How to use scala.None from Java code

Question

In Java you can create an instance of Some using the constructor, i.e. 'new Some( value)', but None has no partner class. How do you pass None to a Scala function from Java?

Answer 1

I think this ugly bit will work: scala.None$.MODULE$

There is no need for a new instance since one None is as good as another...

Answer 2

You can access the singleton None instance from java using:

scala.None$.MODULE$

Answer 3

The scala.None$.MODULE$ thing doesn't always typecheck, for example this doesn't compile:

scala.Option<String> x = scala.None$.MODULE$;

because javac doesn't know about Scala's declaration-site variance, so you get:

J.java:3: incompatible types
found   : scala.None$
required: scala.Option<java.lang.String>
    scala.Option<String> x = scala.None$.MODULE$ ;

This does compile, though:

scala.Option<String> x = scala.Option.apply(null);

so that's a different way get a None that is usable in more situations.

Answer 4

Faced with this stinkfest, my usual modus operandi is:

Scala:

object SomeScalaObject {
  def it = this
}

Java:

doStuff(SomeScalaObject.it());