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Coding Conventions

This page contains the current coding style for the Kotlin language.

Applying the style guide

To configure the IntelliJ formatter according to this style guide, please install Kotlin plugin version 1.2.20 or newer, go to Settings | Editor | Code Style | Kotlin, click Set from… link in the upper right corner, and select Kotlin style guide from the menu.

To verify that your code is formatted according to the style guide, go to Settings | Editor | Inspections and enable the Kotlin | Style issues | File is not formatted according to project settings inspection. Additional inspections that verify other issues described in the style guide (such as naming conventions) are enabled by default.

Source code organization

Directory structure

In pure Kotlin projects, the recommended directory structure follows the package structure with the common root package omitted. For example, if all the code in the project is in the org.example.kotlin package and its subpackages, files with the org.example.kotlin package should be placed directly under the source root, and files in org.example.kotlin.network.socket should be in the network/socket subdirectory of the source root.

On the JVM: In projects where Kotlin is used together with Java, Kotlin source files should reside in the same source root as the Java source files, and follow the same directory structure: each file should be stored in the directory corresponding to each package statement.

Source file names

If a Kotlin file contains a single class (potentially with related top-level declarations), its name should be the same as the name of the class, with the .kt extension appended. If a file contains multiple classes, or only top-level declarations, choose a name describing what the file contains, and name the file accordingly. Use the camel case with an uppercase first letter (for example, ProcessDeclarations.kt ).

The name of the file should describe what the code in the file does. Therefore, you should avoid using meaningless words such as "Util" in file names.

Source file organization

Placing multiple declarations (classes, top-level functions or properties) in the same Kotlin source file is encouraged as long as these declarations are closely related to each other semantically and the file size remains reasonable (not exceeding a few hundred lines).

In particular, when defining extension functions for a class which are relevant for all clients of this class, put them in the same file where the class itself is defined. When defining extension functions that make sense only for a specific client, put them next to the code of that client. Do not create files just to hold "all extensions of Foo".

Class layout

Generally, the contents of a class is sorted in the following order:

Property declarations and initializer blocks

Secondary constructors

Method declarations

Companion object

Do not sort the method declarations alphabetically or by visibility, and do not separate regular methods from extension methods. Instead, put related stuff together, so that someone reading the class from top to bottom can follow the logic of what's happening. Choose an order (either higher-level stuff first, or vice versa) and stick to it.

Put nested classes next to the code that uses those classes. If the classes are intended to be used externally and aren't referenced inside the class, put them in the end, after the companion object.

Interface implementation layout

When implementing an interface, keep the implementing members in the same order as members of the interface (if necessary, interspersed with additional private methods used for the implementation)

Overload layout

Always put overloads next to each other in a class.

Naming rules

Package and class naming rules in Kotlin are quite simple:

Names of packages are always lower case and do not use underscores ( org.example.project ). Using multi-word names is generally discouraged, but if you do need to use multiple words, you can either simply concatenate them together or use the camel case ( org.example.myProject ).

Names of classes and objects start with an upper case letter and use the camel case:

open class DeclarationProcessor { /*...*/ } object EmptyDeclarationProcessor : DeclarationProcessor() { /*...*/ }

Function names

Names of functions, properties and local variables start with a lower case letter and use the camel case and no underscores:

fun processDeclarations() { /*...*/ } var declarationCount = 1

Exception: factory functions used to create instances of classes can have the same name as the abstract return type:

interface Foo { /*...*/ } class FooImpl : Foo { /*...*/ } fun Foo(): Foo { return FooImpl() }

Names for test methods

In tests (and only in tests), it's acceptable to use method names with spaces enclosed in backticks. (Note that such method names are currently not supported by the Android runtime.) Underscores in method names are also allowed in test code.

class MyTestCase { @Test fun `ensure everything works`() { /*...*/ } @Test fun ensureEverythingWorks_onAndroid() { /*...*/ } }

Property names

Names of constants (properties marked with const , or top-level or object val properties with no custom get function that hold deeply immutable data) should use uppercase underscore-separated names:

const val MAX_COUNT = 8 val USER_NAME_FIELD = "UserName"

Names of top-level or object properties which hold objects with behavior or mutable data should use camel-case names:

val mutableCollection: MutableSet<String> = HashSet()

Names of properties holding references to singleton objects can use the same naming style as object declarations:

val PersonComparator: Comparator<Person> = /*...*/

For enum constants, it's OK to use either uppercase underscore-separated names ( enum class Color { RED, GREEN } ) or regular camel-case names starting with an uppercase first letter, depending on the usage.

Names for backing properties

If a class has two properties which are conceptually the same but one is part of a public API and another is an implementation detail, use an underscore as the prefix for the name of the private property:

class C { private val _elementList = mutableListOf<Element>() val elementList: List<Element> get() = _elementList }

Choosing good names

The name of a class is usually a noun or a noun phrase explaining what the class is: List , PersonReader .

The name of a method is usually a verb or a verb phrase saying what the method does: close , readPersons . The name should also suggest if the method is mutating the object or returning a new one. For instance sort is sorting a collection in place, while sorted is returning a sorted copy of the collection.

The names should make it clear what the purpose of the entity is, so it's best to avoid using meaningless words ( Manager , Wrapper etc.) in names.

When using an acronym as part of a declaration name, capitalize it if it consists of two letters ( IOStream ); capitalize only the first letter if it is longer ( XmlFormatter , HttpInputStream ).

Formatting

Use four spaces for indentation. Do not use tabs.

For curly braces, put the opening brace in the end of the line where the construct begins, and the closing brace on a separate line aligned horizontally with the opening construct.

if (elements != null) { for (element in elements) { // ... } }

In Kotlin, semicolons are optional, and therefore line breaks are significant. The language design assumes Java-style braces, and you may encounter surprising behavior if you try to use a different formatting style.

Horizontal whitespace

Put spaces around binary operators ( a + b ). Exception: don't put spaces around the "range to" operator ( 0..i ).

Do not put spaces around unary operators ( a++ )

Put spaces between control flow keywords ( if , when , for and while ) and the corresponding opening parenthesis.

Do not put a space before an opening parenthesis in a primary constructor declaration, method declaration or method call.

class A(val x: Int) fun foo(x: Int) { ... } fun bar() { foo(1) }

Never put a space after ( , [ , or before ] , ) .

Never put a space around . or ?. : foo.bar().filter { it > 2 }.joinToString() , foo?.bar()

Put a space after // : // This is a comment

Do not put spaces around angle brackets used to specify type parameters: class Map<K, V> { ... }

Do not put spaces around :: : Foo::class , String::length

Do not put a space before ? used to mark a nullable type: String?

As a general rule, avoid horizontal alignment of any kind. Renaming an identifier to a name with a different length should not affect the formatting of either the declaration or any of the usages.

Colon

Put a space before : in the following cases:

when it's used to separate a type and a supertype;

when delegating to a superclass constructor or a different constructor of the same class;

after the object keyword.

Don't put a space before : when it separates a declaration and its type.

Always put a space after : .

abstract class Foo<out T : Any> : IFoo { abstract fun foo(a: Int): T } class FooImpl : Foo() { constructor(x: String) : this(x) { /*...*/ } val x = object : IFoo { /*...*/ } }

Class header formatting

Classes with a few primary constructor parameters can be written in a single line:

class Person(id: Int, name: String)

Classes with longer headers should be formatted so that each primary constructor parameter is in a separate line with indentation. Also, the closing parenthesis should be on a new line. If we use inheritance, then the superclass constructor call or list of implemented interfaces should be located on the same line as the parenthesis:

class Person( id: Int, name: String, surname: String ) : Human(id, name) { /*...*/ }

For multiple interfaces, the superclass constructor call should be located first and then each interface should be located in a different line:

class Person( id: Int, name: String, surname: String ) : Human(id, name), KotlinMaker { /*...*/ }

For classes with a long supertype list, put a line break after the colon and align all supertype names horizontally:

class MyFavouriteVeryLongClassHolder : MyLongHolder<MyFavouriteVeryLongClass>(), SomeOtherInterface, AndAnotherOne { fun foo() { /*...*/ } }

To clearly separate the class header and body when the class header is long, either put a blank line following the class header (as in the example above), or put the opening curly brace on a separate line:

class MyFavouriteVeryLongClassHolder : MyLongHolder<MyFavouriteVeryLongClass>(), SomeOtherInterface, AndAnotherOne { fun foo() { /*...*/ } }

Use regular indent (four spaces) for constructor parameters.

Rationale: This ensures that properties declared in the primary constructor have the same indentation as properties declared in the body of a class.

Modifiers

If a declaration has multiple modifiers, always put them in the following order:

public / protected / private / internal expect / actual final / open / abstract / sealed / const external override lateinit tailrec vararg suspend inner enum / annotation / fun // as a modifier in `fun interface` companion inline infix operator data

Place all annotations before modifiers:

@Named("Foo") private val foo: Foo

Unless you're working on a library, omit redundant modifiers (e.g. public ).

Annotation formatting

Annotations are typically placed on separate lines, before the declaration to which they are attached, and with the same indentation:

@Target(AnnotationTarget.PROPERTY) annotation class JsonExclude

Annotations without arguments may be placed on the same line:

@JsonExclude @JvmField var x: String

A single annotation without arguments may be placed on the same line as the corresponding declaration:

@Test fun foo() { /*...*/ }

File annotations

File annotations are placed after the file comment (if any), before the package statement, and are separated from package with a blank line (to emphasize the fact that they target the file and not the package).

/** License, copyright and whatever */ @file:JvmName("FooBar") package foo.bar

Function formatting

If the function signature doesn't fit on a single line, use the following syntax:

fun longMethodName( argument: ArgumentType = defaultValue, argument2: AnotherArgumentType, ): ReturnType { // body }

Use regular indent (4 spaces) for function parameters.

Rationale: Consistency with constructor parameters

Prefer using an expression body for functions with the body consisting of a single expression.

fun foo(): Int { // bad return 1 } fun foo() = 1 // good

Expression body formatting

If the function has an expression body that doesn't fit in the same line as the declaration, put the = sign on the first line. Indent the expression body by four spaces.

fun f(x: String) = x.length

Property formatting

For very simple read-only properties, consider one-line formatting:

val isEmpty: Boolean get() = size == 0

For more complex properties, always put get and set keywords on separate lines:

val foo: String get() { /*...*/ }

For properties with an initializer, if the initializer is long, add a line break after the equals sign and indent the initializer by four spaces:

private val defaultCharset: Charset? = EncodingRegistry.getInstance().getDefaultCharsetForPropertiesFiles(file)

Formatting control flow statements

If the condition of an if or when statement is multiline, always use curly braces around the body of the statement. Indent each subsequent line of the condition by four spaces relative to statement begin. Put the closing parentheses of the condition together with the opening curly brace on a separate line:

if (!component.isSyncing && !hasAnyKotlinRuntimeInScope(module) ) { return createKotlinNotConfiguredPanel(module) }

Rationale: Tidy alignment and clear separation of condition and statement body

Put the else , catch , finally keywords, as well as the while keyword of a do/while loop, on the same line as the preceding curly brace:

if (condition) { // body } else { // else part } try { // body } finally { // cleanup }

In a when statement, if a branch is more than a single line, consider separating it from adjacent case blocks with a blank line:

private fun parsePropertyValue(propName: String, token: Token) { when (token) { is Token.ValueToken -> callback.visitValue(propName, token.value) Token.LBRACE -> { // ... } } }

Put short branches on the same line as the condition, without braces.

when (foo) { true -> bar() // good false -> { baz() } // bad }

Method call formatting

In long argument lists, put a line break after the opening parenthesis. Indent arguments by 4 spaces. Group multiple closely related arguments on the same line.

drawSquare( x = 10, y = 10, width = 100, height = 100, fill = true )

Put spaces around the = sign separating the argument name and value.

Chained call wrapping

When wrapping chained calls, put the . character or the ?. operator on the next line, with a single indent:

val anchor = owner ?.firstChild!! .siblings(forward = true) .dropWhile { it is PsiComment || it is PsiWhiteSpace }

The first call in the chain usually should have a line break before it, but it's OK to omit it if the code makes more sense that way.

Lambda formatting

In lambda expressions, spaces should be used around the curly braces, as well as around the arrow which separates the parameters from the body. If a call takes a single lambda, it should be passed outside of parentheses whenever possible.

list.filter { it > 10 }

If assigning a label for a lambda, do not put a space between the label and the opening curly brace:

fun foo() { ints.forEach lit@{ // ... } }

When declaring parameter names in a multiline lambda, put the names on the first line, followed by the arrow and the newline:

appendCommaSeparated(properties) { prop -> val propertyValue = prop.get(obj) // ... }

If the parameter list is too long to fit on a line, put the arrow on a separate line:

foo { context: Context, environment: Env -> context.configureEnv(environment) }

Trailing commas

A trailing comma is a comma symbol after the last item of a series of elements:

class Person( val firstName: String, val lastName: String, val age: Int, // trailing comma )

Using trailing commas has several benefits:

It makes version-control diffs cleaner – as all the focus is on the changed value.

It makes it easy to add and reorder elements – there is no need to add or delete the comma if you manipulate elements.

It simplifies code generation, for example, for object initializers. The last element can also have a comma.

Trailing commas are entirely optional – your code will still work without them. The Kotlin style guide encourages the use of trailing commas at the declaration site and leaves it at your discretion for the call site.

To enable trailing commas in the IntelliJ IDEA formatter, go to Settings | Editor | Code Style | Kotlin, open the Other tab and select the Use trailing comma option.

Kotlin supports trailing commas in the following cases:

Enumerations

enum class Direction { NORTH, SOUTH, WEST, EAST, // trailing comma }

Value arguments

fun shift(x: Int, y: Int) { /*...*/ } shift( 25, 20, // trailing comma ) val colors = listOf( "red", "green", "blue", // trailing comma )

Class properties and parameters

class Customer( val name: String, val lastName: String, // trailing comma ) class Customer( val name: String, lastName: String, // trailing comma )

Function value parameters

fun powerOf( number: Int, exponent: Int, // trailing comma ) { /*...*/ } constructor( x: Comparable<Number>, y: Iterable<Number>, // trailing comma ) {} fun print( vararg quantity: Int, description: String, // trailing comma ) {}

Parameters with optional type (including setters)

val sum: (Int, Int, Int) -> Int = fun( x, y, z, // trailing comma ): Int { return x + y + x } println(sum(8, 8, 8))

Indexing suffix

class Surface { operator fun get(x: Int, y: Int) = 2 * x + 4 * y - 10 } fun getZValue(mySurface: Surface, xValue: Int, yValue: Int) = mySurface[ xValue, yValue, // trailing comma ]

Lambda parameters

fun main() { val x = { x: Comparable<Number>, y: Iterable<Number>, // trailing comma -> println("1") } println(x) }

when entry

fun isReferenceApplicable(myReference: KClass<*>) = when (myReference) { Comparable::class, Iterable::class, String::class, // trailing comma -> true else -> false }

Collection literals (in annotations)

annotation class ApplicableFor(val services: Array<String>) @ApplicableFor([ "serializer", "balancer", "database", "inMemoryCache", // trailing comma ]) fun run() {}

Type arguments

fun <T1, T2> foo() {} fun main() { foo< Comparable<Number>, Iterable<Number>, // trailing comma >() }

Type parameters

class MyMap< MyKey, MyValue, // trailing comma > {}

Destructuring declarations

data class Car(val manufacturer: String, val model: String, val year: Int) val myCar = Car("Tesla", "Y", 2019) val ( manufacturer, model, year, // trailing comma ) = myCar val cars = listOf<Car>() fun printMeanValue() { var meanValue: Int = 0 for (( _, _, year, // trailing comma ) in cars) { meanValue += year } println(meanValue/cars.size) } printMeanValue()

For longer documentation comments, place the opening /** on a separate line and begin each subsequent line with an asterisk:

/** * This is a documentation comment * on multiple lines. */

Short comments can be placed on a single line:

/** This is a short documentation comment. */

Generally, avoid using @param and @return tags. Instead, incorporate the description of parameters and return values directly into the documentation comment, and add links to parameters wherever they are mentioned. Use @param and @return only when a lengthy description is required which doesn't fit into the flow of the main text.

// Avoid doing this: /** * Returns the absolute value of the given number. * @param number The number to return the absolute value for. * @return The absolute value. */ fun abs(number: Int) { /*...*/ } // Do this instead: /** * Returns the absolute value of the given [number]. */ fun abs(number: Int) { /*...*/ }

Avoiding redundant constructs

In general, if a certain syntactic construction in Kotlin is optional and highlighted by the IDE as redundant, you should omit it in your code. Do not leave unnecessary syntactic elements in code just "for clarity".

Unit

If a function returns Unit, the return type should be omitted:

fun foo() { // ": Unit" is omitted here }

Semicolons

Omit semicolons whenever possible.

String templates

Don't use curly braces when inserting a simple variable into a string template. Use curly braces only for longer expressions.

println("$name has ${children.size} children")

Idiomatic use of language features

Immutability

Prefer using immutable data to mutable. Always declare local variables and properties as val rather than var if they are not modified after initialization.

Always use immutable collection interfaces ( Collection , List , Set , Map ) to declare collections which are not mutated. When using factory functions to create collection instances, always use functions that return immutable collection types when possible:

// Bad: use of mutable collection type for value which will not be mutated fun validateValue(actualValue: String, allowedValues: HashSet<String>) { ... } // Good: immutable collection type used instead fun validateValue(actualValue: String, allowedValues: Set<String>) { ... } // Bad: arrayListOf() returns ArrayList<T>, which is a mutable collection type val allowedValues = arrayListOf("a", "b", "c") // Good: listOf() returns List<T> val allowedValues = listOf("a", "b", "c")

Default parameter values

Prefer declaring functions with default parameter values to declaring overloaded functions.

// Bad fun foo() = foo("a") fun foo(a: String) { /*...*/ } // Good fun foo(a: String = "a") { /*...*/ }

Type aliases

If you have a functional type or a type with type parameters which is used multiple times in a codebase, prefer defining a type alias for it:

typealias MouseClickHandler = (Any, MouseEvent) -> Unit typealias PersonIndex = Map<String, Person>

Lambda parameters

In lambdas which are short and not nested, it's recommended to use the it convention instead of declaring the parameter explicitly. In nested lambdas with parameters, parameters should be always declared explicitly.

Returns in a lambda

Avoid using multiple labeled returns in a lambda. Consider restructuring the lambda so that it will have a single exit point. If that's not possible or not clear enough, consider converting the lambda into an anonymous function.

Do not use a labeled return for the last statement in a lambda.

Named arguments

Use the named argument syntax when a method takes multiple parameters of the same primitive type, or for parameters of Boolean type, unless the meaning of all parameters is absolutely clear from context.

drawSquare(x = 10, y = 10, width = 100, height = 100, fill = true)

Using conditional statements

Prefer using the expression form of try , if and when . Examples:

return if (x) foo() else bar() return when(x) { 0 -> "zero" else -> "nonzero" }

The above is preferable to:

if (x) return foo() else return bar() when(x) { 0 -> return "zero" else -> return "nonzero" }

if versus when

Prefer using if for binary conditions instead of when . Instead of

when (x) { null -> // ... else -> // ... }

use if (x == null) ... else ...

Prefer using when if there are three or more options.

Using nullable Boolean values in conditions

If you need to use a nullable Boolean in a conditional statement, use if (value == true) or if (value == false) checks.

Using loops

Prefer using higher-order functions ( filter , map etc.) to loops. Exception: forEach (prefer using a regular for loop instead, unless the receiver of forEach is nullable or forEach is used as part of a longer call chain).

When making a choice between a complex expression using multiple higher-order functions and a loop, understand the cost of the operations being performed in each case and keep performance considerations in mind.

Loops on ranges

Use the until function to loop over an open range:

for (i in 0..n - 1) { /*...*/ } // bad for (i in 0 until n) { /*...*/ } // good

Using strings

Prefer using string templates to string concatenation.

Prefer to use multiline strings instead of embedding

escape sequences into regular string literals.

To maintain indentation in multiline strings, use trimIndent when the resulting string does not require any internal indentation, or trimMargin when internal indentation is required:

assertEquals( """ Foo Bar """.trimIndent(), value ) val a = """if(a > 1) { | return a |}""".trimMargin()

Functions vs Properties

In some cases functions with no arguments might be interchangeable with read-only properties. Although the semantics are similar, there are some stylistic conventions on when to prefer one to another.

Prefer a property over a function when the underlying algorithm:

does not throw

is cheap to calculate (or cached on the first run)

returns the same result over invocations if the object state hasn't changed

Using extension functions

Use extension functions liberally. Every time you have a function that works primarily on an object, consider making it an extension function accepting that object as a receiver. To minimize API pollution, restrict the visibility of extension functions as much as it makes sense. As necessary, use local extension functions, member extension functions, or top-level extension functions with private visibility.

Using infix functions

Declare a function as infix only when it works on two objects which play a similar role. Good examples: and , to , zip . Bad example: add .

Don't declare a method as infix if it mutates the receiver object.

Factory functions

If you declare a factory function for a class, avoid giving it the same name as the class itself. Prefer using a distinct name making it clear why the behavior of the factory function is special. Only if there is really no special semantics, you can use the same name as the class.

Example:

class Point(val x: Double, val y: Double) { companion object { fun fromPolar(angle: Double, radius: Double) = Point(...) } }

If you have an object with multiple overloaded constructors that don't call different superclass constructors and can't be reduced to a single constructor with default argument values, prefer to replace the overloaded constructors with factory functions.

Platform types

A public function/method returning an expression of a platform type must declare its Kotlin type explicitly:

fun apiCall(): String = MyJavaApi.getProperty("name")

Any property (package-level or class-level) initialised with an expression of a platform type must declare its Kotlin type explicitly:

class Person { val name: String = MyJavaApi.getProperty("name") }

A local value initialized with an expression of a platform type may or may not have a type declaration:

fun main() { val name = MyJavaApi.getProperty("name") println(name) }

Using scope functions apply/with/run/also/let

Kotlin provides a variety of functions to execute a block of code in the context of a given object: let , run , with , apply , and also . For the guidance on choosing the right scope function for your case, refer to Scope Functions.

Coding conventions for libraries

When writing libraries, it's recommended to follow an additional set of rules to ensure API stability: