What is the use case for flatMap vs map in kotlin

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Consider the following example: You have a simple data structure Data with a single property of type List.

class Data(val items : List<String>)

val dataObjects = listOf(
    Data(listOf("a", "b", "c")), 
    Data(listOf("1", "2", "3"))
)

flatMap vs. map

With flatMap, you can “flatten” multiple Data::items into one collection as shown with the items variable.

val items: List<String> = dataObjects
    .flatMap { it.items } //[a, b, c, 1, 2, 3]

Using map, on the other hand, simply results in a list of lists.

val items2: List<List<String>> = dataObjects
    .map { it.items } //[[a, b, c], [1, 2, 3]]

flatten

There’s also a flatten extension on Iterable<Iterable<T>> and also Array<Array<T>> which you can use alternatively to flatMap when using those types:

val nestedCollections: List<Int> = 
    listOf(listOf(1,2,3), listOf(5,4,3))
        .flatten() //[1, 2, 3, 5, 4, 3]

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