Why is KClass declared as KClass<T : Any> and not KClass<T> so that the type argument can be nullable


Andrey Loginov

I'm trying to implement checked Stack<E> based on java array in Kotlin. But I have a problem with using KClass with my generic parameter type <E> which allows null values.

Java Generic types are not available in runtime, but array types are available. I want to use this feature so that there is built-in type checking in runtime.
interface Stack<E> {
fun push(elem: E)
fun pop(): E

class CheckedStack<E>(elementType: Class<out E>, size: Int) : Stack<E> {

companion object {
inline fun <reified E> create(size: Int): CheckedStack<E> {
//**compile error here**
return CheckedStack(E::class.javaObjectType, size)

private val array: Array<E?> = java.lang.reflect.Array.newInstance(elementType, size) as Array<E?>
private var index: Int = -1

override fun push(elem: E) {
check(index < array.size - 1)
array[++index] = elem

override fun pop(): E {
check(index >= 0);
return array[index--] as E

I expect that this code will work like this:

fun main() {
val intStack = CheckedStack.create<Int>(12) // Stack must store only Integer.class values
intStack.push(1); //[1]
intStack.push(2); //[1, 2]

val stackOfAny: Stack<Any?> = intStack as Stack<Any?>;
stackOfAny.push("str") // There should be a runtime error

But I have the compile error

Error:(39, 42) Kotlin: Type parameter bound for T in val <T : Any> KClass<T>.javaObjectType: Class<T>
is not satisfied: inferred type E is not a subtype of Any

In order to fix it, I need to bound the type parameter <E : Any> but I need the stack to be able to work with nullable values <T : Any?>. How to fix it?

Why is KClass declared as KClass<T : Any> not KClass<T : Any?>?

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