Main.java

// plain old Java
import rx.Observable;
import rx.util.functions.Action1;

public class Main {
  public static void main(String[] args) {
    String[] names = new String[] {
      "foo", "bar", "baz"
    };

    // Compiles, but does nothing... (!)
    // The compiler really should only accept Action1<String> here.
    Observable.toObservable(names).subscribe(new Action1<Integer>() {
      @Override public void call(Integer i) {
          System.out.println(i);
      }
    });
  }
}

MainScala.scala

// Scala, without using the language adaptor
import rx.Observable
import rx.util.functions.Action1
import scala.collection.JavaConverters._

object MainScala extends App {
  val names = Seq("foo", "bar", "baz")

  // Works as expected
  Observable toObservable names.asJava subscribe new Action1[String]() {
    def call(str: String): Unit = println(str)
  }
  
  // Compiles, but fails silently...
  // The compiler really should only accept Action1[String] here.
  Observable toObservable names.asJava subscribe new Action1[Int]() {
    def call(i: Int): Unit = println(i)
  }
}

MainScalaAdaptor.scala

// Scala, using the language adaptor
// How I want to be able to use "subscribe"
import rx.Observable
import rx.util.functions.Action1
import scala.collection.JavaConverters._

object MainScalaAdaptor extends App {
  val names = Seq("foo", "bar", "baz")
  
  // Leads to runtime exception: "Unsupported closure type: BoxedUnit"
  Observable toObservable names.asJava subscribe println
}

MainScalaAdaptor2.scala

// Scala, using the language adaptor
import rx.Observable
import rx.util.functions.Action1
import scala.collection.JavaConverters._

object MainScalaAdaptor2 extends App {
  val names = Seq("foo", "bar", "baz")
  
  // not exactly pretty, but works
  Observable toObservable names.asJava subscribe { (str: String) => println(str) }

  // compiles, but silently fails...
  Observable toObservable names.asJava subscribe { (i: Int) => println(i) }
}