Writing Documentation for Kotlin Projects: A Complete Guide

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Introduction: Why Do You Need Quality Documentation for Kotlin Projects?

Kotlin is a modern, concise, and expressive programming language that is actively used for developing Android applications, server-side solutions, and cross-platform projects. However, even the most elegant code becomes useless if developers cannot understand how to use it. Quality documentation is the bridge between the code author and its users. It accelerates the onboarding of new team members, reduces the number of errors, and increases overall productivity.

In this article, we will break down how to properly write documentation for Kotlin projects using built-in tools such as KDoc and best documentation practices. You will learn how to structure comments, describe classes and functions, and generate professional documentation using Dokka.

Basics of Documentation in Kotlin: KDoc and Its Syntax

Kotlin uses its own documentation format — KDoc, which is very similar to JavaDoc but adapted to the language's capabilities. KDoc supports Markdown for text formatting, making the documentation more readable.

KDoc Syntax

KDoc comments start with /** and end with */. The main elements are:

Description — the first paragraph after the opening tag.
Tags — @param, @return, @throws, @see, @property, and others.
Markdown — supports lists, links, code, and bold text.

Basic KDoc Example

/** * Calculates the factorial of a given number. * * Uses an iterative approach to avoid stack overflow. * * @param n A non-negative integer. * @return The factorial of the number n. * @throws IllegalArgumentException if n < 0. * @see Math */fun factorial(n: Int): Long {    require(n >= 0) { "n must be non-negative" }    var result = 1L    for (i in 2..n) {        result *= i    }    return result}

In this example, we use @param to describe the argument, @return for the return value, and @throws for the exception. Markdown allows you to highlight keywords and add links.

Documenting Various Kotlin Components

Each Kotlin element requires its own approach to documentation. Let's look at the main cases: classes, functions, properties, and extensions.

Documenting Classes and Interfaces

For classes, it is important to describe their purpose, usage, and examples. Use the @property tag to document class properties.

/** * Represents a system user. * * Usage example: * ``` * val user = User("john_doe", "john@example.com") * println(user.displayName) * ``` * * @property username The unique username. * @property email The email address. * @constructor Creates a new user with the specified parameters. */data class User(    val username: String,    val email: String) {    /**     * Returns the display name of the user.     */    val displayName: String        get() = "$username ($email)"}

Documenting Extension Functions

Extensions are one of Kotlin's strong points. Document them like regular functions, but specify which type they apply to.

/** * Checks if the string is a valid email address. * * @receiver The String to be checked. * @return `true` if the string matches the email format, otherwise `false`. */fun String.isValidEmail(): Boolean {    return this.matches(Regex("^[A-Za-z0-9+_.-]+@[A-Za-z0-9.-]+\\\\.[A-Za-z]{2,}$"))}