Question

So I’m playing with writing a battlecode player in Scala. In battlecode certain classes are disallowed and there is a runtime exception if you ever try to access them. When I use the Array.fill function I get a message from the battlecode server saying [java] Illegal class: scala/reflect/Manifest$. This is the offending line:

val g_score = Array.fill[Int](rc.getMapWidth(), rc.getMapHeight())(0)

The method takes an implicit ClassManifest argument which has the following documentation:

A ClassManifest[T] is an opaque descriptor for type T. It is used by the compiler
to preserve information necessary for instantiating Arrays in those cases where
the element type is unknown at compile time.

But I do know the type of the array elements at compile time, as shown above I explicitly state that they will be Int. Is there a way to avoid this? To workaround I’ve written my own version of Array.fill. This seems like a hack. As an aside, does Scala have real 2D arrays? Array.fill seems to return an Array[Array[T]] which is the only way I found to write my own. This also seems inelegant.

Edit: Using Scala 2.9.1

Answer 1

For background information, see this related question: What is a Manifest in Scala and when do you need it?. In this answer, you will find an explanation why manifests are needed for arrays.

In short: Although the JVM uses type erasure, arrays are an exception and need a manifest. Since you could compile your code, that manifest was found (manifests are always available for proper types). Your error occurs at runtime.

I don’t know the details of the battlecode server, but there are two possibilities: Either you are running your compiled classes with a binary incompatible version of Scala (difference in major version, e.g. compiled with Scala 2.9 and server uses 2.10). Or the server doesn’t even have the scala-library.jar on its class path.

As said in the comment, manifests are deprecated in Scala 2.10 and replaced by ClassTag.

---

EDIT: So it seems the class loader is artificially restricting the allowed classes. My suggestion would be: Add a helper Java class. You can easily mix Java and Scala code. If it’s just about the Int–Array instantiation, you could provide something like:

public static class Helper {
    public static int[][] makeArray(int d1, int d2) { return new int[d1][d2](); }
}

(hope that’s valid java code, a bit rusty)

Also, have you tried to create the outer array with new Array[Array[Int]](d1), and then iterate to create the inner arrays?