Can I use scala List directly in Java?

Question

Can I use scala List in Java, like :

import scala.collection.immutable.List;
class HelloScalaList {
    public static void main (String[] args) {
        List xs = List(1, 2, 3);
        System.out.println(xs);
    }
}

It does not seem to compile. can't find List$.apply method.

when I change it to

List xs = Dir.ls()

where Dir is my scala class, and ls() returns a scala List, the compiler complaints about

"Internal compiler error: java.lang.ClassCastException: org.eclipse.jdt.internal.compiler.lookup.BaseTypeBinding cannot be cast to org.eclipse.jdt.internal.compiler.lookup.ReferenceBinding at org.eclipse.jdt.internal.compiler.lookup.BinaryTypeBinding.initializeTypeVariable(BinaryTypeBinding.java:927)"

which I have no idea what it means.

---

I want to write some library in scala, but would also like it to be used in Java.

In my scala class there are methods that return scala List, for java code to use them, I have two options:

1. use scala List in java directly

2. write a wrapper class that returns java.util.List for those methods.

I'd rather like option 1, because otherwise I'll have to write a wrapper class for nearly ALL my scala classes.

But I just can't get scala List running in Java.

Answer 1

The problem with this syntax:

List xs = List(1, 2, 3);

is that it's Scala, not Java. When you instantiate an object like that in Scala, syntactic sugar calls the apply() method of the class' companion object. You would have to do that manually in Java.

And you aren't actually creating a scala.collection.immutable.List (which is abstract):

scala> val list = List(1,2,3)                      
list: List[Int] = List(1, 2, 3)
scala> list.getClass                               
res12: java.lang.Class[_] = class scala.collection.immutable.$colon$colon

Calling Scala from Java isn't nearly as fun as calling Java from Scala. I think you'll have an easier time converting the Scala List to a Java Collection. Using any of the Scala List higher order functions would be difficult in Java and without those, you pretty much have a Java Collection anyway.

Answer 2

A little java-side helper method does the trick:

import scala.collection.immutable.List;
import scala.collection.immutable.List$;
import scala.collection.immutable.$colon$colon;

public class HelloScalaList {

    public static void main (String[] args) {
        List xs = list(1,2,3);
        System.out.println(xs);
    }

    public static <T> List<T> list(T ... ts) {
        List<T> result = List$.MODULE$.empty();
        for(int i = ts.length; i > 0; i--) {
            result = new $colon$colon(ts[i - 1], result);
        }
        return result;
    }
}

[Update]

As a result of this question, I started a little project called "Scava" in order to support calls from Java to Scala: http://code.google.com/p/scava-org/

Answer 3

As of Scala 2.10, I didn't succeed with the tricks above.

But you can use the following code:

  public static <T> scala.collection.immutable.List<T> scalaList(List<T> javaList) {
    return scala.collection.JavaConversions.asScalaIterable(javaList).toList();
  }