Kotlin Reference

This section documents Kotlin 2.4.x on the JVM, as published at kotlinlang.org, which is the reference these pages are written and verified against.

This content was generated with the assistance of AI and should be verified against kotlinlang.org before being relied on in production.

This section’s bibliography lists the reference material consulted while preparing these pages.

Welcome to the Kotlin reference. Kotlin is a modern, statically-typed language for the JVM (and, via Kotlin Multiplatform, JS/Native/Wasm) created by JetBrains — concise syntax, null safety built into the type system, and full Java interoperability. This section documents Kotlin 2.4.x on the JVM, written and verified against kotlinlang.org.

If you are new to Kotlin, read Getting Started and Kotlin and the JVM first, then the Language fundamentals pages in order, followed by Object-oriented and functional programming — from there the remaining groups (the type system, collections and metaprogramming, error handling, coroutines, and Kotlin on Android and tooling) can be read in any order as you need them. For Kotlin’s server-side use, see Java or Kotlin for Spring Boot?.

What’s covered

Getting started

Getting Started — what Kotlin is, installing it, kotlinc, the Kotlin REPL, the Kotlin Playground, and "Hello, World".
Kotlin and the JVM — Kotlin/JVM bytecode, Java interoperability, where Kotlin/JVM runs, and Kotlin Multiplatform’s other targets.

Language fundamentals

Lexical Structure and Style — file/package structure, identifiers, comments, optional semicolons, literals, and the official coding conventions.
Basic Types and Variables — val/var, type inference, the built-in number/Boolean/Char types, Any/Unit/Nothing, arrays, and unsigned integers.
Strings and Text — string templates, raw/triple-quoted strings, and common String operations.
Operators and Ranges — arithmetic/comparison/logical operators, ranges and progressions, and infix notation.
Control Flow — if/when as expressions, for/while, and labeled break/continue.
Functions — default and named arguments, single-expression functions, vararg, local functions, and tailrec.

Object-oriented and functional programming

Classes and Objects — primary/secondary constructors, properties, init blocks, visibility modifiers, and "classes are final by default."
Inheritance and Interfaces — open/override, abstract classes, interfaces with default/property members, and class delegation via by.
Data Classes and Destructuring — data class, generated members, copy(), componentN(), and destructuring declarations.
Sealed Classes and Enums — sealed class/interface, exhaustive when, and enum class.
Objects and Companion Objects — singletons, companion objects, and object expressions.
Extension Functions and Scope Functions — extension functions/properties, and let/run/with/apply/also.
Lambdas and Higher-Order Functions — lambda syntax, closures, function types, and inline/noinline/crossinline.

Type system

Generics and Variance — declaration-site variance (in/out), star projections, and reified type parameters.
Null Safety — nullable types, ?., ?:, !!, as?, platform types, and smart casts.
Equality and Operator Overloading — structural vs. referential equality, and the overloadable operator conventions.

Collections and metaprogramming

Collections and Sequences — read-only vs. mutable collections, functional operations, and Sequence (lazy) vs. eager collections.
Annotations and Reflection — declaring annotations, meta-annotations, and basic kotlin.reflect.
Type-Safe Builders and DSLs — lambdas with receiver and building a small type-safe builder.

Error handling

Exceptions and Error Handling — try as an expression, no checked exceptions, custom exceptions, and Result/runCatching.

Coroutines

Coroutines Basics — suspend functions, coroutine builders, CoroutineScope, and structured concurrency.
Coroutine Context, Cancellation and Exceptions — Dispatchers, cancellation and timeouts, SupervisorJob, and CoroutineExceptionHandler.
Flows — cold Flow basics, operators, StateFlow vs. SharedFlow, and buffering.

Kotlin on Android & tooling

Kotlin for Android — Android KTX, Jetpack pointers, and coroutines on Android.
Build and Tooling — the Gradle Kotlin DSL, Maven’s Kotlin plugin, IDE support, and static analysis.
Testing — kotlin.test, JUnit 5 with Kotlin, MockK, and kotlinx-coroutines-test.

Reference

Cheat Sheet (PDF) — a single-page, printable summary of everything in this section, with a downloadable PDF.

Bibliography

kotlinlang.org — the official Kotlin documentation home, and the primary source for every page in this section.
the Kotlin standard library API reference — the reference every stdlib type and function in this section links to conceptually.
the Kotlin language specification — the authoritative reference for the language-level pages.
the Kotlin coroutines guide — the primary source for the Coroutines group.
Kotlin docs — Kotlin for server-side development.
the Android Kotlin developer guide — the primary source for Kotlin for Android.
Aigner, Sebastian, Roman Elizarov, Svetlana Isakova, and Dmitry Jemerov. Kotlin in Action, 2nd ed. Manning Publications, 2024. ISBN 9781617299599. Consulted as part of the bibliography for this section, cross-checked against its public table of contents only (the full text was not reachable while preparing these pages) — see the publisher’s book page and manning.com.

The book above is a consulted reference, not the primary source for this section; on any discrepancy, or wherever it is silent on a feature introduced after it was written, the official Kotlin documentation wins.