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?
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.
{ 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.
LocalEnvironment seems to be a standalone singleton object, it should probably be return LocalEnvironment.MODULE$ (no $ in LocalEnvironment).LocalEnvironment class will contain exactly and only those static methods needed to copy the (non-static) methods in the Scala singleton. MODULE$ is not one of those methods--it is a field of the LocalEnvironment$ class that is instantiated in a single copy (i.e. LocalEnvironment$ is the class that contains implementation details, and MODULE$ is an implementation detail within it).