How to check generic type in Kotlin?

Question

I have class

class Generic<T : SuperType>() 

and this code is't correct

fun typeCheck(s: SuperType): Unit {             when(s){                 is T -> //do some thin             }         } 

but cast s to type T s as T show warning - unsafe cast.
How check that s is type T?

Answer 1

If you need to check if something is of generic type T you need to to have an instance of Class<T> to check against. This is a common technique in Java however in Kotlin we can make use of an inlined factory method that gets us the class object.

class Generic<T : Any>(val klass: Class<T>) {     companion object {         inline operator fun <reified T : Any>invoke() = Generic(T::class.java)     }      fun checkType(t: Any) {         when {             klass.isAssignableFrom(t.javaClass) -> println("Correct type")             else -> println("Wrong type")        }      } }  fun main(vararg args: String) {     Generic<String>().checkType("foo")     Generic<String>().checkType(1) } 

Answer 2

Generic types are not reified on the JVM at runtime, so there's no way to do this in Kotlin. The warning is correct because the compiler can't possibly generate any instruction that will fail when the cast is done, so the cast is unchecked, meaning that the program may or may not break at some point later instead.

A related feature which might be of use is reified type parameters in inline functions. Classes can't have reified type parameters though, so if you elaborate a bit more on your use case, I can try helping you achieve what you seem to need.