Kotlin Standard Library: A Complete Overview

What is the Kotlin Standard Library?

The Kotlin Standard Library (kotlin-stdlib) is a set of modules included in the language that provide developers with ready-made solutions for everyday tasks. Unlike Java, where many things have to be written manually or require third-party libraries, Kotlin offers powerful built-in tools for working with collections, strings, streams, coroutines, and serialization. The library is fully compatible with Java, but extends its capabilities, making code more concise and safe.

Main Modules of the Standard Library

• kotlin-stdlib — the core of the library: collections, ranges, sequences, standard functions (let, apply, also, run).
• kotlinx-coroutines — support for asynchronous programming via coroutines.
• kotlinx-serialization — serialization/deserialization to JSON, Protobuf, and other formats.
• kotlin-test — tools for unit testing.
• kotlin-reflect — reflection and working with types at runtime.

Collections and Sequences

One of Kotlin's strongest features is working with collections. The library offers both mutable and immutable collections. All basic operations (filtering, transformation, grouping) are implemented as extension functions.

Example: Working with Lists

val numbers = listOf(1, 2, 3, 4, 5, 6)val evenSquares = numbers    .filter { it % 2 == 0 }    .map { it * it }    .sortedDescending()println(evenSquares) // [36, 16, 4]

In this example, we use a chain of calls: filtering, transformation, and sorting. All operations are performed lazily until a terminal operation is called (in this case, printing). For large datasets, it is recommended to use Sequence.

Sequences for Lazy Evaluation

val largeList = (1..1_000_000).toList()val result = largeList.asSequence()    .filter { it % 2 == 0 }    .map { it * 2 }    .take(10)    .toList()println(result) // [4, 8, 12, 20, 24, 28, 32, 36, 40]

Unlike regular collections, Sequence does not create intermediate lists, saving memory when working with large amounts of data.

Coroutines: Asynchrony Without Pain

The kotlinx-coroutines module is the standard way to work with asynchronous code in Kotlin. It replaces callbacks and RxJava, providing a simple sequential syntax.

Simple Coroutine Example

import kotlinx.coroutines.*

fun main() = runBlocking {    val result = async {        delay(1000L)        "Done!"    }    println("Waiting...")    println(result.await()) // "Done!" after 1 second}

Key concepts:

• runBlocking — blocks the current thread until the coroutine completes (used in main).
• async — launches a coroutine and returns a Deferred (analogous to Future).
• await — suspends the coroutine until the result is obtained.

Structured Concurrency

fun main() = runBlocking {    launch {        repeat(3) { i ->            delay(500L)            println("Task $i completed")        }    }    println("Main coroutine continues working")}

Thanks to structured concurrency, all child coroutines are automatically completed when the parent coroutine completes. This prevents memory leaks and makes the code predictable.

Data Serialization (kotlinx.serialization)

The kotlinx-serialization module allows converting Kotlin objects to JSON, XML, Protobuf, and other formats without using reflection. Serialization works at compile time, making it fast and