Generic abstract method override issue

Language Design
1

Hi, I not really sure how Kotlin handle the generic abstract method, here is example:

21

It runs fine, but the it keep said uncheck cast: Student to T.

But if we do this in java, we will not need to cast into T.

31

Is that bug or there is some Kotlin tricks for this?

Any help will be appreciated

2

Please consider this code:

fun main() {
    val main: MainAbstract = Main()
    val stranger = main.initPerson<Stranger>()
}


abstract class MainAbstract {
    abstract fun <T : Person> initPerson(): T
}

class Main : MainAbstract() {
    override fun <T : Person> initPerson(): T = Student() as T
}

open class Person
class Student : Person()
class Stranger : Person()

Where wish you put a warning?

3

thanks for reply,

It will keep give me the warning

4

Of course, because T is Stranger in this case. You can’t cast Student to Stanger. The value of T declared like this will be inferred from the location where you call the method, like val stranger : Stranger = initPerson(), not from the thing you actually return.

If you just want to return something that extends person, write abstract fun initPerson() : Person instead.

5

Thank you for reply, but I want that similar to Java, so the subclass can return the subtype. like :

override fun initPerson() : Student → but this is not complier at all

6

That works in Java completely the same as in Kotlin.

In Java you could override an
abstract Person initPerson()
with an
@Override Student initPerson()
and in Kotlin you can override an
abstract fun initPerson() : Person
with an
override fun initPerson() : Student.
In both cases, you don’t need generics.

7

Sorry, we cannot do that, it is got error

47
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But if you do that in java, that is fine.

32

8

Copyable code for reference:

public abstract class MainAbstract {
	public abstract <T extends Person> T initPerson();
}

class Main extends MainAbstract{
	@Override
	public Student initPerson() {
		return new Student();
	}

	
}

class Person {}

class Student extends Person {}

That gives a warning in Java and is completely unsafe for the reasons we told you:

Unchecked overriding: return type requires unchecked conversion. Found 'Student', required 'T'

Kotlin is completely right in not allowing that.

Is there a reason why you can’t simply use Person as return value in AbstractMain instead of doing this generic stuff.

9

Thank you so much for you help, I think you are right, Java will got warning as well. Ok, I will remove the generic type will do the job. Thank you again