Scala cast to generic type

Question

I'm confused about the generic type. I expect that 2.asInstanceOf[A] is cast to the type A, meanwhile, it's cast to Int. Besides that, the input is java.lang.Long whereas the output is a list of Int (according to the definition the input and the output should be the same type). Why is that?

def whatever[A](x: A): List[A] = {
  val two = 2.asInstanceOf[A]
  val l = List(1.asInstanceOf[A],2.asInstanceOf[A])

  println(f"Input type inside the function for 15L: ${x.getClass}")
  println(f"The class of two: ${two.getClass}, the value of two: $two")
  println(f"The class of the first element of l: ${l.head.getClass}, first element value: ${l.head}")
  l
}

println(f"Returned from whatever function: ${whatever(15L)}")

the outupt:

Input type inside the function for 15L: class java.lang.Long
The class of two: class java.lang.Integer, the value of two: 2
The class of the first element of l: class java.lang.Integer, first element value: 1
Returned from whatever function: List(1, 2)

Answer 1

a.asInstanceOf[B] means:

Dear compiler;

Please forget what you think the type of a is. I know better. I know that if a isn't actually type B then my program could blow up, but I'm really very smart and that's not going to happen.

Sincerely yours, Super Programmer

In other words val b:B = a.asInstanceOf[B] won't create a new variable of type B, it will create a new variable that will be treated as if it were type B. If the actual underlying type of a is compatible with type B then everything is fine. If a's real type is incompatible with B then things blow up.