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 Inst[T] { 
  def is(x: Any) = scala.util.Try { as(x) }.isSuccess
  def as(x: Any): T = x.asInstanceOf[T]
}

scala> new Inst[String].is(3)
res17: Boolean = true

scala> new Inst[String].as(3)
java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String
 ... 33 elided

What is going on here? Why does only the second call to as throw, but not the first one?

Answer 1

This is because the class-cast-exception is only thrown when you do something with the value, call a method on it after the cast. In the REPL for example, you would have a toString call in the second case. Note:

new Inst[String].as(3); ()           // nothing happens
new Inst[String].as(3).toString; ()  // exception

The reason why this takes the extra step is that Inst[T] is generic with type parameter T which is erased at runtime; only when the call-site (that has a static knowledge of type T) tries to call a method on the result, the actual type check occurs.

---

For your follow-up question, toString is defined on any object and since T is generic you have a boxed integer (<: AnyRef) and toString and println succeed within the is method. So another example where the Try would fail is this:

class Inst[T] { 
  def foo(x: Any)(implicit ev: T <:< String) = scala.util.Try {
    ev(as(x)).reverse
  }.isSuccess
}

new Inst[String].foo(3)  // false!

Answer 2

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)
...