Question

I have a class structure like this

abstract class A
class B extends A
class C extends A
class D extends A
class E extends A

and I have a collection of the various instances, for example:

val xs = List(new D, new B, new E, new E, new C, new B)

My question is, is there an elegant way to filter out some of the subclasses from the List?

Let’s say I want all instances except B’s and C’s. I can do it with a bunch of isInstanceOf’s, or using collect like this:

val ys = (xs collect {
    case b: B => None
    case c: C => None
    case notBorC => notBorC
}).filter(_ != None).asInstanceOf[List[A]]

This works but it feels awkward, mostly because of the filter and cast. Is there a more elegant way? Less code is preferred, and I’d like to have a solution that doesn’t need to be updated if I add more subclasses of A.

Answer 1

flatMap that shit! (as they say):

scala> val ys = xs flatMap {
     |   case _: B | _: C => None
     |   case other => Some(other)
     | }
ys: List[A] = List(D@7ecdc97b, E@2ce07e6b, E@468bb9d1)

In your case you were getting a List[ScalaObject] because ScalaObject is the least upper bound of None, D, and E.