Having trouble creating Scala delegate for java.util.Collection#toArray

Question

The method defined in the Java API:

interface Collection<T> {
  <X> X[] toArray(X[] a);
}

I attempt to do something like this in Scala:

class SCollection[T] extends Collection[T] {
    val queue = new LinkedBlockingQueue[T]

    def toArray[X](a: Array[X]) : Array[X] = queue.toArray(a)
}

I've left out the other methods in the interface for clarity. The compiler complains with:

overloaded method value toArray with alternatives:
  [T](x$1: Array[T with java.lang.Object])Array[T with java.lang.Object] <and>
  ()Array[java.lang.Object]
 cannot be applied to (Array[X])
  def toArray[X](a: Array[X]) : Array[X] = queue.toArray(a)
                                                 ^

How do I successfully override the toArray(..) method?

Moritz · Accepted Answer · 2011-03-01 23:49:36Z

4

The compiler complains because you have to make sure that you do not pass primitive arrays like e.g. Array[Int] (which maps to int[] in Java).

You can write your method like this:

   override def toArray[X](a: Array[X with AnyRef]) = queue.toArray[X](a)

answered Mar 1, 2011 at 23:49

Moritz

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1 Comment

Jeffrey Aguilera Over a year ago

That just generates a different compiler error: Expression of type Array[X with AnyRef] does not conform to expected type Array[X]

Kevin Wright · Accepted Answer · 2011-03-02 00:47:46Z

Arrays are the only reified "collection" type on the jvm, so to build an array you need to know the exact type at runtime. Not such an easy thing to do after erasure...

If you look at how toArrayis implemented on Scala collections, you'll see that it uses a Manifestto recover the erased type information. You basically need to duplicate this, and ensure a Manifest is available for your type T.

However, why are you implementing this class in the first place? If you're trying to create a custom collection class, you'll find it quickly becomes very challenging to add features such as CanBuildFrom. If you simply want a wrapper to offer a java Collection to some other library, then I'd recommend JavaConvertors instead. If you don't need interop, then there's no reason to implement Collection at all, as Scala has a distinct collection framework and typically avoids Java's collections, which are broken for use in fully type safe functional programming.

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