Type mismatch with generic parameter - kotlin

Question

I'm trying to run the following code, but I'm getting the following compiler error: Error:(12, 9) Type mismatch: inferred type is Child but Parent<Any> was expected.
abstract class Parent<T> {
abstract fun hi()
}
class Child: Parent<String>() {
override fun hi() {
println("Hi from child")
}
}
fun main(args: Array<String>) {
println("Hello, world!")
test(Child())
}
fun test(parent: Parent<Any>) {
parent.hi()
}
But the Java's equivalent, works as expected:
public class HelloWorld {
public static void main(String[] args) {
test(new Child());
}
public static void test(Parent object) {
object.hi();
}
}
abstract class Parent<T> {
public abstract void hi();
}
class Child extends Parent<String> {
public void hi() {
System.out.println("Hi from child");
}
}
What is wrong with the Kotlin code?

Answer 1

You want Parent<String> to be a subtype of Parent<Any>. Since String is a subtype of Any, what you're looking for is called covariance. You can mark the type parameter of Parent to behave this way with the out keyword:
abstract class Parent<out T> {
abstract fun hi()
}
See more about variance in the official documentation.
As for why the Java example works, #Carcigenicate already mentioned in the comment above that you're using raw types there, e.g. the parent parameter of your test function doesn't have a type parameter at all. You can achieve something similar with a star projection in Kotlin, if you really have to:
fun test(parent: Parent<*>) {
parent.hi()
}