Kotlin – How to convert a list of objects into a single one after map operation?

I’m trying to wrap my head around map and reduce operations in Kotlin. At least, I guess it’s reduce what I’m trying to do.

Let’s say that I have a class called Car that takes any number (varargs constructor) of CarPart. Then, I have a list of CarPart which I’ll do a map operation and from the result of the operation I need to build one Car using each subelement, something along these lines:

class CarPart(val description: String)
class Car(vararg val carPart: CarPart)

val carParts = listOf(CarPart("Engine"), CarPart("Steering Wheel")))

carParts.map { it.description.toUpperCase() }
    .map { CarPart(it) }
    .reduce { acc, carPart -> Car(carPart) }  <--- I'm struggling here, is reduce what I should be doing 
                                                   to construct one car from all the subelement?

PS.1: I know that the class design could be better and not take a varargs, this is just an example of a legacy application I’m refactoring and originally that’s a Java class taking varargs which I can’t change now.

PS.2: The example of mapping to a String and then creating an object out of that String is just for the sake of the example. The actual code grabs an object within the list.

>Solution :

You can simply use a the spread operator (*) over an array:

val mappedCarParts = carParts
    .map { it.description.toUpperCase() }
    .map { CarPart(it) }
    .toTypedArray()

val car = Car(*mappedCarParts)

// Or even:

val car = carParts
    .map { it.description.toUpperCase() }
    .map { CarPart(it) }
    .toTypedArray()
    .let{ Car(*it) }