Skip to content

Standard rules

Annotation formatting

Multiple annotations should be on a separate line than the annotated declaration; annotations with parameters should each be on separate lines; annotations should be followed by a space

// A single annotation (without parameters) is allowed on same line as annotated construct
@FunctionalInterface class FooBar {
    @JvmField var foo: String
    @Test fun bar() {}
}
// A class or function parameter may have a single annotation with parameter(s) on the same line
class Foo(@Path("fooId") val fooId: String)
class Bar(
    @NotNull("fooId") val fooId: String,
    @NotNull("bar") bar: String
)
// Multiple annotations (without parameters) are allowed on the same line
@Foo @Bar
class FooBar {
    @Foo @Bar
    var foo: String
    @Foo @Bar
    fun bar() {}
}
// An array of annotations (without parameters) is allowed on same line as annotated construct
@[Foo Bar] class FooBar2 {
    @[Foo Bar] var foo: String
    @[Foo Bar] fun bar() {}
}

// An annotation with parameter(s) is not allowed on same line as annotated construct
@Suppress("Unused") class FooBar {
    @Suppress("Unused") var foo: String
    @Suppress("Unused") fun bar() {}
}
// Multiple annotation on same line as annotated construct are not allowed
@Foo @Bar class FooBar {
    @Foo @Bar var foo: String
    @Foo @Bar fun bar() {}
}

Rule-id: annotation (standard rule set)

Blank line before declarations

Requires a blank line before any class or function declaration. No blank line is required between the class signature and the first declaration in the class. In a similar way, a blank line is required before any list of top level or class properties. No blank line is required before local properties or between consecutive properties.

const val foo1 = "foo1"

class FooBar {
    val foo2 = "foo2"
    val foo3 = "foo3"

    fun bar1() {
       val foo4 = "foo4"
       val foo5 = "foo5"
    }

    fun bar2() = "bar"

    val foo6 = "foo3"
    val foo7 = "foo4"

    enum class Foo {}
}

const val foo1 = "foo1"
class FooBar {
    val foo2 = "foo2"
    val foo3 = "foo3"
    fun bar1() {
       val foo4 = "foo4"
       val foo5 = "foo5"
    }
    fun bar2() = "bar"
    val foo6 = "foo3"
    val foo7 = "foo4"
    enum class Foo {}
}

Rule id: blank-line-before-declaration (standard rule set)

Note

This rule is only run when ktlint_code_style is set to ktlint_official or when the rule is enabled explicitly.

Block comment initial star alignment

Lines in a block comment which (exclusive the indentation) start with a * should have this * aligned with the * in the opening of the block comment.

/*
 * This comment is formatted well.
 */

/*
      * This comment is not formatted well.
    */

Rule id: block-comment-initial-star-alignment (standard rule set)

Discouraged comment location

Detect discouraged comment locations (no autocorrect).

Note

Kotlin allows comments to be placed almost everywhere. As this can lead to code which is hard to read, most of them will never be used in practice. Ideally each rule takes comments at all possible locations into account. Sometimes this is really hard and not worth the effort. By explicitly forbidding such comment locations, the development of those rules becomes a bit easier.

fun <T> /* some comment */ foo(t: T) = "some-result"

fun foo() {
    if (true)
        // some comment
        bar()
}

Rule id: discouraged-comment-location (standard rule set)

Enum entry

Enum entry names should be uppercase underscore-separated or upper camel-case separated.

enum class Bar {
    FOO,
    Foo,
    FOO_BAR,
    FooBar
}

enum class Bar {
    foo,
    bAr,
    Foo_Bar,
}

Rule id: enum-entry-name-case (standard rule set)

File name

A file containing only one visible (e.g. non-private) class, and visible declarations related to that class only, should be named according to that element. The same applies if the file does not contain a visible class but exactly one type alias or one object declaration. Otherwise, the PascalCase notation should be used.

Rule id: filename (standard rule set)

Final newline

Ensures consistent usage of a newline at the end of each file.

This rule can be configured with .editorconfig property insert_final_newline.

Rule id: final-newline (standard rule set)

Function signature

Rewrites the function signature to a single line when possible (e.g. when not exceeding the max_line_length property) or a multiline signature otherwise. In case of function with a body expression, the body expression is placed on the same line as the function signature when not exceeding the max_line_length property.

In ktlint-official code style, a function signature is always rewritten to a multiline signature in case the function has 2 or more parameters. This number of parameters can be set via .editorconfig property ktlint_function_signature_rule_force_multiline_when_parameter_count_greater_or_equal_than.

// Assume that the last allowed character is
// at the X character on the right           X
fun foooooooo(
    a: Any,
    b: Any,
    c: Any
): String {
    // body
}

// Assume that the last allowed character is
// at the X character on the right           X
fun bar(a: Any, b: Any, c: Any): String {
    // body
}

// When wrapping of body is set to 'default'.
// Assume that the last allowed character is
// at the X character on the right           X
fun f(a: Any, b: Any): String = "some-result"
    .uppercase()

// When wrapping of body is set to 'multiline'
// or 'always'.
// Assume that the last allowed character is
// at the X character on the right           X
fun f(a: Any, b: Any): String =
    "some-result"
        .uppercase()

// Assume that the last allowed character is
// at the X character on the right           X
fun foooooooo(a: Any, b: Any, c: Any): String {
    // body
}

// Assume that the last allowed character is
// at the X character on the right           X
fun bar(
    a: Any,
    b: Any,
    c: Any
): String {
    // body
}

// When wrapping of body is set to 'default'.
// Assume that the last allowed character is
// at the X character on the right           X
fun f(a: Any, b: Any): String =
    "some-result"
        .uppercase()

// When wrapping of body is set to 'multiline'
// or 'always'.
// Assume that the last allowed character is
// at the X character on the right           X
fun f(a: Any, b: Any): String = "some-result"
    .uppercase()

Rule id: function-signature (standard rule set)

If else bracing

If at least one branch of an if-else statement or an if-else-if statement is wrapped between curly braces then all branches should be wrapped between braces.

fun foo(value: int) {
    if (value > 0) {
        doSomething()
    } else if (value < 0) {
        doSomethingElse()
    } else {
        doSomethingElse2()
    }
}

fun foo(value: int) {
    if (value > 0)
        doSomething()
    else if (value < 0) {
        doSomethingElse()
    } else
        doSomethingElse2()
}

Rule id: if-else-bracing (standard rule set)

Note

This rule is only run when ktlint_code_style is set to ktlint_official or when the rule is enabled explicitly.

Import ordering

Ensures that imports are ordered consistently (see Import Layouts for configuration).

import com.bar.Bar
import com.foo.Foo
import org.foo.bar.FooBar
import java.util.concurrent.ConcurrentHashMap

import com.bar.Bar
import java.util.concurrent.ConcurrentHashMap
import org.foo.bar.FooBar
import com.foo.Foo

Rule id: import-ordering (standard rule set)

Indentation

Indentation formatting - respects .editorconfig indent_size with no continuation indent (see EditorConfig section for more).

fun main() {
    foobar(
        a,
        b,
        c
    )
}

fun main() {
    foobar(
          a,
          b,
          c
          )
}

Note

This rule handles indentation for many different language constructs which can not be summarized with a few examples. See the unit tests for more details.

Rule id: indent (standard rule set)

Naming

Class naming

Enforce naming of class and objects.

class Foo
class Foo1

@Nested
inner class `Some descriptive class name` {
    @Test
    fun `Some descriptive test name`() {
        // do something
    }
}

class foo
class Foo_Bar
class `Some class in the production code`

Note

Functions in files which import a class from package org.junit.jupiter.api are considered to be test functions and are allowed to have a name specified between backticks and do not need to adhere to the normal naming convention. Although, the Kotlin coding conventions does not allow this explicitly for class identifiers, ktlint does allow it.

This rule can also be suppressed with the IntelliJ IDEA inspection suppression ClassName.

Rule id: class-naming (standard rule set)

Function naming

Enforce naming of function.

fun foo() {}
fun fooBar() {}

@Test
fun `Some name`() {}

@Test
fun do_something() {}

fun Foo() {}
fun Foo_Bar() {}
fun `Some name`() {}
fun do_something() {}

Note

Functions in files which import a class from package org.junit, org.testng or kotlin.test are considered to be test functions. Functions in such classes are allowed to have underscores in the name. Or function names can be specified between backticks and do not need to adhere to the normal naming convention.

This rule can also be suppressed with the IntelliJ IDEA inspection suppression FunctionName.

Rule id: function-naming (standard rule set)

Package name

Validates that the package name matches the regular expression [a-z][a-zA-Z\d]*(\.[a-z][a-zA-Z\d]*)*.

package foo
package foo.bar

package Foo
package foo.Foo
package `foo bar`
package foo.`foo bar`

Rule id: package-name (standard rule set)

Property naming

Enforce naming of property.

Note

This rule can not reliably detect all situations in which incorrect property naming is used. So it only detects in which it is certain that naming is incorrect.

val foo1 = Foo() // In case developer want to communicate that Foo is mutable
val FOO1 = Foo() // In case developer want to communicate that Foo is deeply immutable

const val FOO_BAR = "FOO-BAR" // By definition deeply immutable

var foo2: Foo = Foo() // By definition not immutable

class Bar {
    val foo1 = Foo() // In case developer want to communicate that Foo is mutable
    val FOO1 = Foo() // In case developer want to communicate that Foo is deeply immutable

    const val FOO_BAR = "FOO-BAR" // By definition deeply immutable

    var foo2: Foo = Foo() // By definition not immutable

    // Backing property
    private val _elementList = mutableListOf<Element>()
    val elementList: List<Element>
        get() = _elementList
}

const val fooBar = "FOO-BAR" // By definition deeply immutable

var FOO2: Foo = Foo() // By definition not immutable

class Bar {
    val FOO_BAR = "FOO-BAR" // Class properties always start with lowercase, const is not allowed

    // Incomplete backing property as public property 'elementList1' is missing
    private val _elementList1 = mutableListOf<Element>()

    // Invalid backing property as '_elementList2' is not a private property
    val _elementList2 = mutableListOf<Element>()
    val elementList2: List<Element>
        get() = _elementList2
}

This rule can also be suppressed with the IntelliJ IDEA inspection suppression PropertyName.

Rule id: property-naming (standard rule set)

No blank lines in list

Disallow blank lines to be used in lists before the first element, between elements, and after the last element.

Super type

class FooBar:
    Foo,
    Bar {
    // body
}

class FooBar:

    Foo,

    Bar

{
    // body
}

Type argument list

val foobar: FooBar<
    Foo,
    Bar,
    > = FooBar(Foo(), Bar())

val foobar: FooBar<

    Foo,

    Bar,

    > = FooBar(Foo(), Bar())

Type constraint list

class BiAdapter<C : RecyclerView.ViewHolder, V1 : C, V2 : C, out A1, out A2>(
    val adapter1: A1,
    val adapter2: A2
) : RecyclerView.Adapter<C>()
    where A1 : RecyclerView.Adapter<V1>, A1 : ComposableAdapter.ViewTypeProvider,
          A2 : RecyclerView.Adapter<V2>, A2 : ComposableAdapter.ViewTypeProvider {
    // body
}

class BiAdapter<C : RecyclerView.ViewHolder, V1 : C, V2 : C, out A1, out A2>(
    val adapter1: A1,
    val adapter2: A2
) : RecyclerView.Adapter<C>()
    where
          A1 : RecyclerView.Adapter<V1>, A1 : ComposableAdapter.ViewTypeProvider,

          A2 : RecyclerView.Adapter<V2>, A2 : ComposableAdapter.ViewTypeProvider
{
    // body
}

Type parameter list

fun <
    Foo,
    Bar,
    > foobar()

fun <

    Foo,

    Bar,

    > foobar()

Value argument list

val foobar = foobar(
    "foo",
    "bar",
)

val foobar = foobar(

    "foo",

    "bar",

)

Value parameter list

fun foobar(
    foo: String,
    bar: String,
)

fun foobar(

    foo: String,

    bar: String,

)

Rule id: no-blank-line-in-list (standard rule set)

Note

This rule is only run when ktlint_code_style is set to ktlint_official or when the rule is enabled explicitly.

No consecutive comments

Consecutive comments are disallowed in following cases: - Any mix of a consecutive kdoc, a block comment or an EOL comment unless separated by a blank line in between - Consecutive KDocs (even when separated by a blank line) - Consecutive block comments (even when separated by a blank line)

Consecutive EOL comments are always allowed as they are often used instead of a block comment.

// An EOL comment
// may be followed by another EOL comment
val foo = "foo"

// Different comment types (including KDoc) may be consecutive ..

/*
 * ... but do need to be separated by a blank line ...
 */

/**
  * ... but a KDoc can not be followed by an EOL or a block comment or another KDoc
  */
fun bar() = "bar"

/*
 * Block comments can not be consecutive ...
 */
/*
 * ... even not when separated by a new line.
 */
val bar = "bar" 

/**
  * A KDoc can not be followed by a block comment or an EOL comment or another KDOC
  */

// ... even not when separated by a new line.

Rule id: no-consecutive-comments (standard rule set)

Note

This rule is only run when ktlint_code_style is set to ktlint_official or when the rule is enabled explicitly.

No empty file

A kotlin (script) file should not be empty. It needs to contain at least one declaration. Files only contain a package and/or import statements are as of that disallowed.

Rule id: no-empty-file

No empty first line at start in class body

Detect blank lines at start of a class body.

class Foo {
    val foo = "foo"
}

class Foo {

    val foo = "foo"
}

Rule id: no-empty-first-line-in-class-body (standard rule set)

Note

This rule is only run when ktlint_code_style is set to ktlint_official or when the rule is enabled explicitly.

No single line block comment

A single line block comment should be replaced with an EOL comment when possible.

/*
 * Some comment
 */
val foo = "foo" // Some comment
val foo = { /* no-op */ }

/* Some comment */
val foo = "foo" /* Some comment */

Rule id: no-single-line-block-comment (standard rule set)

Ktlint-suppression rule

The ktlint-disable and ktlint-enable directives are no longer supported as of ktlint version 0.50.0. This rule migrates the directives to Suppress or SuppressWarnings annotations.

Identifiers in the @Suppress and @SuppressWarnings annotations to suppress ktlint rules are checked for validity and autocorrected when possible. 

@file:Suppress("ktlint:standard:no-wildcard-imports")

class FooBar {
    @Suppress("ktlint:standard:max-line-length")
    val foo = "some longggggggggggggggggggg text"

    fun bar() =
        @Suppress("ktlint:standard:no-multi-spaces")
        listOf(
            "1   One", 
            "10  Ten", 
            "100 Hundred", 
        )
}

/* ktlint-disable standard:no-wildcard-imports */

class FooBar {
    val foo = "some longggggggggggggggggggg text" // ktlint-disable standard:max-line-length

    fun bar() =
        listOf(
            /* ktlint-disable standard:no-multi-spaces */
            "1   One", 
            "10  Ten", 
            "100 Hundred", 
            /* ktlint-enable standard:no-multi-spaces */
        )
}

Rule id: ktlint-suppression (standard rule set)

Note

This rule can not be disabled in the .editorconfig.

Max line length

Ensures that lines do not exceed the given length of .editorconfig property max_line_length (see EditorConfig section for more). This rule does not apply in a number of situations. For example, in the case a line exceeds the maximum line length due to a comment that disables ktlint rules then that comment is being ignored when validating the length of the line. The .editorconfig property ktlint_ignore_back_ticked_identifier can be set to ignore identifiers which are enclosed in backticks, which for example is very useful when you want to allow longer names for unit tests.

// Assume that the last allowed character is
// at the X character on the right           X
// Lines below are accepted although the max
// line length is exceeded.
package com.toooooooooooooooooooooooooooo.long
import com.tooooooooooooooooooooooooooooo.long
val foo =
    """
    fooooooooooooooooooooooooooooooooooooooooo
    """
@Test
fun `Test description which is toooooooooooo long`() {
}

// Assume that the last allowed character is
// at the X character on the right           X
val fooooooooooooooo = "fooooooooooooooooooooo"
val foooooooooooooo = "foooooooooooooooooooo" // some comment
val fooooooooooooo =
    "foooooooooooooooooooooooooooooooooooooooo"

Rule id: max-line-length (standard rule set)

Modifier order

Consistent order of modifiers

abstract class A {
    protected open val v = ""
    internal open suspend fun f(v: Any): Any = ""
    protected lateinit var lv: String
}

abstract class A {
    open protected val v = ""
    open suspend internal fun f(v: Any): Any = ""
    lateinit protected var lv: String
}

Rule id: modifier-order (standard rule set)

Multiline if-else

Braces required for multiline if/else statements.

val foo =
    if (true) {
        return 0
    } else {
        return 1
    }

val foo =
    if (true)
        return 0
    else
        return 1

Rule id: multiline-if-else (standard rule set)

No blank lines before }

No blank lines before }.

fun main() {
    fun a() {
    }
    fun b()
}

fun main() {
    fun a() {

    }
    fun b()

}

Rule id: no-blank-line-before-rbrace (standard rule set)

No blank lines in chained method calls

fun foo(inputText: String) {
    inputText
        .lowercase(Locale.getDefault())
}

fun foo(inputText: String) {
    inputText

        .lowercase(Locale.getDefault())
}

Rule id: no-blank-lines-in-chained-method-calls (standard rule set)

No consecutive blank lines

package com.test

import com.test.util

val a = "a"

fun b() {
}

fun c()

package com.test


import com.test.util


val a = "a"


fun b() {
}


fun c()

Rule id: no-consecutive-blank-lines (standard rule set)

No empty ({}) class bodies

class C
data class DC(val v: Any)
interface I
object O

class C {}
data class DC(val v: Any) { }
interface I {
}
object O{}

Rule id: no-empty-class-body (standard rule set)

No leading empty lines in method blocks

fun bar() {
   val a = 2
}

fun bar() {

   val a = 2
}

Rule id: no-empty-first-line-in-method-block (standard rule set)

No line break after else

Disallows line breaks after the else keyword if that could lead to confusion, for example:

fun funA() {
    if (conditionA()) {
        doSomething()
    } else if (conditionB()) {
        doAnotherThing()
    }
}

fun funA() {
    if (conditionA()) {
        doSomething()
    } else
    if (conditionB()) {
        doAnotherThing()
    }
}

Rule id: no-line-break-after-else (standard rule set)

No line break before assignment

When a line is broken at an assignment (=) operator the break comes after the symbol.

val valA =
    ""

val valA
    = ""

Rule id: no-line-break-before-assignment (standard rule set)

No multi spaces

Except in indentation and in KDoc's it is not allowed to have multiple consecutive spaces.

fun main() {
    x(1, 3)
}

fun  main()  {
    x(1,  3)
}

Rule id: no-multi-spaces (standard rule set)

No semicolons

No semicolons (unless used to separate multiple statements on the same line).

fun foo() {
    bar()

    bar()
}

fun foo() {
    ;
    bar()
    ;

    bar()

    ;
}

Rule id: no-semi (standard rule set)

No trailing whitespaces

Rule id: no-trailing-spaces (standard rule set)

No Unit as return type

The Unit type is not allowed as return type of a function. returns (fun fn {} instead of fun fn: Unit {})

fun fn() {}

fun fn(): Unit {}

Rule id: no-unit-return (standard rule set)

No unused imports

Warning

This rule is not able to detect all unused imports as mentioned in this issue comment.

Rule id: no-unused-imports (standard rule set)

No wildcard imports

No wildcard imports except imports listed in .editorconfig property ij_kotlin_packages_to_use_import_on_demand.

import foobar.Bar
import foobar.Foo

import foobar.*

Warning

In case property ij_kotlin_packages_to_use_import_on_demand is not explicitly set, it allows wildcards imports like java.util.* by default to keep in sync with IntelliJ IDEA behavior. To disallow all wildcard imports, add property below to your .editorconfig: 

[*.{kt,kts}]
ij_kotlin_packages_to_use_import_on_demand = unset

Rule id: no-wildcard-imports (standard rule set)

Spacing

Angle bracket spacing

No spaces around angle brackets when used for typing.

val a: Map<Int, String> = mapOf()
val b: Map<Int, String> = mapOf()
val c: Map<Int, String> = mapOf()

val a: Map< Int, String> = mapOf()
val b: Map<Int, String > = mapOf()
val c: Map <Int, String> = mapOf()

Rule id: spacing-around-angle-brackets (standard rule set)

Annotation spacing

Annotations should be separated by a single line break.

@JvmField
fun foo() {}

/**
 * block comment
 */
@Foo @Bar
class FooBar {
}

@JvmField

fun foo() {}

@Foo @Bar
/**
 * block comment
 */
class FooBar {
}

Rule id: annotation-spacing (standard rule set)

Blank line between declarations with annotations

Declarations with annotations should be separated by a blank line.

fun a()

@Bar
fun b()

fun a()
@Bar
fun b()

Rule id: spacing-between-declarations-with-annotations (standard rule set)

Blank line between declaration with comments

Declarations with comments should be separated by a blank line.

// some comment 1
bar()

/*
 * some comment 2
 */
foo()

// some comment 1
bar()
/*
 * some comment 2
 */
foo()

Rule id: spacing-between-declarations-with-comments (standard rule set)

Colon spacing

Consistent spacing around colon.

class A : B
class A2 : B2

class A:B
class A2  :  B2

Rule id: colon-spacing (standard rule set)

Comma spacing

Consistent spacing around comma.

val foo1 = Foo(1, 3)
val foo2 = Foo(1, 3)

val foo1 = Foo(1 ,3)
val foo2 = Foo(1,3)

Rule id: comma-spacing (standard rule set)

Comment spacing

The end of line comment sign // should be preceded and followed by exactly a space.

// comment
var debugging = false // comment
var debugging = false // comment
var debugging = false // comment
fun main() {
    System.out.println( // 123
        "test"
    )
}
    // comment

//comment
var debugging = false// comment
var debugging = false //comment
var debugging = false//comment
fun main() {
    System.out.println(//123
        "test"
    )
}
    //comment

Rule id: comment-spacing (standard rule set)

Curly spacing

Consistent spacing around curly braces.

val foo = if (true) { 0 } else { 1 }

val foo = if (true){0}else{1}

Rule id: curly-spacing (standard rule set)

Dot spacing

Consistent spacing around dots.

fun String.foo() = "foo"

fun String . foo() = "foo"

Rule id: dot-spacing (standard rule set)

Double colon spacing

No spaces around ::.

val foo = Foo::class

val foo1 = Foo ::class
val foo2 = Foo:: class
val foo3 = Foo :: class
val foo4 = Foo::
    class

Rule id: double-colon-spacing (standard rule set)

Function return type spacing

Consistent spacing around the function return type.

fun foo(): String = "some-result"

fun foo1() : String = "some-result"
fun foo2():  String = "some-result"
fun foo3():String = "some-result"
fun foo4():
    String = "some-result"

Rule id: function-return-type-spacing (standard rule set)

Function start of body spacing

Consistent spacing before start of function body.

fun foo1() = "some-result"
fun foo2() =
    "some-result"
fun foo3() {
    // do something
}
fun bar1(): String = "some-result"
fun bar2(): String =
    "some-result"
fun bar3(): String {
    return "some-result"
}

fun foo1()= "some-result"
fun foo2()
    = "some-result"
fun foo3()
{
    // do something
}
fun bar1(): String= "some-result"
fun bar2(): String
    = "some-result"
fun bar3(): String
{
    return "some-result"
}

Rule id: function-start-of-body-spacing (standard rule set)

Function type reference spacing

Consistent spacing in the type reference before a function.

fun String.foo() = "some-result"

fun String .foo() = "some-result"
fun String
    .foo() = "some-result"
fun String? .foo() = "some-result"
fun String?
    .foo() = "some-result"

Rule id: function-type-reference-spacing (standard rule set)

Fun keyword spacing

Consistent spacing after the fun keyword.

fun foo() = "some-result"

fun  foo() = "some-result"
fun
foo() = "some-result"

Rule id: fun-keyword-spacing (standard rule set)

Kdoc wrapping

A KDoc comment should start and end on a line that does not contain any other element.

/** Some KDoc comment 1 */
val foo1 = "foo1"

/** Some KDoc comment 1 */ val foo1 = "foo1"
val foo2 = "foo2" /** Some KDoc comment
                   * with a newline
                   */

Rule id: kdoc-wrapping (standard rule set)

Keyword spacing

Consistent spacing around keywords.

fun main() {
    if (true) {}
}

fun main() {
    if(true){}
}

Rule id: keyword-spacing (standard rule set)

Modifier list spacing

Consistent spacing between modifiers in and after the last modifier in a modifier list.

abstract class Foo {
    protected abstract suspend fun execute()
}

abstract  class Foo {
    protected  abstract  suspend  fun execute()
}
abstract
class Foo {
    protected
    abstract
    suspend
    fun execute()
}

Rule id: modifier-list-spacing (standard rule set)

Nullable type spacing

No spaces in a nullable type.

val foo: String? = null
val foo: List<String?> = listOf(null)

val foo: String ? = null
val foo: List<String ?> = listOf(null)

Rule id: nullable-type-spacing (standard rule set)

Operator spacing

Consistent spacing around operators.

val foo1 = 1 + 2
val foo2 = 1 - 2
val foo3 = 1 * 2
val foo4 = 1 / 2

val foo1 = 1+2
val foo2 = 1- 2
val foo3 = 1 *2
val foo4 = 1  /  2

Rule id: op-spacing (standard rule set)

Parameter list spacing

Consistent spacing inside the parameter list.

fun foo(a: Any ) = "some-result"
fun foo() = "some-result"

fun foo( a : Any ) = "some-result"
fun foo(
) = "some-result"

Rule id: parameter-list-spacing (standard rule set)

Parenthesis spacing

Consistent spacing around parenthesis.

class Foo : Bar {
    constructor(string: String) : super()
}
val foo1 = ((1 + 2) / 3)

class Foo : Bar {
    constructor(string: String) : super ()
}
val foo1 = ( (1 + 2 ) / 3)

Rule id: paren-spacing (standard rule set)

Range spacing

Consistent spacing around range operators.

val foo1 = (1..12 step 2).last
val foo2 = (1..12 step 2).last
val foo3 = (1..12 step 2).last

val foo1 = (1.. 12 step 2).last
val foo2 = (1 .. 12 step 2).last
val foo3 = (1 ..12 step 2).last

Rule id: range-spacing (standard rule set)

Spacing between function name and opening parenthesis

Consistent spacing between function name and opening parenthesis.

fun foo() = "foo"

fun foo () = "foo"

Rule id: spacing-between-function-name-and-opening-parenthesis (standard rule set)

Try catch finally spacing

Enforce consistent spacing in try { .. } catch { .. } finally { .. }.

fun foo() =
    try {
        // do something
    } catch (exception: Exception) {
        // handle exception
    } finally {
        // clean up
    }

fun foo1() = try { /* ... */ } catch (exception: Exception) { /* ... */ } finally { /* ... */ }
fun foo2() = 
    try {
        // do something
    }
    catch (exception: Exception) {
        // handle exception
    }
    finally {
        // clean up
    }

Rule id: try-catch-finally-spacing (standard rule set)

Note

This rule is only run when ktlint_code_style is set to ktlint_official or when the rule is enabled explicitly.

Type argument list spacing

Spacing before and after the angle brackets of a type argument list.

val res = ArrayList<LintError>()
class B<T> : A<T>() {
    override fun x() = super<A>.x()
}

val res = ArrayList < LintError > ()
class B<T> : A< T >() {
    override fun x() = super< A >.x()
}

Rule id: type-argument-list-spacing (standard rule set)

Type parameter list spacing

Spacing after a type parameter list in function and class declarations.

fun <T> foo1(t: T) = "some-result"
fun <T> foo2(t: T) = "some-result"
fun <T> foo3(t: T) = "some-result"

fun<T> foo1(t: T) = "some-result"
fun <T>foo2(t: T) = "some-result"
fun<T>foo3(t: T) = "some-result"

Rule id: type-parameter-list-spacing (standard rule set)

Unary operator spacing

No spaces around unary operators.

fun foo1(i: Int) = i++
fun foo2(i: Int) = ++i
fun foo3(i: Int) = ++i

fun foo1(i: Int) = i ++
fun foo2(i: Int) = ++ i
fun foo3(i: Int) = ++
    i

Rule id: unary-op-spacing (standard rule set)

String template

Consistent string templates ($v instead of ${v}, ${p.v} instead of ${p.v.toString()})

val foo = "$foo hello"

val foo = "${foo} hello"

Rule id: string-template (standard rule set)

String template indent

Enforce consistent string template indentation for multiline string templates which are post-fixed with .trimIndent(). The opening and closing """ are placed on separate lines and the indentation of the content of the template is aligned with the """.

val foo =
    """
    line1
    line2
    """.trimIndent()
fun foo() {
    // The opening """ can not be wrapped to next line as that would result in a compilation error
    return """
        line1
        line2
        """.trimIndent()
}

val foo = """
          line1
          line2
          """.trimIndent()
fun foo() {
    return """
        line1
        line2
    """.trimIndent()
}

Rule id: string-template-indent (standard rule set)

Note

This rule is only run when ktlint_code_style is set to ktlint_official or when the rule is enabled explicitly.

Trailing comma on call site

Consistent removal (default) or adding of trailing commas on call site.

Important

KtLint uses the IntelliJ IDEA .editorconfig property ij_kotlin_allow_trailing_comma_on_call_site to configure the rule. When this property is enabled, KtLint enforces the usage of the trailing comma at call site while IntelliJ IDEA default formatter only allows to use the trailing comma but leaves it to the developer's discretion to actually use it (or not). KtLint values consistent formatting more than a per-situation decision.

FooWrapper(
    Foo(
        a = 3,
        b = 4,
    ),
)

FooWrapper(Foo(
    a = 3,
    b = 4,
),) // it's weird to insert "," between unwrapped (continued) parenthesis

Note

In KtLint 0.48.x the default value for using the trailing comma on call site has been changed to true except when codestyle android is used.

Although the Kotlin coding conventions leaves it to the developer's discretion to use trailing commas on the call site, it also states that usage of 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.

Note

Trailing comma on call site is automatically disabled if the Wrapping rule (or, before version 0.45.0, the Indentation rule) is disabled or not loaded. Because it cannot provide proper formatting with unwrapped calls. (see dependencies).

Rule id: trailing-comma-on-call-site (standard rule set)

Trailing comma on declaration site

Consistent removal (default) or adding of trailing commas on declaration site.

Important

KtLint uses the IntelliJ IDEA .editorconfig property ij_kotlin_allow_trailing_comma to configure the rule. When this property is enabled, KtLint enforces the usage of the trailing comma at declaration site while IntelliJ IDEA default formatter only allows to use the trailing comma but leaves it to the developer's discretion to actually use it (or not). KtLint values consistent formatting more than a per-situation decision.

class FooWrapper(
    val foo = Foo(
        a = 3,
        b = 4,
    ),
)

class FooWrapper(val foo = Foo(
    a = 3,
    b = 4,
),) // it's weird to insert "," between unwrapped (continued) parenthesis

Note

In KtLint 0.48.x the default value for using the trailing comma on declaration site has been changed to true except when codestyle android is used.

The Kotlin coding conventions encourages the usage of trailing commas on the declaration site, but leaves it to the developer's discretion to use trailing commas on the call site. But next to this, it also states that usage of 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.

Note

Trailing comma on declaration site is automatically disabled if the Wrapping rule (or, before version 0.45.0, the Indentation rule) is disabled or not loaded. Because it cannot provide proper formatting with unwrapped declarations. (see dependencies).

Rule id: trailing-comma-on-declaration-site (standard rule set)

Unnecessary parenthesis before trailing lambda

An empty parentheses block before a lambda is redundant.

"some-string".count { it == '-' }

"some-string".count() { it == '-' }

Rule id: unnecessary-parentheses-before-trailing-lambda (standard rule set)

Wrapping

Argument list wrapping

All arguments should be on the same line, or every argument should be on a separate line.

val x = f(
    a,
    b,
    c
)

val x = f(
    a,
    b, c
)

Rule-id: argument-list-wrapping (standard rule set)

Chain wrapping

When wrapping chained calls ., ?. and ?: should be placed on the next line

val foo = listOf(1, 2, 3)
    .filter { it > 2 }!!
    .takeIf { it.count() > 100 }
    ?.sum()
val foobar = foo()
    ?: bar

val foo = listOf(1, 2, 3).
    filter { it > 2 }!!.
    takeIf { it.count() > 100 }?.
    sum()
val foobar = foo() ?:
    bar

Rule id: chain-wrapping (standard rule set)

Comment wrapping

A block comment should start and end on a line that does not contain any other element.

/* Some comment 1 */
val foo1 = "foo1"
val foo2 = "foo" // Some comment
val foo3 = { /* no-op */ } 

/* Some comment 1 */ val foo1 = "foo1"
val foo2 = "foo" /* Block comment instead of end-of-line comment */
val foo3 = "foo" /* Some comment
                  * with a newline
                  */

Rule id: comment-wrapping (standard rule set)

Content receiver wrapping

Wraps the content receiver list to a separate line regardless of maximum line length. If the maximum line length is configured and is exceeded, wrap the context receivers and if needed its projection types to separate lines.

// ALways wrap regardless of whether max line length is set
context(Foo)
fun fooBar()

// Wrap each context receiver to a separate line when the
// entire context receiver list does not fit on a single line
context(
    Fooooooooooooooooooo1,
    Foooooooooooooooooooooooooooooo2
)
fun fooBar()

// Wrap each context receiver to a separate line when the
// entire context receiver list does not fit on a single line.
// Also, wrap each of it projection types in case a context
// receiver does not fit on a single line after it has been
// wrapped.
context(
    Foooooooooooooooo<
        Foo,
        Bar
        >
)
fun fooBar()

// Should be wrapped regardless of whether max line length is set
context(Foo) fun fooBar()

// Should be wrapped when the entire context receiver list does not
// fit on a single line
context(Fooooooooooooooooooo1, Foooooooooooooooooooooooooooooo2)
fun fooBar()

// Should be wrapped when the entire context receiver list does not
// fit on a single line. Also, it should wrap each of it projection
// type in case a context receiver does not fit on a single line 
// after it has been wrapped.
context(Foooooooooooooooo<Foo, Bar>)
fun fooBar()

Rule id: context-receiver-wrapping (standard rule set)

Enum wrapping

An enum should be a single line, or each enum entry has to be placed on a separate line. In case the enumeration contains enum entries and declarations those are to be separated by a blank line.

enum class Foo { A, B, C, D }

enum class Foo {
    A,
    B,
    C,
    D,
    ;

    fun foo() = "foo"
}

enum class Foo {
    A,
    B, C,
    D
}

enum class Foo {
    A;
    fun foo() = "foo"
}

Rule id: enum-wrapping (standard rule set)

If else wrapping

A single line if-statement should be kept simple. It may contain no more than one else-branch. The branches may not be wrapped in a block.

fun foobar() {
    if (true) foo()
    if (true) foo() else bar()
}

fun foobar() {
    if (true) if (false) foo() else bar()
    if (true) bar() else if (false) foo() else bar()
    if (true) { foo() } else bar()
    if (true) bar() else { if (false) foo() else bar() }
}

Rule id: if-else-wrapping (standard rule set)

Note

This rule is only run when ktlint_code_style is set to ktlint_official or when the rule is enabled explicitly.

Multiline expression wrapping

Multiline expression on the right hand side of an expression are forced to start on a separate line. Expressions in return statement are excluded as that would result in a compilation error.

val foo =
    foo(
        parameterName =
            "The quick brown fox "
                .plus("jumps ")
                .plus("over the lazy dog"),
    )

val foo = foo(
    parameterName = "The quick brown fox "
        .plus("jumps ")
        .plus("over the lazy dog"),
)

Rule id: multiline-expression-wrapping (standard rule set)

Note

This rule is only run when ktlint_code_style is set to ktlint_official or when the rule is enabled explicitly.

Parameter list wrapping

When class/function signature doesn't fit on a single line, each parameter must be on a separate line

class ClassA(paramA: String, paramB: String, paramC: String)
class ClassA(
    paramA: String,
    paramB: String,
    paramC: String
)
fun f(a: Any, b: Any, c: Any)
fun f(
    a: Any,
    b: Any,
    c: Any
)
fun foo(
    @Bar fooBar: FooBar
)

class ClassA(
    paramA: String, paramB: String,
    paramC: String
)
fun f(
    a: Any,
    b: Any, c: Any
)
fun foo(@Bar fooBar: FooBar)

class ClassA(
    paramA: String, paramB: String,
    paramC: String
)
fun f(
    a: Any,
    b: Any, c: Any
)

Rule id: parameter-list-wrapping (standard rule set)

Parameter wrapping

When a function or class parameter doesn't fit on a single line, wrap the type or value to a separate line

// Assume that the last allowed character is
// at the X character on the right           X
class Bar(
    val fooooooooooooooooooooooooTooLong:
        Foo,
)
fun bar(
    fooooooooooooooooooooooooTooLong:
        Foo,
)

// Assume that the last allowed character is
// at the X character on the right           X
class Bar(
    val fooooooooooooooooooooooooTooLong:
    Foo,
)
fun bar(
    fooooooooooooooooooooooooTooLong:
    Foo,
)

// Assume that the last allowed character is
// at the X character on the right           X
class Bar(
    val fooooooooooooooooooooooooTooLong: Foo,
)
fun bar(
    fooooooooooooooooooooooooooooTooLong: Foo,
)

Rule id: parameter-wrapping (standard rule set)

Property wrapping

When a property doesn't fit on a single line, wrap the type or value to a separate line

// Assume that the last allowed character is
// at the X character on the right           X
val aVariableWithALooooooooooooongName:
    String

// Assume that the last allowed character is
// at the X character on the right           X
val aVariableWithALooooooooooooongName: String

Rule id: property-wrapping (standard rule set)

Statement wrapping

A function, class/object body or other block body statement has to be placed on different line than the braces of the body block.

fun foo() {
    if (true) {
        // do something
    }
}
class A {
    val a = 0
    val b = 1
}
enum class FooBar1 { FOO, BAR }
enum class FooBar2 {
    FOO,
    BAR,
}

fun foo() { if (true) {
        // do something
    }
}
class A { val a = 0
    val b = 1 }

Rule id: statement-wrapping

Wrapping

Inserts missing newlines (for example between parentheses of a multi-line function call).

val x = f(
    a,
    b,
    c
)

val x = f(
    a,
    b,
    c)

Rule id: wrapping (standard rule set)