Scala class inheritance and method parameters

Question

In Scala, I want to have the following traits and classes

trait Paper {
    def paint(args: Material): Unit
}

class WhitePaper extends Paper {
    override def paint(args: DarkMaterial): Unit = {
        darkMaterials.open()
    }
}


trait Material {
   def open() : Unit = {}
}

class DarkMaterial extends Material{
   override def open() : Unit = {
       print("Dark material")
   }
}

However, my IDE shows error for the override of method paint(args: DarkMaterial), even though DarkMaterial extends Material. Why is this wrong? Anyone knows how to fix it?

Thank you so much.

paint(DarkMaterial) does not override paint(Material), because paint(Material) can accept any Material; but paint(DarkMaterial) can only accept DarkMaterial. — khelwood
– khelwood, Commented Jul 27, 2016 at 18:36
Thx! But how can I fix it so that def paint(args: DarkMaterial) is acceptable? — lentebloem
– lentebloem, Commented Jul 27, 2016 at 18:53
Well, it's not an override. That's why you're getting the error. Whatever you're trying to do, this isn't it. Either paint() is supposed to accept any Material, or it isn't. — khelwood
– khelwood, Commented Jul 27, 2016 at 18:56

jwvh · Accepted Answer · 2016-07-27 19:18:32Z

1

You could use type parameters to nail things down.

trait Paper[M] {
  def paint(args: M): Unit
}

class WhitePaper extends Paper[DarkMaterial] {
  override def paint(args: DarkMaterial): Unit = { // "override" not needed
    args.open()
  }
}
// Material and DarkMaterial are unchanged

edited Jul 27, 2016 at 19:18

answered Jul 27, 2016 at 19:05

jwvh

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2 Comments

Ian McLaird Over a year ago

We might want to consider making that [M <: Material] to indicate that the M type is intended to be a material, so you don't end up with a Paper[String], depending on the use cases, of course.

lentebloem Over a year ago

@IanMcLaird Thanks! Your suggestion is very helpful.

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