Automatic casting in Scala

Question

I have a class that inherits the Actor trait. In my code, I have a method that creates x numbers of this actor using a loop and another method that simply sends the Finish message to all of them to tell them to terminate. I made the kill method just take an array of Actor since I want to be able to use it with an array of any type of Actor. For some reason, however, when I pass a value of type Array[Producer], where Producer extends Actor, to a method that accepts the type Array[Actor], I get a type error. Shouldn't Scala see that Producer is a type of Actor and automatically cast this?

Answer 1

What you are describing is called covariance, and it is a property of most of the collection classes in Scala--a collection of a subtype is a subtype of the collection of the supertype. However, since Array is a Java primitive array, it is not covariant--a collection of a subtype is simply different. (The situation is more complicated in 2.7 where it's almost a Java primitive array; in 2.8 Array is just a plain Java primitive array, since the 2.7 complications turned out to have unfortunate corner cases.)

If you try the same thing with an ArrayBuffer (from collection.mutable <- edit: this part is wrong, see comments) or a List (<- edit: this is true) or a Set (<- edit: no, Set is also invariant), you'll get the behavior you want. You could also create an Array[Actor] to begin with but always feed it Producer values.

If for some reason you really must use Array[Producer], you can still cast it using .asInstanceOf[Array[Actor]]. But I suggest using something other than primitive arrays--anything you could possibly be doing with actors will be far slower than the tiny overhead of using a more full-featured collection class.