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?

You could define implicit conversions in the package object, avoiding the extra import. — axel22, Feb 07 '11 at 13:41
Java classes don't have a companion object. Do you mean you're trying to cheat and make it look like a Java class has a companion object? — Rex Kerr, Feb 07 '11 at 13:48
@Rex Kerr: Yeah, well, I didn't know it was considered cheating. @axel22: Good idea, I think I'll go ahead with that! — Jean-Philippe Pellet, Feb 07 '11 at 13:56
I actually would like to ask a similar question when the purpose is to write a pattern extractor (`unapply`) for the Java class, without having to use a different name for the object. — Ken Bloom, Feb 07 '11 at 14:18

score 10 · Answer 1 · answered Feb 07 '11 at 15:01

10

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)

answered Feb 07 '11 at 15:01

oxbow_lakes

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That would a "standalone object", not a companion object, no? (the end result is the same here) – VonC Feb 07 '11 at 15:13
3

it, at least, won't work for implicit conversions (which are a sweet spot of companion objects) – Vasil Remeniuk Feb 07 '11 at 15:34
That is true - but it is still an object in its own right and can be passed around – oxbow_lakes Feb 07 '11 at 15:44
5

This is not a companion object. – James Iry Feb 07 '11 at 15:46
You'll never be able to get a **real** companion object because a real companion object has access to the private members of the class that it's the companion to. – Ken Bloom Feb 07 '11 at 17:02

score 9 · Accepted Answer · answered Feb 07 '11 at 16:05

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.

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

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