What is the Kotlin equivalent of Java Stream.collect?

Question

If I want to collect my Kotlin collection into something that isn't built into the stdlib, how do I do it?

Answer 1

For scenarios not covered by built in operations toList() etc, you can use the fact that collect is just a fold. So given

val list: List<Pair<String, Int>> = listOf("Ann" to 19, "John" to  23)

you can collect to a collection of your choice with a fold

val map: Map<String, Int> = list.fold(HashMap(), { accumulator, item ->
    accumulator.put(item.first, item.second); accumulator})

If you then define an extension function

fun <T, R> Iterable<T>.collectTo(accumulator: R, accumulation: (R, T) -> Unit) = 
    this.fold(accumulator, { accumulator, item -> accumulation(accumulator, item); accumulator } )

you can further simplify

val map2: Map<String, Int> = list.collectTo(HashMap(), { accumulator, item -> 
    accumulator.put(item.first, item.second) })

Although in this case of course you could just use the .toMap extension function.