + (x._2.mkString("\n    "))))

to print out all the stack traces of all the threads currently running. Without this import, the map function would fail as getAllStackTraces returns Map<Thread, StackTracesElement[]> - a Java Map that doesn't have the map method.