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

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.

CodePudding user response：

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) }

CodePudding user response：

You could just extract the constructor of the Car outside of the creation of the list. I don't see any reason as to why you'd want it inside.

val car = Car(
        *carParts
            .map { CarPart(it.description.uppercase(Locale.getDefault())) } //keep the .toUpperCase() if you are using an old version of Kotlin
            .toTypedArray()
    )

We need the spread operator there in order for the vararg to know that we are passing it the elements of the list and not the list itself.