Hi folks, somewhat new to Kotlin and I'm strugglin...

# getting-started

[Aadi Deshpande]

Hi folks, somewhat new to Kotlin and I'm struggling with understanding how generics work. For my piece of work, I'm working on extending a java class with generics ( https://micronaut-projects.github.io/micronaut-data/2.3.0/api/io/micronaut/data/repository/CrudRepository.html ) , which I've defined like so :

open class FavoritesRepository (val db: Datastore) : CrudRepository<Favorite, UUID> {

where db is an instance of a Cloud Datastore ( https://cloud.google.com/java/docs/reference/google-cloud-datastore/latest/com.google.cloud.datastore.Datastore )

In particular, I'm having a hard time understanding why my override of the save function doesn't work as I expect it to.

The method is:

override fun <S: Favorite> save(entity: S): S {
        val key = db.newKeyFactory().newKey(entity.id.toString())
        val favorite = Entity.newBuilder(key).apply {
            set("cityID", entity.cityID)
            set("guid", entity.guid)
            set("email", entity.email)
        }.build()

        return Favorite(cityID=entity.cityID, cguid = entity.cguid,  email = entity.email)
}

In particular the last line produces a Type mismatch error, which I can overcome by adding an as S to the end of it

But, I'd really like to figure out why it doesn't see S and the Favorite as being compatible types.

Thanks in advance for any insight you can provide.

[Klitos Kyriacou]

You can't return a Favorite because that is a supertype of the expected return type S. Imagine code like:

val entity: SubtypeOfFavorite = ...
val foo: SubtypeOfFavorite = save(entity)

If your save function could compile without error, it would cause chaos because what you would return would be a Favorite type, which is not a SubtypeOfFavorite.

The save function is meant to return a value similar to the passed entity, perhaps with a key property added.

By the way, you don't need to call apply { set... - you can just use .set in a chain, as set returns the receiver object.

[hho]

I guess you're coming from Java – and there it also wouldn't work:

public class SomeClass {
	public static class Favorite {}

	public <S extends Favorite> S save(S entity) {
		return new Favorite();
	}
}

Reason being exactly what @Klitos Kyriacou explained 🙂