Is this really the way to pass void functions to Scala methods from Java?

Question

I've been playing with Scala/Java interop lately, specifically, calling Scala (2.10.4) code from Java (7). It's been more pleasant than I expected, but a few things puzzle me.

E.g., in scala.runtime I have a nice collection of AbstractFunction abstract classes. But I don't see anything for methods with no return value. E.g., suppose I have the following Scala code:

class MyClass(name: String) {
  def SayWhat(say_fn: String => Unit) = say_fn(name)
}

My understanding is that Java's void is more or less Scala's Unit, so I can pass something vaguely lambda-like with the following Java anonymous class:

import scala.Function1;
import scala.runtime.AbstractFunction1;
import scala.runtime.BoxedUnit;

public class MyProgram {
  public static void main(String[] args) {
    MyClass mc = new MyClass("Dan");

    Function1<String, BoxedUnit> f = new AbstractFunction1<String, BoxedUnit>() {
      public BoxedUnit apply(String s) {
        System.out.println("Why, hello there " + s);
          return null;
      }
    };

    mc.SayWhat(f);
  }
}

This obviously not the prettiest thing, but I appreciate the AbstractFunction stuff, really, compared to what I would have to do otherwise! But is there really no AbstractProcedure or something? Also, why does my "lambda" have to return BoxedUnit?

Answer 1

Your function actually has to return BoxedUnit.UNIT, which is the unit in question.

Unit is an AnyVal, not an AnyRef, so it doesn't include null.

It is so much nicer when functions return interesting values.