Hi all, I have a few questions about kotlin. Why does kotlin not complain about unsafe casting from nullable? like

var a: String? = null

    var b: String = a as String

it doesn’t warn that this kind of cast might not be safe

It's the same kind of cast as:

val a: Any = Any()
val b: String = a as String

It is a checked cast, so it is not unsafe. It can throw an exception, yes, but that is the nature of casts.

hmm fair, for some reason I assumed that your code would complain about unchecked cast, but it doesn’t

for reference, unchecked casts aren’t unsafe in the sense that they’re guaranteed to fail, they’re unsafe because the compiler has no way to guarantee type safety afterwards

That is because the cast is not unchecked. The cast is checked at runtime. An unchecked cast on the other hand is not checked at runtime and you can end up with heap pollution. This happens due to generic erasure.

like you can “cast”

List<Int>

to

List<String>

technically, but it’s an unchecked cast since the JVM

javac

erases types at compile-time

(Technically it's the compiler that erases, not the JVM </nitpick>)

ok cool that makes sense