For Scala and Java this is a solved problem; scalac understands just enough Java that you can run scalac, passing it all the source, then run javac, passing it all the Java source.
Beyond that, there's probably some sanity to be gained by *not* combining them in one module, except perhaps very briefly or on a separate branch while converting one way or the other.
Date: Tue, 6 Dec 2011 06:06:29 -0800 (PST)
Subject: [The Java Posse] Polyglot compiling from Episode 371
In 371, there was talk about the desire to have a build framework that would allow Java code to rely on Scala code, and that Scala code to rely on other Java code or maybe code in another JVM language.
Wouldn't the different build frameworks like Maven or Gradle enable this (or at least could be updated to enable this)? Wouldn't you make sure your Java code was in a package that had a dependency on a package that contains the Scala (or whatever) code?