Inheritance and method overriding in Scala

Question

When we have the following basic classes

class X {
  def f = println("X")
}

class Y extends X {
  override def f = println("Y")
}

val a : X = Y

I think I am happy with why we get

scala> a.f
Y

But then I don't understand why we then have

scala> val b : AnyRef = new Array(10)
scala> b(0)
<console>:9: error: AnyRef does not take parameters
              b(0)

since as far as I can tell AnyRef is a superclass of Array in a similar way to how X was a superclass of Y. If someone could explain this I would be very grateful.

Answer 1

If you look at the API documentation for AnyRef you will note that it does not provide an apply method - so the error makes sense.

You can see the same behavior if you change your example to add a method to Y that is not available on X:

class X {
  def f = println("X")
}

class Y extends X {
  override def f = println("Y")
  def f2 = println("Not in X")
}

val a : X = new Y

scala> a.f2
<console>:11: error: value f2 is not a member of X
              a.f2
                ^