Passing Java array to Scala

Question

Although I've been using Scala for a while and have mixed it with Java before, I bumped on a problem.

How can I pass a Java array to Scala? I know that the other way around is fairly straightforward. Java to Scala is not so however.

Should I declare my method in Scala?

Here is a small example of what I'm trying to achieve:

Scala:

def sumArray(ar: Array[Int]) = ...

Java:

RandomScalaClassName.sumArray(new int[]{1,2,3});

Is this possible?

Solution 1

absolutely!

The Array[T] class in Scala is mapped directly to the Java type T[]. They both have exactly the same representation in bytecode.

At least, this is the case in 2.8. Things were a little different in 2.7, with lots of array boxing involved, but ideally you should be working on 2.8 nowadays.

So yes, it'll work exactly as you've written it.

Solution 2

Yes, it is totally possible and in fact very easy. The following code will work as expected.

// TestArray.scala
object TestArray {
    def test (array: Array[Int]) = array.foreach (println _)
}

-

// Sample.java
public class Sample
{
    public static void main (String [] args) {
        int [] x = {1, 2, 3, 4, 5, 6, 7};
        TestArray.test (x);
    }
}

Use the following command to compile/run.

$scalac TestArray.scala
$javac -cp .:/opt/scala-2.8.0/lib/scala-library.jar Sample.java
$java -cp .:/opt/scala-2.8.0/lib/scala-library.jar Sample

Comments

[halfwarp]

Thanks. It was Eclipse's plugin fault.

[MarnixKlooster ReinstateMonica]

From this comment I understand that you had the same problem I see right now. In a Java class I have a call App.main(args), where args is of type String[], and App is a Scala object with a def main(args: Array[String]). Now the Eclipse JDT source analyzer marks that call as an error: "The method main(Array) in the type App is not applicable for the arguments (String[])". This is harmless, and the error doesn't even show up in the Problems view. I don't know whether this is a problem in the JDT, in the Scala plug-in, in the Scala compiler, or in multiple of the above.