im really unsure why this is happening

Your function returns type

Single<HashMap<String, List<Product>>>

,

toMap

will return the Kotlin stdlib Map interface. HashMap is based on Java collection, and the Kotlin compiler won't know how to cast a Kotlin Map to a HashMap

ahh...yes I thought its weird why it kept prepending

Collections....

before the datatype

if your function returned

Single<Map<String, List<Products>>>

then

toMap

should work?

Map doesnt have constructors

you can use

mutableMapOf

I think the main issue here is that the datatype returned also doesnt match the right hand side of whats being assigned in that mapping function

class Repository(
    private val apiService: ApiService
) : IRepository {
    
    override fun getProducts(): Single<Map<String, List<Product>>> =
        apiService.getProducts().map { productListings ->
            productListings.groupBy {it.name}.toMap()
        }
}

Should that not suffice?

class Repository(
    private val apiService: ApiService
) : IRepository {

    override fun getProducts(): Single<Map<String, List<Product>>> =
        apiService.getProducts().map { productListings ->
            val productsMap = mutableMapOf<String, List<Product>>()
            productListings.groupBy {it.name}
            productListings.foreach { listing ->
                productsMap.put(listing.name, listing.product)
            }
            productsMap
        }
}

I'm sure there is a more functional way of doing it but I think this should work

thank you it makes total sense

no problem, glad I could help

and it resolved all of my issues, went back to what i originally wanted, a String/List<Product> map

data class Asas(val id: Int, val name: String)
            val list = listOf<Asas>()
            val asas: HashMap<String, List<Asas>> = HashMap(list.groupBy { it.name })

Can create a

HashMap

from the

groupBy

?! (Not tested the code above)