In Scala, how can I define a companion object for a class defined in Java?

Question

I'd like to add implicit conversions to Java classes generated by a modeling tool. So I want to add them to the companion object of those classes, so that the compiler automatically finds them. But I cannot add them in a separate file, because the companion has to be defined in the same file. Is there anything I can do about this?

Of course, I can define all my implicit conversions in another object and then bring it into scope, but this requires an extra import. Any other solution?

Answer 1

With the Scala compiler as it stands now there is no way to define companion objects other than by putting them in the same file. The best you can do is a non-companion object with the same package and name and an extra import.

If you can think of a good way to create post-hoc companionship without breaking assumptions about encapsulation please come post on http://groups.google.com/group/scala-debate because it would clearly be a very useful feature.

Answer 2

You can define your own companion object of course, which I often do in my own project-specific Predef-like arrangement. For example:

object domain {

  type TimeUnit = java.util.concurrent.TimeUnit
  object TimeUnit {
    def valueOf(s : String) = java.util.concurrent.TimeUnit.valueOf(str)
    val Millis = java.util.concurrent.TimeUnit.MILLISECONDS
    //etc
  }

Then this can be used:

import my.domain._
val tu : TimeUnit = TimeUnit.valueOf("MILLISECONDS")

But your domain.TimeUnit is a module (i.e. scala object)