Hello how could i do this better

You can further simplify this by providing just the key selector to associateBy and avoid referencing the concrete implementation:

val mapSomeDTO = getSomeDTOList().associateBy { it.id }

hello @Matteo Mirk good morning, thx for aswering

why do you need to reference the concrete class instead of its interface?

What i really want is returning a map because later i have to used for complete another response, in order to do not use a find on a list i rather a hashmap, but really i want to decrease from O(n^2) to O(n)

I see, so there’s no reason to couple the client code to a specific implementation (HashMap), you can safely return a

Map

reference if all you need is a fast lookup by id. [By the way, a linear search on a list has an average complexity of O(n), but you can use a binary search which is O(log n)]

yes for sure but elements on list are not ordered

Ah yes, sorry I overlooked that. Anyway you don’t start from O(n^2) complexity