Hello!
fun <T> cast(from: Any): T? = from as? T
val x = cast<String>(42) // java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String
Is it expected behaviour? I thought as? operator should suppress exception and return null.
Thanks!
Warning: Unchecked cast: Any to T
This is caused by type erasure.
Try:
inline fun <reified T> cast(from: Any): T? = from as? TThe question mark is usually used in Kotlin for nullability. The “as?” also allows the cast to return null when the left-hand side is null. See the documentation:
https://kotlinlang.org/docs/reference/typecasts.html#safe-nullable-cast-operator
It will not allow the above code.
Thank you @fvasco!
But this is simplified example of real code to show the problem.
I can’t use reified T in real code.
Thank you @Tim_van_der_Leeuw.1!
It is written in docs : To avoid an exception being thrown, one can use a safe cast operator as? that returns null on failure.
I thought that “failure” means not only case when left-side is null, for example this works well:
val x = 42 as? String // x initialised to nullUnless parameter T is reified, it’s treated as Any, so basically you wrote the following function: fun cast(from: Any): Any? = from as? Any. Obviously it works for anything and compiler adds a different check at call site which throws exception. Basically that’s limitation of type erasure inherited from JVM. In Java you would pass Class<T> instance, in Kotlin you can do the same or use reified modifier which will do the same under the hood,
Thank you @vbezhenar!
But I still don’t understand why this works:
val x = cast<String>("some str")
val x = cast<Int>(42)
What is more I thought that as? operator is some kinda shorthand for this:
fun <T> cast(from: Any): T? = try {
from as T
} catch (e: ClassCastException) {
null
}
Which works perfectly. Am I wrong?
The cast which throws the ClassCastException in your example is not the as? operator, it’s a cast inserted by the compiler where you assign the return value of a generic method (which is actually Any because of type erasure) to the variable of a specific type.
@yole thanks for your reply!
If I understand you correctly, you tell that ClassCastException raised after the as? operator.
But at that time result should already be null.
Why ClassCastException message tells : java.lang.Integer cannot be cast to java.lang.String ?
No, the result will not be null because generic types are erased at runtime. Your cast function performs a cast to Any, which always succeeds.
Thank you @yole!
Now I understand. So there is no way to write such function, maybe if I pass KClass as argument?
It works if you use inline and reified type T. That prevents type-erasure.
The following worked from me in the Kotlin REPL:
inline fun <reified T> cast(any: Any?) : T? = any as? T?
cast<String>(14)
null