Why does `.asInstanceOf` sometimes throw, and sometimes not?

Question

I was trying to answer this question, as I thought I knew the answer. Turns out, I did not know quite enough :/

Here is a test I have done:

class In         


        

Answer 1

While @0__'s answer explains why it doesn't work, here is how to make it work:

class Inst[T](implicit tag: scala.reflect.ClassTag[T]) {
  def is(x: Any) = tag.runtimeClass.isInstance(x) 
  // scala.util.Try { as(x) }.isSuccess will work as well
  def as(x: Any): T = tag.runtimeClass.cast(x).asInstanceOf[T]
}

object Main extends App {
  println(new Inst[String].is(3))
  println(new Inst[String].as(3))
}


false
java.lang.ClassCastException: Cannot cast java.lang.Integer to java.lang.String
    at java.lang.Class.cast(Class.java:3369)
...