Exploring Kotlin: Useful Standard Library Functions

As others have written about this before Kotlin comes with a lot of handy functions like let, apply, with or also. Less is written about what comes with the collections, ranges, and other packages of the standard library. I think that a lot more can be done using just the Kotlin standard library, so let’s explore it in depth!


Kotlin comes with Pair and Triple which are basic generic tuples:

Pair("foo", "bar")

Triple(1, "wom", "bat")

Java does not have them so you might ask why are these useful?

Ever wanted to return two values from a function? With Pairs this is rather easy to accomplish, you just have to use it as a return type. What’s more useful is that we can use destructuring to split a Pair into two values:

fun someFunction(): Pair<String, Int> = Pair("foo", 1)

// ...

val (foo, one) = someFunction() // foo will be the first, one will be the second value of a Pair

We can put tuples to good use when we work with Maps as well.

Pair comes with an useful infix function, to which lets us create a Pair like this:

1 to 3

Then we can use this syntax to create Maps in a much more readable way:

mapOf(1 to 3, 4 to 2)

If you come from Java these might be a bit odd at first, but once you get used to it using infix functions and tuples will become second nature!


If you have worked with the Kotlin stdlib for a while you probably bumped into a bunch of library functions which are either improvements over the Java versions or new additions by Kotlin. You have map, filter, reduce for example which are defined on Iterable objects and there are a bunch of others which are defined on immutable Lists, or Sets like plus or minus.

Note that from now on we’ll talk about operations which are defined for immutable collections.

Creating collections never have been easier. We have listOf, setOf, mapOf and even arrayOf to create the corresponding collection.

What’s interesting is that most of these operations are also defined for Maps.

Note that a Map in Kotlin does not implement the Collection interface, but defines some operations for Maps which are counterparts for the ones in Collection.


Kotlin treats Maps as collections of Pairs and we can create Maps from any collection which holds them:

listOf(1 to 2, 2 to 3, 5 to 1).toMap()

In addition to map, filter and reduce we also have mapValues, mapKeys, filterKeys and filterValues for Maps.

mapValues and mapKeys will create new Maps when we call them. They are useful when we have a Map and we only want to transform either the keys or the values. The filter variants follow the same logic but with filtering. We can also combine them:

val dirtyData = mapOf("1" to "foo", "2" to "bar", "baz" to "qux")

val cleanData = dirtyData
        .filterKeys { it.toIntOrNull() != null }
        .mapKeys { it.key.toInt() }

// {1=foo, 2=bar}

If we want to perform operations other than these we can turn our Map into a collection by calling toList or asIterable.

These operations might be a bit odd if you come from Java, but after a while it makes sense if you start to think about Maps as a sequence of key-value Pairs.


In addition to toList, toMap and other conversion functions there are also some specialized ones which are defined on only some selected types like Byte, Boolean or Char. For example we can turn a List of Bytes to a ByteArray like this:

listOf<Byte>(1, 2, 3).toByteArray()

There is a to*Array defined for each primitive type. They all return a corresponding *Array type which is optimized.

Immutable Collections

Note that these collections in Kotlin are not immutable in fact, it is only the interface which does not allow mutation. There are some pitfalls to this. Take a look at my other article about the topic where I explain this problem.

Immutable collections are perfect for functional programming since every operation defined on them returns a new version of the old collection without changing it. This also means that they are safe from a concurrency perspective since we don’t need locks to work with them. A problem though is that we lose operations like removeAll or retainAll.

Luckily most operations which work with mutable collections have an immutable counterpart. plus and minus work like add and remove and we also have subtract, union and intersect. They work like removeAll, addAll and retainAll:

val mySet = setOf(1, 2, 3, 4, 5, 6)

val union = mySet.union(setOf(7, 8, 9))

// [1, 2, 3, 4, 5, 6, 7, 8, 9]

val intersection = mySet.intersect(setOf(3, 4, 5, 11, 12))

// [3, 4, 5]

val difference = mySet.subtract(setOf(1, 2, 3))

// [4, 5, 6]

Drop and take

We can also work with collections in a way offset and limit works in RDBMSes. drop will return with a List without the first n elements:

val myList = listOf(1, 2, 3, 4, 5, 6)


// [5, 6]

dropLast works in the same way but drops elements from the end:


// [1, 2]

There is also dropWhile and dropLastWhile which drops elements until a certain condition is met:

myList.dropWhile { it < 5 }

// [5, 6]

myList.dropLastWhile { it > 3 }

// [1, 2, 3]

For all of the above functions there is a take variant which works like drop but it takes elements:


// [1, 2, 3]


// [4, 5, 6]

myList.takeWhile { it < 5 }

// [1, 2, 3, 4]

myList.takeLastWhile { it > 5 }

// [6]

If you come from LISP these functions might be familiar to you: they are like first and rest in Clojure for example.

In Kotlin you can define first as take(1) and rest as drop(1).

Calculating distinct values

These are useful but sometimes we only want to pick distinct values. We can do so by calling distinct or distinctBy. With distinctBy we can write our own selector function:

val listWithDuplicates = listOf(1, 1, 2, 2, 3, 4)


// [1, 2, 3, 4]

val chars = listOf("a", "A", "b", "B", "c")

chars.distinctBy { it.toLowerCase() }

// [a, b, c]

Grouping collections

We’ve already seen ways we can turn Maps to Lists but can we do it the other way around? The answer is yes. Kotlin comes with groupBy, associate and associateBy which lets us split our Lists in different ways.

groupBy separates our List into multiple Lists grouped by keys, so the result is a multimap. We only need to provide a key selector function to do so. In this example we group a List of Ints into even and odd groups:

val items = listOf(1, 2, 3, 4)

items.groupBy { if (it % 2 == 0) "even" else "odd" }

// {odd=[1, 3], even=[2, 4]}

associate is different in a way that it transforms each element to a key-value pair and if multiple values map to the same key only the last one is returned. In our previous list of Ints which are sorted this will effectively give us the greatest odd and even numbers:

items.associate { (if (it % 2 == 0) "greatest_even" else "greatest_odd") to it }

// {greatest_odd=3, greatest_even=4}

A variant to associate is associateBy which does not transform the original values but takes a key selector function. If multiple elements would have the same key only the last one is added to the resulting Map. This is an example which does the same as the previous one but with associateBy:

items.associateBy { if(it % 2 == 0) "greatest_even" else "greatest_odd" }

// {greatest_odd=3, greatest_even=4}

partition is a special transformation function which groups to only a Pair of Lists based on the result of a predicate:

val items = listOf(1, 2, 3, 4)

val x = items.partition { it % 2 == 0 }

// ([2, 4], [1, 3])

Joining collections

We’ve seen how we can split things but let’s see what we have for joining them!

zip will work exactly like the zipper of your trousers: it zips two Lists into a List of Pairs:

val names = listOf("Jon", "John", "Jane")
val ages = listOf(23, 32, 28)


// [(Jon, 23), (John, 32), (Jane, 28)]

We can be a bit more sophisticated if we provide a transform function to zip:

data class User(val name: String, val age: Int)

val names = listOf("Jon", "John", "Jane")
val ages = listOf(23, 32, 28)

names.zip(ages, { name, age -> User(name, age)})

// [User(name=Jon, age=23), User(name=John, age=32), User(name=Jane, age=28)]

zipWithNext will pair each element with the next:

val nodes = listOf("A", "B", "C", "D")

val x = nodes.zipWithNext()

// [(A, B), (B, C), (C, D)]

and it can also take a transform function:

data class Node(val value: String, val edges: List<String>)

val nodes = listOf("A", "B", "C", "D")

nodes.zipWithNext { node, edge -> Node(node, listOf(edge))}

// [Node(value=A, edges=[B]), Node(value=B, edges=[C]), Node(value=C, edges=[D])]

Sometimes we want to transform our collections to a String representation. This is useful if we want to log the contents of them for example. For this purpose we have joinTo which takes an Appendable and some extra arguments (like a separator) and returns the Appendable with the contents of the collection appended:

val numbers = listOf(1, 2, 3, 4)

val stringBuilder = StringBuilder()

val x = numbers.joinTo(stringBuilder, ", ")

// 1, 2, 3, 4

Since joining to a String is so common we also have joinToString:

val numbers = listOf(1, 2, 3, 4)

numbers.joinToString(", ")

// 1, 2, 3, 4

The takeaway

As you have seen from the previous examples Kotlin collections work a bit differently from Java ones. When we use Java we have the Stream API which lets us perform most of these operations but they come at a price: we need to convert between streams and collections, and they also come with more boilerplate.

Kotlin does not differentiate between streams and collections. All of the above funtions are defined for Iterables, Collections, Maps or Lists. This lets us write programs more fluently and at the end of the day we’ll end up with more readable code, by doing less work.

Immutable collections are an added bonus: they help us write code which is less error-prone, without the need to write more. And since we can’t mutate them we’ll have less concurrency issues, like race conditions or deadlocks.

Honorable mentions

The examples above are far from exhaustive but there are some interesting functions which are really useful sometimes. For example we have the most commonly used String transformations as extension functions:

"hello, world".capitalize()

// Hello, world

"HELLO, WORLD".toLowerCase().capitalize()

// Hello, world

"good bye".toUpperCase()




There are also Ranges which are very useful for iteration. We can create them directly from numbers with useful infix operations:

(0 until 10).forEach {
    print("$it ")

// 0 1 2 3 4 5 6 7 8 9

(0 .. 10).forEach {
    print("$it ")

// 0 1 2 3 4 5 6 7 8 9 10

(10 downTo 0).forEach {
    print("$it ")

// 10 9 8 7 6 5 4 3 2 1 0

(10 downTo 0 step 2).forEach {
    print("$it ")

// 10 8 6 4 2 0


We’ve only scratched the surface with the examples above but I hope that you now have an idea about what the Kotlin stdlib has to offer. I strongly encourage you to open your IDE and take a look at these functions from the source. They are documented very well so you can get started in no time.