How can I pass a Scala object reference around in Java?

Question

I want to return from a Java method a reference to a Scala object. How can I do that?

My Scala objects are like this:

trait Environment 

object LocalEnvironment extends Environment {...}
object ServerEnvironment extends Environment {...}

... and I want my Java method to be like this:

Environment getEnvironment() { return LocalEnvironment; }  // DOES NOT COMPILE

Is there a way to do this?

Answer 1

While the $.MODULE$ method works, a slightly less jarring way to get Java-interop with Scala objects is to expose the object as a method on itself.

The Scala:

object LocalEnvironment extends Environment{
   def instance = this
}

The Java:

Environment getEnvironment() { return LocalEnvironment.instance(); }  

This works because under the covers, .instance() is implemented as a static method on class LocalEnvironment. There has been some discussion about Scala objects getting an "instance" method by default, for just this purpose.

Answer 2

{ return LocalEnvironment$.MODULE$; }

should work.

---

Edit: the reason why this works is that this is how Scala represents singleton objects. The class ObjectName$ has a field in it called MODULE$ that is populated with the single valid instance of that class. But there is also a class called ObjectName that copies all the methods as static methods. That way you can use it like Java (just call ObjectName.methodName) in most cases, and Scala gets to have a real class to pass around.

But when Java needs to pass the class around--not something normally done with a bunch of static methods, which is what object is designed to emulate in Java--you then have to know how Scala represents it internally.