why are there different results in two scala running environments? - scala

Question

I am reading Section 19.3 of the book "programming in scala 2nd",
there is a snippet code and some description around it in page 431:
NOTE: FC17 x86_64 Scala-2.9.2
class Cell[T](init: T) {
private[this] var current = init
def get = current
def set(x: T) { current = x }
}
I modifyed this sample in two different enviroments:
in first one, I wrote the following code in a file Cell.scala
class A
class B extends A
class C extends B
class Cell[+T](init: T) {
private[this] var current = init
def get = current
def set[U >: T](x: U) {
current = x.asInstanceOf[T]
println("current " + current)
}
}
object Cell {
def main(args: Array[String]) {
val a1 = new Cell[B](new B)
a1.set(new B)
a1.set(new String("Dillon"))
a1.get
}
}
and using the following command, and got nothing error:
[abelard <at> localhost lower-bound]$ scalac Cell.scala
[abelard <at> localhost lower-bound]$ scala -cp . Cell
current B <at> 67591ba4
current Dillon
Dillon
[abelard <at> localhost lower-bound]$
in the second, I directly wrote the code under the REPL:
scala> class Cell[+T](init: T) {
| private[this] var current = init
| def get = current
| def set[U >: T](x: U) {current = x.asInstanceOf[T]
| }}
defined class Cell
scala> val a1 = new Cell[B](new B)
a1: Cell[B] = Cell <at> 6717f3cb
scala> a1.set(new B)
scala> a1.set(new String("Dillon"))
scala> a1.get
java.lang.ClassCastException:
java.lang.String cannot be cast to B
at .<init>(<console>:25)
at .<clinit>(<console>)
accordding to my uderstanding to covariant and lower-bound,
I think the second result is right, but I do not know why
the first one did not throw any error?
I know I must missing something, I want to get a compiling
error as the second, what should I do?

Answer 1

You're not going to get a compile error, because there's no problem at compile time. A ClassCastException is a runtime exception.
You should notice that the exception occurs after you evaluate a1.get, rather that when you perform the cast in a1.set. More precisely, it occurs when you try to assign that return value to a variable.
In your first scenario a1.get is not assigned to a value. In your second, you're assigning it to a value such as res0 etc.
You can show that this is the problem by trying the following in the REPL:
scala> a1.get
java.lang.ClassCastException: java.lang.String cannot be cast to B
scala> println(a1.get)
Dillon