How can I infer generic type in kotlin? (covariance problem) - spring-boot

Question

I have two class which extends Data, interface.
A: Data
B: Data
Then I have two repositories. TestRepository is interface which get generic class.
TestRepository<T: Data> {
fun save(data: T): T
}
#Repository
ARepository: TestRepository<A> {
override fun save(data: A): A
}
#Repository
BRepository: TestRepository<B> {
override fun save(data: B): B
}
it all have save method which gets data from generic type, and returns generic type.
ARepo and BRepo gets data from A: Data, B:Data and returns corresponding type.
Then we have new Service,
#Service
CService(
private aRepository: ARepository,
private bRepository: BRepository
) {
fun test(t: String): TestRepository<out Data> =
when (t) {
'1' -> aRepository
'2' -> bRepository
else -> throw Error("error")
}
}
it returns aRepository or bRepository, so return type of test function is TestRepository<out Data>. But when I try to use that class with DI,
#Service
class TestClass(
private val cService: CService
) {
fun cServiceTest() {
...
val saveObject = Data('')
val repo = cService.test("1") // or "2"
repo.save(saveObject) <-- error
}
}
repo.save emits error,
Type mismatch.
Required:
Nothing
Found:
Data
How can I solve this error?

Answer 1

How about this?
import kotlin.reflect.KClass
interface Data {
val name: String
}
data class A(
override val name: String = "A"
) : Data
data class B(
override val name: String = "B"
) : Data
interface DataRepository<T : Data> {
fun save(data: T): T
}
class ARepository : DataRepository<A> {
override fun save(data: A): A {
println("Saved - (A)")
return data.copy()
}
}
class BRepository : DataRepository<B> {
override fun save(data: B): B {
println("Saved - (B)")
return data.copy()
}
}
class DataService(
private val aClassRepository: ARepository = ARepository(),
private val bClassRepository: BRepository = BRepository(),
) {
#Suppress("UNCHECKED_CAST")
fun <T : Data> getRepository(_class: KClass<T>): DataRepository<T> =
when (_class) {
A::class -> aClassRepository
B::class -> bClassRepository
else -> throw RuntimeException()
} as? DataRepository<T> ?: throw RuntimeException()
}
fun main() {
val service = DataService()
val repository = service.getRepository(A::class)
val saved = repository.save(A())
println(saved)
}
I don't know what kind of problem you are trying to solve.
So I think the solution I came up with is not good.