Kotlin Standard Library: Complete Overview

The Kotlin Standard Library (stdlib) is a powerful set of tools that comes with the compiler and provides basic functionality for everyday development. Unlike Java, where many operations require external libraries, Kotlin offers built-in solutions for working with collections, strings, coroutines, serialization, and file input/output. In this article, we will thoroughly examine the key modules and functions of the Kotlin Standard Library with practical examples.

1. Basic Types and Their Capabilities

The library includes extended functions for primitive types and strings:

Numeric types — Int, Double, Long and their extensions (.toInt(), .coerceIn()).
Strings — templates ("Hello, $name"), multiline strings ("""..."""), methods replace(), split(), trimIndent().
Ranges — 1..10, step 2, downTo.

// Example of working with strings and rangesfun main() {    val name = "Kotlin"    val greeting = "Hello, $name!"    println(greeting) // Hello, Kotlin!

    for (i in 1..5 step 2) {        println(i) // 1, 3, 5    }}

2. Collections: List, Set, Map

Kotlin separates collections into mutable (MutableList, MutableSet, MutableMap) and immutable (List, Set, Map). Main capabilities:

Creation — listOf(), setOf(), mapOf().
Filtering and transformation — filter(), map(), flatMap(), groupBy().
Aggregation — sum(), count(), reduce(), fold().
Sorting — sorted(), sortedBy().

// Example of working with collectionsfun main() {    val numbers = listOf(1, 2, 3, 4, 5)    val evenSquares = numbers        .filter { it % 2 == 0 }        .map { it * it }    println(evenSquares) // [4, 16]

    val grouped = numbers.groupBy { if (it % 2 == 0) "even" else "odd" }    println(grouped) // {odd=[1, 3, 5], even=[2, 4]}}

3. Sequences

For lazy evaluation, Kotlin offers Sequence. This is especially useful when working with large volumes of data:

fun main() {    val result = (1..1_000_000)        .asSequence()        .filter { it % 2 == 0 }        .map { it * 2 }        .take(5)        .toList()    println(result) // [4, 8, 12, 16, 20]}

4. Null-safety and Elvis Operator

One of the key features of Kotlin is safe null handling. The library provides:

Elvis operator (?:) — sets a default value.
Safe call (?.) — prevents NullPointerException.
let function — executes code only if the object is not null.

fun main() {    val name: String? = null    val length = name?.length ?: 0    println(length) // 0

    name?.let {         println("Name is not null: $it")    }}