Version: Next

Style Rule Set

The Style ruleset provides rules that assert the style of the code. This will help keep code in line with the given code style guidelines.

AbstractClassCanBeConcreteClass

This rule inspects abstract classes. Abstract classes which do not define any abstract members should instead be refactored into concrete classes.

Active by default: Yes - Since v1.2.0

Requires Type Resolution

Configuration options:

~~excludeAnnotatedClasses~~ (default: [])

Deprecated: Use ignoreAnnotated instead

Allows you to provide a list of annotations that disable this check.

Noncompliant Code:

abstract class OnlyConcreteMembersInAbstractClass { // violation: no abstract members

    val i: Int = 0
    fun f() { }
}

Compliant Code:

interface OnlyAbstractMembersInInterface {
    val i: Int
    fun f()
}

class OnlyConcreteMembersInClass {
    val i: Int = 0
    fun f() { }
}

AbstractClassCanBeInterface

This rule inspects abstract classes. In case an abstract class does not define any abstract members, it should instead be refactored into an interface.

Active by default: Yes - Since v1.23.0

Requires Type Resolution

Noncompliant Code:

abstract class OnlyAbstractMembersInAbstractClass { // violation: no concrete members

    abstract val i: Int
    abstract fun f()
}

Compliant Code:

interface Interface {
    val i: Int
    fun f()
}

abstract class NonAbstractMembersInAbstractClass {

    abstract val i: Int
    fun f() {
    }
}

AlsoCouldBeApply

Detects when an also block contains only it-started expressions.

By refactoring the also block to an apply block makes it so that all its can be removed thus making the block more concise and easier to read.

Active by default: No

Noncompliant Code:

Buzz().also {
it.init()
it.block()
}

Compliant Code:

Buzz().apply {
init()
block()
}

// Also compliant
fun foo(a: Int): Int {
return a.also { println(it) }
}

BracesOnIfStatements

This rule detects if statements which do not comply with the specified rules. Keeping braces consistent will improve readability and avoid possible errors.

The available options are:

always: forces braces on all if and else branches in the whole codebase.
consistent: ensures that braces are consistent within each if-else if-else chain. If there's a brace on one of the branches, all branches should have it.
necessary: forces no braces on any if and else branches in the whole codebase except where necessary for multi-statement branches.
never: forces no braces on any if and else branches in the whole codebase.

Single-line if-statement has no line break (\n):

if (a) b else c

Multi-line if-statement has at least one line break (\n):

if (a) b
else c

Active by default: No

Configuration options:

singleLine (default: 'never')

single-line braces policy
multiLine (default: 'always')

multi-line braces policy

Noncompliant Code:

// singleLine = 'never'
if (a) { b } else { c }

if (a) { b } else c

if (a) b else { c; d }

// multiLine = 'never'
if (a) {
    b
} else {
    c
}

// singleLine = 'always'
if (a) b else c

if (a) { b } else c

// multiLine = 'always'
if (a) {
    b
} else
    c

// singleLine = 'consistent'
if (a) b else { c }
if (a) b else if (c) d else { e }

// multiLine = 'consistent'
if (a)
    b
else {
    c
}

// singleLine = 'necessary'
if (a) { b } else { c; d }

// multiLine = 'necessary'
if (a) {
    b
    c
} else if (d) {
    e
} else {
    f
}

Compliant Code:

// singleLine = 'never'
if (a) b else c

// multiLine = 'never'
if (a)
    b
else
    c

// singleLine = 'always'
if (a) { b } else { c }

if (a) { b } else if (c) { d }

// multiLine = 'always'
if (a) {
    b
} else {
    c
}

if (a) {
    b
} else if (c) {
    d
}

// singleLine = 'consistent'
if (a) b else c

if (a) { b } else { c }

if (a) { b } else { c; d }

// multiLine = 'consistent'
if (a) {
    b
} else {
    c
}

if (a) b
else c

// singleLine = 'necessary'
if (a) b else { c; d }

// multiLine = 'necessary'
if (a) {
    b
    c
} else if (d)
    e
else
    f

BracesOnWhenStatements

This rule detects when statements which do not comply with the specified policy. Keeping braces consistent will improve readability and avoid possible errors.

Single-line when statement is: a when where each of the branches are single-line (has no line breaks \n).

Multi-line when statement is: a when where at least one of the branches is multi-line (has a break line \n).

Available options are:

never: forces no braces on any branch. Tip: this is very strict, it will force a simple expression, like a single function call / expression. Extracting a function for "complex" logic is one way to adhere to this policy.
necessary: forces no braces on any branch except where necessary for multi-statement branches.
consistent: ensures that braces are consistent within when statement. If there are braces on one of the branches, all branches should have it.
always: forces braces on all branches.

Active by default: No

Configuration options:

singleLine (default: 'necessary')

single-line braces policy
multiLine (default: 'consistent')

multi-line braces policy

Noncompliant Code:

// singleLine = 'never'
when (a) {
    1 -> { f1() } // Not allowed.
    2 -> f2()
}
// multiLine = 'never'
when (a) {
    1 -> { // Not allowed.
        f1()
    }
    2 -> f2()
}
// singleLine = 'necessary'
when (a) {
    1 -> { f1() } // Unnecessary braces.
    2 -> f2()
}
// multiLine = 'necessary'
when (a) {
    1 -> { // Unnecessary braces.
        f1()
    }
    2 -> f2()
}

// singleLine = 'consistent'
when (a) {
    1 -> { f1() }
    2 -> f2()
}
// multiLine = 'consistent'
when (a) {
    1 ->
        f1() // Missing braces.
    2 -> {
        f2()
        f3()
    }
}

// singleLine = 'always'
when (a) {
    1 -> { f1() }
    2 -> f2() // Missing braces.
}
// multiLine = 'always'
when (a) {
    1 ->
        f1() // Missing braces.
    2 -> {
        f2()
        f3()
    }
}

Compliant Code:

// singleLine = 'never'
when (a) {
    1 -> f1()
    2 -> f2()
}
// multiLine = 'never'
when (a) {
    1 ->
        f1()
    2 -> f2()
}
// singleLine = 'necessary'
when (a) {
    1 -> f1()
    2 -> { f2(); f3() } // Necessary braces because of multiple statements.
}
// multiLine = 'necessary'
when (a) {
    1 ->
        f1()
    2 -> { // Necessary braces because of multiple statements.
        f2()
        f3()
    }
}

// singleLine = 'consistent'
when (a) {
    1 -> { f1() }
    2 -> { f2() }
}
when (a) {
    1 -> f1()
    2 -> f2()
}
// multiLine = 'consistent'
when (a) {
    1 -> {
        f1()
    }
    2 -> {
        f2()
        f3()
    }
}

// singleLine = 'always'
when (a) {
    1 -> { f1() }
    2 -> { f2() }
}
// multiLine = 'always'
when (a) {
    1 -> {
        f1()
    }
    2 -> {
        f2()
        f3()
    }
}

CanBeNonNullable

This rule inspects variables marked as nullable and reports which could be declared as non-nullable instead.

It's preferred to not have functions that do "nothing". A function that does nothing when the value is null hides the logic, so it should not allow null values in the first place. It is better to move the null checks up around the calls, instead of having it inside the function.

This could lead to less nullability overall in the codebase.

Active by default: No

Requires Type Resolution

Noncompliant Code:

class A {
    var a: Int? = 5

    fun foo() {
        a = 6
    }
}

class A {
    val a: Int?
        get() = 5
}

fun foo(a: Int?) {
    val b = a!! + 2
}

fun foo(a: Int?) {
    if (a != null) {
        println(a)
    }
}

fun foo(a: Int?) {
    if (a == null) return
    println(a)
}

Compliant Code:

class A {
    var a: Int = 5

    fun foo() {
        a = 6
    }
}

class A {
    val a: Int
        get() = 5
}

fun foo(a: Int) {
    val b = a + 2
}

fun foo(a: Int) {
    println(a)
}

CascadingCallWrapping

Requires that all chained calls are placed on a new line if a preceding one is.

Active by default: No

Configuration options:

includeElvis (default: true)

require trailing elvis expressions to be wrapped on a new line

Noncompliant Code:

foo()
.bar().baz()

Compliant Code:

foo().bar().baz()

foo()
.bar()
.baz()

ClassOrdering

This rule ensures class contents are ordered as follows as recommended by the Kotlin Coding Conventions:

Property declarations and initializer blocks
Secondary constructors
Method declarations
Companion object

Active by default: No

Noncompliant Code:

class OutOfOrder {
    companion object {
        const val IMPORTANT_VALUE = 3
    }

    fun returnX(): Int {
        return x
    }

    private val x = 2
}

Compliant Code:

class InOrder {
    private val x = 2

    fun returnX(): Int {
        return x
    }

    companion object {
        const val IMPORTANT_VALUE = 3
    }
}

CollapsibleIfStatements

This rule detects if statements which can be collapsed. This can reduce nesting and help improve readability.

However, carefully consider whether merging the if statements actually improves readability, as collapsing the statements may hide some edge cases from the reader.

Active by default: No

Noncompliant Code:

val i = 1
if (i > 0) {
    if (i < 5) {
        println(i)
    }
}

Compliant Code:

val i = 1
if (i > 0 && i < 5) {
    println(i)
}

DataClassContainsFunctions

This rule reports functions inside data classes which have not been marked as a conversion function.

Data classes should mainly be used to store data. This rule assumes that they should not contain any extra functions aside functions that help with converting objects from/to one another. Data classes will automatically have a generated equals, toString and hashCode function by the compiler.

Active by default: No

Configuration options:

conversionFunctionPrefix (default: ['to'])

allowed conversion function names
allowOperators (default: false)

allows overloading an operator

Noncompliant Code:

data class DataClassWithFunctions(val i: Int) {
    fun foo() { }
}

DataClassShouldBeImmutable

This rule reports mutable properties inside data classes.

Data classes should mainly be used to store immutable data. This rule assumes that they should not contain any mutable properties.

Active by default: No

Noncompliant Code:

data class MutableDataClass(var i: Int) {
    var s: String? = null
}

Compliant Code:

data class ImmutableDataClass(
    val i: Int,
    val s: String?
)

DestructuringDeclarationWithTooManyEntries

Destructuring declarations with too many entries are hard to read and understand. To increase readability they should be refactored to reduce the number of entries or avoid using a destructuring declaration.

Active by default: Yes - Since v1.21.0

Configuration options:

maxDestructuringEntries (default: 3)

maximum allowed elements in a destructuring declaration

Noncompliant Code:

data class TooManyElements(val a: Int, val b: Int, val c: Int, val d: Int)
val (a, b, c, d) = TooManyElements(1, 2, 3, 4)

Compliant Code:

data class FewerElements(val a: Int, val b: Int, val c: Int)
val (a, b, c) = TooManyElements(1, 2, 3)

DoubleNegativeExpression

Detects expressions with two or more calls of operator not could be simplified.

Active by default: No

Requires Type Resolution

Noncompliant Code:

isValid.not().not()
!isValid.not()
!!isValid

Compliant Code:

isValid

DoubleNegativeLambda

Detects negation in lambda blocks where the function name is also in the negative (like takeUnless). A double negative is harder to read than a positive. In particular, if there are multiple conditions with && etc. inside the lambda, then the reader may need to unpack these using DeMorgan's laws. Consider rewriting the lambda to use a positive version of the function (like takeIf).

Active by default: No

Configuration options:

negativeFunctions (default: ['takeUnless', 'none'])

Function names expressed in the negative that can form double negatives with their lambda blocks. These are grouped together with a recommendation to use a positive counterpart, or null if this is unknown.
negativeFunctionNameParts (default: ['not', 'non'])

Function name parts to look for in the lambda block when deciding if the lambda contains a negative.

Noncompliant Code:

fun Int.evenOrNull() = takeUnless { it % 2 != 0 }

Compliant Code:

fun Int.evenOrNull() = takeIf { it % 2 == 0 }

EqualsNullCall

To compare an object with null prefer using ==. This rule detects and reports instances in the code where the equals() method is used to compare a value with null.

Active by default: Yes - Since v1.2.0

Noncompliant Code:

fun isNull(str: String) = str.equals(null)

Compliant Code:

fun isNull(str: String) = str == null

EqualsOnSignatureLine

Requires that the equals sign, when used for an expression style function, is on the same line as the rest of the function signature.

Active by default: No

Noncompliant Code:

fun stuff(): Int
    = 5

fun <V> foo(): Int where V : Int
    = 5

Compliant Code:

fun stuff() = 5

fun stuff() =
    foo.bar()

fun <V> foo(): Int where V : Int = 5

ExplicitCollectionElementAccessMethod

In Kotlin functions get or set can be replaced with the shorter operator — [], see Indexed access operator. Prefer the usage of the indexed access operator [] for map or list element access or insert methods.

Active by default: No

Requires Type Resolution

Noncompliant Code:

val map = mutableMapOf<String, String>()
map.put("key", "value")
val value = map.get("key")

Compliant Code:

val map = mutableMapOf<String, String>()
map["key"] = "value"
val value = map["key"]

ExplicitItLambdaParameter

Lambda expressions are one of the core features of the language. They often include very small chunks of code using only one parameter. In this cases Kotlin can supply the implicit it parameter to make code more concise. It fits most use cases, but when faced larger or nested chunks of code, you might want to add an explicit name for the parameter. Naming it just it is meaningless and only makes your code misleading, especially when dealing with nested functions.

Active by default: Yes - Since v1.21.0

Noncompliant Code:

a?.let { it -> it.plus(1) }
foo.flatMapObservable { it -> Observable.fromIterable(it) }
listOfPairs.map(::second).forEach { it ->
    it.execute()
}
collection.zipWithNext { it, next -> Pair(it, next) }

Compliant Code:

a?.let { it.plus(1) } // Much better to use implicit it
a?.let { value: Int -> value.plus(1) } // Better as states the type more clearly
foo.flatMapObservable(Observable::fromIterable) // Here we can have a method reference

// For multiline blocks it is usually better come up with a clear and more meaningful name
listOfPairs.map(::second).forEach { apiRequest ->
    apiRequest.execute()
}

// Lambdas with multiple parameter should be named clearly, using it for one of them can be confusing
collection.zipWithNext { prev, next ->
    Pair(prev, next)
}

ExpressionBodySyntax

Functions which only contain a return statement can be collapsed to an expression body. This shortens and cleans up the code.

Active by default: No

Configuration options:

includeLineWrapping (default: false)

include return statements with line wraps in it

Noncompliant Code:

fun stuff(): Int {
    return 5
}

Compliant Code:

fun stuff() = 5

fun stuff() {
    return
        moreStuff()
            .getStuff()
            .stuffStuff()
}

ForbiddenAnnotation

This rule allows to set a list of forbidden annotations. This can be used to discourage the use of language annotations which do not require explicit import.

Active by default: No

Requires Type Resolution

Configuration options:

annotations (default: ['java.lang.SuppressWarnings', 'java.lang.Deprecated', 'java.lang.annotation.Documented', 'java.lang.annotation.Target', 'java.lang.annotation.Retention', 'java.lang.annotation.Repeatable', 'java.lang.annotation.Inherited'])

List of fully qualified annotation classes which are forbidden.

Noncompliant Code:

@SuppressWarnings("unused")
class SomeClass()

Compliant Code:

@Suppress("unused")
class SomeClass()

ForbiddenComment

This rule allows to set a list of comments which are forbidden in the codebase and should only be used during development. Offending code comments will then be reported.

The regular expressions in comments list will have the following behaviors while matching the comments:

Each comment will be handled individually.
- single line comments are always separate, consecutive lines are not merged.
- multi line comments are not split up, the regex will be applied to the whole comment.
- KDoc comments are not split up, the regex will be applied to the whole comment.
The following comment delimiters (and indentation before them) are removed before applying the regex: //, // , /*, /* , /**, * aligners, */, */
The regex is applied as a multiline regex, see Anchors for more info. To match the start and end of each line, use ^ and $. To match the start and end of the whole comment, use \A and \Z. To turn off multiline, use (?-m) at the start of your regex.
The regex is applied with dotall semantics, meaning . will match any character including newlines, this is to ensure that freeform line-wrapping doesn't mess with simple regexes. To turn off this behavior, use (?-s) at the start of your regex, or use [^\r\n]* instead of .*.
The regex will be searched using "contains" semantics not "matches", so partial comment matches will flag forbidden comments. In practice this means there's no need to start and end the regex with .*.

The rule can be configured to add extra comments to the list of forbidden comments, here are some examples:

ForbiddenComment:
  comments:
    # Repeat the default configuration if it's still needed.
    - reason: 'Forbidden FIXME todo marker in comment, please fix the problem.'
      value: 'FIXME:'
    - reason: 'Forbidden STOPSHIP todo marker in comment, please address the problem before shipping the code.'
      value: 'STOPSHIP:'
    - reason: 'Forbidden TODO todo marker in comment, please do the changes.'
      value: 'TODO:'
    # Add additional patterns to the list.

    - reason: 'Authors are not recorded in KDoc.'
      value: '@author'

    - reason: 'REVIEW markers are not allowed in production code, only use before PR is merged.'
      value: '^\s*(?i)REVIEW\b'
      # Non-compliant: // REVIEW this code before merging.
      # Compliant: // Preview will show up here.

    - reason: 'Use @androidx.annotation.VisibleForTesting(otherwise = VisibleForTesting.PRIVATE) instead.'
      value: '^private$'
      # Non-compliant: /*private*/fun f() { }

      - reason: 'KDoc tag should have a value.'
        value: '^\s*@(?!suppress|hide)\w+\s*$'
        # Non-compliant: /** ... @see */
    # Compliant: /** ... @throws IOException when there's a network problem */

    - reason: 'include an issue link at the beginning preceded by a space'
      value: 'BUG:(?! https://github\.com/company/repo/issues/\d+).*'

By default the commonly used todo markers are forbidden: TODO:, FIXME: and STOPSHIP:.

Active by default: Yes - Since v1.0.0

Configuration options:

~~values~~ (default: [])

Deprecated: Use comments instead, make sure you escape your text for Regular Expressions.

forbidden comment strings
comments (default: ['FIXME:', 'STOPSHIP:', 'TODO:'])

forbidden comment string patterns
allowedPatterns (default: '')

ignores comments which match the specified regular expression. For example Ticket|Task.
~~customMessage~~ (default: '')

Deprecated: Use comments and provide reason against each value.

error message which overrides the default one

Noncompliant Code:

val a = "" // TODO: remove please
/**
* FIXME: this is a hack
*/
fun foo() { }
/* STOPSHIP: */

ForbiddenImport

Reports all imports that are forbidden.

This rule allows to set a list of forbidden [imports]. This can be used to discourage the use of unstable, experimental or deprecated APIs.

Active by default: No

Configuration options:

imports (default: [])

imports which should not be used
forbiddenPatterns (default: '')

reports imports which match the specified regular expression. For example net.*R.

Noncompliant Code:

import kotlin.jvm.JvmField
import kotlin.SinceKotlin

ForbiddenMethodCall

Reports all method or constructor invocations that are forbidden.

This rule allows to set a list of forbidden [methods] or constructors. This can be used to discourage the use of unstable, experimental or deprecated methods, especially for methods imported from external libraries.

Active by default: No

Requires Type Resolution

Configuration options:

methods (default: ['kotlin.io.print', 'kotlin.io.println', 'java.math.BigDecimal.<init>(kotlin.Double)'])

List of fully qualified method signatures which are forbidden. Methods can be defined without full signature (i.e. java.time.LocalDate.now) which will report calls of all methods with this name or with full signature (i.e. java.time.LocalDate(java.time.Clock)) which would report only call with this concrete signature. If you want to forbid an extension function like fun String.hello(a: Int) you should add the receiver parameter as the first parameter like this: hello(kotlin.String, kotlin.Int). To forbid constructor calls you need to define them with <init>, for example java.util.Date.<init>. To forbid calls involving type parameters, omit them, for example fun hello(args: Array<Any>) is referred to as simply hello(kotlin.Array) (also the signature for vararg parameters). To forbid methods from the companion object reference the Companion class, for example as TestClass.Companion.hello() (even if it is marked @JvmStatic).

Noncompliant Code:

fun main() {
println()
val myPrintln : () -> Unit = ::println
kotlin.io.print("Hello, World!")
}

ForbiddenSuppress

Report suppressions of all forbidden rules.

This rule allows to set a list of [rules] whose suppression is forbidden. This can be used to discourage the abuse of the Suppress and SuppressWarnings annotations.

This rule is not capable of reporting suppression of itself, as that's a language feature with precedence.

Active by default: No

Configuration options:

rules (default: [])

Rules whose suppression is forbidden.

Noncompliant Code:

package foo

// When the rule "MaximumLineLength" is forbidden
@Suppress("MaximumLineLength", "UNCHECKED_CAST")
class Bar

Compliant Code:

package foo

// When the rule "MaximumLineLength" is forbidden
@Suppress("UNCHECKED_CAST")
class Bar

ForbiddenVoid

This rule detects usages of Void and reports them as forbidden. The Kotlin type Unit should be used instead. This type corresponds to the Void class in Java and has only one value - the Unit object.

Active by default: Yes - Since v1.21.0

Requires Type Resolution

Configuration options:

ignoreOverridden (default: false)

ignores void types in signatures of overridden functions
ignoreUsageInGenerics (default: false)

ignore void types as generic arguments

Noncompliant Code:

runnable: () -> Void
var aVoid: Void? = null

Compliant Code:

runnable: () -> Unit
Void::class

FunctionOnlyReturningConstant

A function that only returns a single constant can be misleading. Instead, prefer declaring the constant as a const val.

Active by default: Yes - Since v1.2.0

Configuration options:

ignoreOverridableFunction (default: true)

if overridden functions should be ignored
ignoreActualFunction (default: true)

if actual functions should be ignored
excludedFunctions (default: [])

excluded functions

Noncompliant Code:

fun functionReturningConstantString() = "1"

Compliant Code:

const val constantString = "1"

LoopWithTooManyJumpStatements

Loops which contain multiple break or continue statements are hard to read and understand. To increase readability they should be refactored into simpler loops.

Active by default: Yes - Since v1.2.0

Configuration options:

maxJumpCount (default: 1)

maximum allowed jumps in a loop

Noncompliant Code:

val strs = listOf("foo, bar")
for (str in strs) {
    if (str == "bar") {
        break
    } else {
        continue
    }
}

MagicNumber

This rule detects and reports usages of magic numbers in the code. Prefer defining constants with clear names describing what the magic number means.

Active by default: Yes - Since v1.0.0

Configuration options:

ignoreNumbers (default: ['-1', '0', '1', '2'])

numbers which do not count as magic numbers
ignoreHashCodeFunction (default: true)

whether magic numbers in hashCode functions should be ignored
ignorePropertyDeclaration (default: false)

whether magic numbers in property declarations should be ignored
ignoreLocalVariableDeclaration (default: false)

whether magic numbers in local variable declarations should be ignored
ignoreConstantDeclaration (default: true)

whether magic numbers in constant declarations should be ignored
ignoreCompanionObjectPropertyDeclaration (default: true)

whether magic numbers in companion object declarations should be ignored
ignoreAnnotation (default: false)

whether magic numbers in annotations should be ignored
ignoreNamedArgument (default: true)

whether magic numbers in named arguments should be ignored
ignoreEnums (default: false)

whether magic numbers in enums should be ignored
ignoreRanges (default: false)

whether magic numbers in ranges should be ignored
ignoreExtensionFunctions (default: true)

whether magic numbers as subject of an extension function should be ignored

Noncompliant Code:

class User {

    fun checkName(name: String) {
        if (name.length > 42) {
            throw IllegalArgumentException("username is too long")
        }
        // ...
    }
}

Compliant Code:

class User {

    fun checkName(name: String) {
        if (name.length > MAX_USERNAME_SIZE) {
            throw IllegalArgumentException("username is too long")
        }
        // ...
    }

    companion object {
        private const val MAX_USERNAME_SIZE = 42
    }
}

MandatoryBracesLoops

This rule detects multi-line for and while loops which do not have braces. Adding braces would improve readability and avoid possible errors.

Active by default: No

Noncompliant Code:

for (i in 0..10)
    println(i)

while (true)
    println("Hello, world")

do
    println("Hello, world")
while (true)

Compliant Code:

for (i in 0..10) {
    println(i)
}

for (i in 0..10) println(i)

while (true) {
    println("Hello, world")
}

while (true) println("Hello, world")

do {
    println("Hello, world")
} while (true)

do println("Hello, world") while (true)

MaxChainedCallsOnSameLine

Limits the number of chained calls which can be placed on a single line.

Active by default: No

Requires Type Resolution

Configuration options:

maxChainedCalls (default: 5)

maximum chained calls allowed on a single line

Noncompliant Code:

a().b().c().d().e().f()

Compliant Code:

a().b().c()
.d().e().f()

MaxLineLength

This rule reports lines of code which exceed a defined maximum line length.

Long lines might be hard to read on smaller screens or printouts. Additionally, having a maximum line length in the codebase will help make the code more uniform.

Active by default: Yes - Since v1.0.0

Configuration options:

maxLineLength (default: 120)

maximum line length
excludePackageStatements (default: true)

if package statements should be ignored
excludeImportStatements (default: true)

if import statements should be ignored
excludeCommentStatements (default: false)

if comment statements should be ignored
excludeRawStrings (default: true)

if raw strings should be ignored

MayBeConstant

This rule identifies and reports properties (val) that may be const val instead. Using const val can lead to better performance of the resulting bytecode as well as better interoperability with Java.

Active by default: Yes - Since v1.2.0

Noncompliant Code:

val myConstant = "abc"

Compliant Code:

const val MY_CONSTANT = "abc"

ModifierOrder

This rule reports cases in the code where modifiers are not in the correct order. The default modifier order is taken from: Modifiers order

Active by default: Yes - Since v1.0.0

Noncompliant Code:

lateinit internal val str: String

Compliant Code:

internal lateinit val str: String

MultilineLambdaItParameter

Lambda expressions are very useful in a lot of cases, and they often include very small chunks of code using only one parameter. In this cases Kotlin can supply the implicit it parameter to make code more concise. However, when you are dealing with lambdas that contain multiple statements, you might end up with code that is hard to read if you don't specify a readable, descriptive parameter name explicitly.

Active by default: No

Requires Type Resolution

Noncompliant Code:

val digits = 1234.let {
    println(it)
    listOf(it)
}

val digits = 1234.let { it ->
    println(it)
    listOf(it)
}

val flat = listOf(listOf(1), listOf(2)).mapIndexed { index, it ->
    println(it)
    it + index
}

Compliant Code:

val digits = 1234.let { explicitParameterName ->
    println(explicitParameterName)
    listOf(explicitParameterName)
}

val lambda = { item: Int, that: String ->
    println(item)
    item.toString() + that
}

val digits = 1234.let { listOf(it) }
val digits = 1234.let {
    listOf(it)
}
val digits = 1234.let { it -> listOf(it) }
val digits = 1234.let { it ->
    listOf(it)
}
val digits = 1234.let { explicit -> listOf(explicit) }
val digits = 1234.let { explicit ->
    listOf(explicit)
}

MultilineRawStringIndentation

This rule ensures that raw strings have a consistent indentation.

The content of a multi line raw string should have the same indentation as the enclosing expression plus the configured indentSize. The closing triple-quotes (""") must have the same indentation as the enclosing expression.

Warning: Rule MultilineRawStringIndentation overlaps with StringTemplateIndent from the formatting rule set.

Active by default: No

Configuration options:

indentSize (default: 4)

indentation size
trimmingMethods (default: ['trimIndent', 'trimMargin'])

allows to provide a list of multiline string trimming methods

Noncompliant Code:

val a = """
Hello World!
How are you?
""".trimMargin()

val a = """
    Hello World!
    How are you?
    """.trimMargin()

Compliant Code:

val a = """
    Hello World!
    How are you?
""".trimMargin()

val a = """
    Hello World!
      How are you?
""".trimMargin()

NestedClassesVisibility

Nested classes inherit their visibility from the parent class and are often used to implement functionality local to the class it is nested in. These nested classes can't have a higher visibility than their parent. However, the visibility can be further restricted by using a private modifier for instance. In internal classes the explicit public modifier for nested classes is misleading and thus unnecessary, because the nested class still has an internal visibility.

Active by default: Yes - Since v1.16.0

Noncompliant Code:

internal class Outer {
    // explicit public modifier still results in an internal nested class
    public class Nested
}

Compliant Code:

internal class Outer {
    class Nested1
    internal class Nested2
}

NewLineAtEndOfFile

This rule reports files which do not end with a line separator.

Active by default: Yes - Since v1.0.0

NoTabs

This rule reports if tabs are used in Kotlin files. According to Google's Kotlin style guide the only whitespace chars that are allowed in a source file are the line terminator sequence and the ASCII horizontal space character (0x20). Strings containing tabs are allowed.

Active by default: No

NullableBooleanCheck

Detects nullable boolean checks which use an elvis expression ?: rather than equals ==.

Per the Kotlin coding conventions converting a nullable boolean property to non-null should be done via != false or == true rather than ?: true or ?: false (respectively).

Active by default: No

Requires Type Resolution

Noncompliant Code:

value ?: true
value ?: false

Compliant Code:

value != false
value == true

ObjectLiteralToLambda

An anonymous object that does nothing other than the implementation of a single method can be used as a lambda.

See SAM conversions, Functional (SAM) interfaces

Active by default: Yes - Since v1.21.0

Requires Type Resolution

Noncompliant Code:

object : Foo {
    override fun bar() {
    }
}

Compliant Code:

Foo {
}

OptionalAbstractKeyword

This rule reports abstract modifiers which are unnecessary and can be removed.

Active by default: Yes - Since v1.0.0

Noncompliant Code:

abstract interface Foo { // abstract keyword not needed

    abstract fun x() // abstract keyword not needed
    abstract var y: Int // abstract keyword not needed
}

Compliant Code:

interface Foo {

    fun x()
    var y: Int
}

OptionalUnit

It is not necessary to define a return type of Unit on functions or to specify a lone Unit statement. This rule detects and reports instances where the Unit return type is specified on functions and the occurrences of a lone Unit statement.

Active by default: No

Requires Type Resolution

Noncompliant Code:

fun foo(): Unit {
    return Unit
}
fun foo() = Unit

fun doesNothing() {
    Unit
}

Compliant Code:

fun foo() { }

// overridden no-op functions are allowed
override fun foo() = Unit

PreferToOverPairSyntax

This rule detects the usage of the Pair constructor to create pairs of values.

Using <value1> to <value2> is preferred.

Active by default: No

Requires Type Resolution

Noncompliant Code:

val pair = Pair(1, 2)

Compliant Code:

val pair = 1 to 2

ProtectedMemberInFinalClass

Kotlin classes are final by default. Thus classes which are not marked as open should not contain any protected members. Consider using private or internal modifiers instead.

Active by default: Yes - Since v1.2.0

Noncompliant Code:

class ProtectedMemberInFinalClass {
    protected var i = 0
}

Compliant Code:

class ProtectedMemberInFinalClass {
    private var i = 0
}

RangeUntilInsteadOfRangeTo

Reports calls to .. operator instead of calls to ..<. ..< is applicable in cases where the upper range value is described as open ended range(or in case of integral types some value subtracted by 1). ..< helps to prevent off-by-one errors.

Active by default: No

Noncompliant Code:

for (i in 0..10 - 1) {}
val range = 0..10 - 1

Compliant Code:

for (i in 0..<10) {}
val range = 0..<10

RedundantConstructorKeyword

This rule checks for redundant constructor keywords.

Active by default: No

Noncompliant Code:

data class Foo constructor(val foo: Int)

Compliant Code:

data class Foo(val foo: Int)

data class Bar private constructor(val bar: String) {
    constructor(bar: Int): this("$foo")
}

RedundantExplicitType

Local properties do not need their type to be explicitly provided when the inferred type matches the explicit type.

Active by default: No

Requires Type Resolution

Noncompliant Code:

fun function() {
    val x: String = "string"
}

Compliant Code:

fun function() {
    val x = "string"
}

RedundantHigherOrderMapUsage

Redundant maps add complexity to the code and accomplish nothing. They should be removed or replaced with the proper operator.

Active by default: Yes - Since v1.21.0

Requires Type Resolution

Noncompliant Code:

fun foo(list: List<Int>): List<Int> {
    return list
        .filter { it > 5 }
        .map { it }
}

fun bar(list: List<Int>): List<Int> {
    return list
        .filter { it > 5 }
        .map {
            doSomething(it)
            it
        }
}

fun baz(set: Set<Int>): List<Int> {
    return set.map { it }
}

Compliant Code:

fun foo(list: List<Int>): List<Int> {
    return list
        .filter { it > 5 }
}

fun bar(list: List<Int>): List<Int> {
    return list
        .filter { it > 5 }
        .onEach {
            doSomething(it)
        }
}

fun baz(set: Set<Int>): List<Int> {
    return set.toList()
}

RedundantVisibilityModifier

This rule checks for redundant visibility modifiers. One exemption is the explicit API mode In this mode, the visibility modifier should be defined explicitly even if it is public. Hence, the rule ignores the visibility modifiers in explicit API mode.

Active by default: No

Noncompliant Code:

public interface Foo { // public per default

    public fun bar() // public per default
}

Compliant Code:

interface Foo {

    fun bar()
}

ReturnCount

Restrict the number of returns allowed in methods.

Having many exit points in a function can be confusing and impacts readability of the code.

Active by default: Yes - Since v1.0.0

Configuration options:

max (default: 2)

define the maximum number of return statements allowed per function
excludedFunctions (default: ['equals'])

define a list of function names to be ignored by this check
excludeLabeled (default: false)

if labeled return statements should be ignored
excludeReturnFromLambda (default: true)

if labeled return from a lambda should be ignored
excludeGuardClauses (default: false)

if true guard clauses at the beginning of a method should be ignored

Noncompliant Code:

fun foo(i: Int): String {
    when (i) {
        1 -> return "one"
        2 -> return "two"
        else -> return "other"
    }
}

Compliant Code:

fun foo(i: Int): String {
    return when (i) {
        1 -> "one"
        2 -> "two"
        else -> "other"
    }
}

SafeCast

This rule inspects casts and reports casts which could be replaced with safe casts instead.

Active by default: Yes - Since v1.0.0

Noncompliant Code:

fun numberMagic(number: Number) {
    val i = if (number is Int) number else null
    // ...
}

Compliant Code:

fun numberMagic(number: Number) {
    val i = number as? Int
    // ...
}

SerialVersionUIDInSerializableClass

Classes which implement the Serializable interface should also correctly declare a serialVersionUID. This rule verifies that a serialVersionUID was correctly defined and declared as private.

More about SerialVersionUID

Active by default: Yes - Since v1.16.0

Noncompliant Code:

class IncorrectSerializable : Serializable {

    companion object {
        val serialVersionUID = 1 // wrong declaration for UID
    }
}

Compliant Code:

class CorrectSerializable : Serializable {

    companion object {
        private const val serialVersionUID = 1L
    }
}

SpacingAfterPackageDeclaration

This rule verifies spacing between package and import statements as well as between import statements and class declarations.

Active by default: No

Noncompliant Code:

package foo
import a.b
class Bar { }

Compliant Code:

package foo

import a.b

class Bar { }

StringShouldBeRawString

This rule reports when the string can be converted to Kotlin raw string. Usage of a raw string is preferred as that avoids the need for escaping strings escape characters like \n, \t, ". Raw string also allows us to represent multiline string without the need of \n. Also, see Kotlin coding convention for recommendation on using multiline strings

Active by default: No

Configuration options:

maxEscapedCharacterCount (default: 2)

maximum escape characters allowed
ignoredCharacters (default: [])

list of characters to ignore

Noncompliant Code:

val windowJson = "{\n" +
    "  \"window\": {\n" +
    "    \"title\": \"Sample Quantum With AI and ML Widget\",\n" +
    "    \"name\": \"main_window\",\n" +
    "    \"width\": 500,\n" +
    "    \"height\": 500\n" +
    "  }\n" +
    "}"

val patRegex = "/^(\\/[^\\/]+){0,2}\\/?\$/gm\n"

Compliant Code:

val windowJson = """
    {
         "window": {
             "title": "Sample Quantum With AI and ML Widget",
             "name": "main_window",
             "width": 500,
             "height": 500
         }
    }
""".trimIndent()

val patRegex = """/^(\/[^\/]+){0,2}\/?$/gm"""

ThrowsCount

Functions should have clear throw statements. Functions with many throw statements can be harder to read and lead to confusion. Instead, prefer limiting the number of throw statements in a function.

Active by default: Yes - Since v1.0.0

Configuration options:

max (default: 2)

maximum amount of throw statements in a method
excludeGuardClauses (default: false)

if set to true, guard clauses do not count towards the allowed throws count

Noncompliant Code:

fun foo(i: Int) {
    when (i) {
        1 -> throw IllegalArgumentException()
        2 -> throw IllegalArgumentException()
        3 -> throw IllegalArgumentException()
    }
}

Compliant Code:

fun foo(i: Int) {
    when (i) {
        1,2,3 -> throw IllegalArgumentException()
    }
}

TrailingWhitespace

This rule reports lines that end with a whitespace.

Note: in KDoc comments we use Markdown, so two spaces at the end of lines should be allowed. However, JetBrains haven't implemented this in their flavour of "standard" Markdown yet (in Dokka, nor in KTIJ), which means Markdown line-breaks in KDoc are really only trailing whitespace for now.

Active by default: No

TrimMultilineRawString

All the Raw strings that have more than one line should be followed by trimMargin() or trimIndent().

Active by default: No

Configuration options:

trimmingMethods (default: ['trimIndent', 'trimMargin'])

allows to provide a list of multiline string trimming methods

Noncompliant Code:

"""
Hello World!
How are you?
"""

Compliant Code:

"""
|  Hello World!
|  How are you?
""".trimMargin()

"""
Hello World!
How are you?
""".trimIndent()

"""Hello World! How are you?"""

UnderscoresInNumericLiterals

This rule detects and reports long base 10 numbers which should be separated with underscores for readability. For Serializable classes or objects, the field serialVersionUID is explicitly ignored. For floats and doubles, anything to the right of the decimal point is ignored.

Active by default: No

Configuration options:

~~acceptableDecimalLength~~ (default: 5)

Deprecated: Use acceptableLength instead

Length under which base 10 numbers are not required to have underscores
acceptableLength (default: 4)

Maximum number of consecutive digits that a numeric literal can have without using an underscore
allowNonStandardGrouping (default: false)

If set to false, groups of exactly three digits must be used. If set to true, 100_00 is allowed.

Noncompliant Code:

const val DEFAULT_AMOUNT = 1000000

Compliant Code:

const val DEFAULT_AMOUNT = 1_000_000

UnnecessaryAnnotationUseSiteTarget

This rule inspects the use of the Annotation use-site Target. In case that the use-site Target is not needed it can be removed. For more information check the kotlin documentation: Annotation use-site targets

Active by default: No

Noncompliant Code:

@property:Inject private val foo: String = "bar" // violation: unnecessary @property:

class Module(@param:Inject private val foo: String) // violation: unnecessary @param:

Compliant Code:

class Module(@Inject private val foo: String)

UnnecessaryAny

Turn on this rule to flag usage of any which can either be replaced with simple contains call or can removed entirely to reduce visual complexity.

Active by default: No

Requires Type Resolution

Noncompliant Code:

val a = 1
list.any { it == a }

Compliant Code:

val a = 1
list.contains(a)

UnnecessaryApply

apply expressions are used frequently, but sometimes their usage should be replaced with an ordinary method/extension function call to reduce visual complexity

Active by default: Yes - Since v1.16.0

Requires Type Resolution

Noncompliant Code:

config.apply { version = "1.2" } // can be replaced with `config.version = "1.2"`
config?.apply { environment = "test" } // can be replaced with `config?.environment = "test"`
config?.apply { println(version) } // `apply` can be replaced by `let`

Compliant Code:

config.apply {
    version = "1.2"
    environment = "test"
}

UnnecessaryBackticks

This rule reports unnecessary backticks.

Active by default: No

Noncompliant Code:

class `HelloWorld`

Compliant Code:

class HelloWorld

UnnecessaryBracesAroundTrailingLambda

In Kotlin functions the last lambda parameter of a function is a function then a lambda expression passed as the corresponding argument can be placed outside the parentheses. see Passing trailing lambdas. Prefer the usage of trailing lambda.

Active by default: No

Requires Type Resolution

Noncompliant Code:

fun test() {
    repeat(10, {
        println(it)
    })
}

Compliant Code:

fun test() {
    repeat(10) {
        println(it)
    }
}

UnnecessaryFilter

Unnecessary filters add complexity to the code and accomplish nothing. They should be removed.

Active by default: Yes - Since v1.21.0

Requires Type Resolution

Noncompliant Code:

val x = listOf(1, 2, 3)
    .filter { it > 1 }
    .count()

val x = listOf(1, 2, 3)
    .filter { it > 1 }
    .isEmpty()

Compliant Code:

val x = listOf(1, 2, 3)
    .count { it > 2 }

val x = listOf(1, 2, 3)
    .none { it > 1 }

UnnecessaryInheritance

This rule reports unnecessary super types. Inheriting from Any or Object is unnecessary and should simply be removed.

Active by default: Yes - Since v1.2.0

Noncompliant Code:

class A : Any()
class B : Object()

UnnecessaryInnerClass

This rule reports unnecessary inner classes. Nested classes that do not access members from the outer class do not require the inner qualifier.

Active by default: No

Requires Type Resolution

Noncompliant Code:

class A {
    val foo = "BAR"

    inner class B {
        val fizz = "BUZZ"

        fun printFizz() {
            println(fizz)
        }
    }
}

UnnecessaryLet

let expressions are used extensively in our code for null-checking and chaining functions, but sometimes their usage should be replaced with an ordinary method/extension function call to reduce visual complexity.

Active by default: No

Requires Type Resolution

Noncompliant Code:

a.let { print(it) } // can be replaced with `print(a)`
a.let { it.plus(1) } // can be replaced with `a.plus(1)`
a?.let { it.plus(1) } // can be replaced with `a?.plus(1)`
a?.let { that -> that.plus(1) }?.let { it.plus(1) } // can be replaced with `a?.plus(1)?.plus(1)`
a.let { 1.plus(1) } // can be replaced with `1.plus(1)`
a?.let { 1.plus(1) } // can be replaced with `if (a != null) 1.plus(1)`

Compliant Code:

a?.let { print(it) }
a?.let { 1.plus(it) } ?.let { msg -> print(msg) }
a?.let { it.plus(it) }
val b = a?.let { 1.plus(1) }

UnnecessaryParentheses

This rule reports unnecessary parentheses around expressions. These unnecessary parentheses can safely be removed.

Added in v1.0.0.RC4

Active by default: No

Configuration options:

allowForUnclearPrecedence (default: false)

allow parentheses when not strictly required but precedence may be unclear, such as (a && b) || c

Noncompliant Code:

val local = (5 + 3)

if ((local == 8)) { }

fun foo() {
    function({ input -> println(input) })
}

Compliant Code:

val local = 5 + 3

if (local == 8) { }

fun foo() {
    function { input -> println(input) }
}

UnusedImport

This rule reports unused imports. Unused imports are dead code and should be removed. Exempt from this rule are imports resulting from references to elements within KDoc and from destructuring declarations (componentN imports).

Active by default: No

Requires Type Resolution

UnusedParameter

An unused parameter can be removed to simplify the signature of the function.

Active by default: Yes - Since v1.23.0

Aliases: UNUSED_VARIABLE, UNUSED_PARAMETER, unused, UnusedPrivateMember

Configuration options:

allowedNames (default: 'ignored|expected')

unused parameter names matching this regex are ignored

Noncompliant Code:

fun foo(unused: String) {
    println()
}

Compliant Code:

fun foo(used: String) {
    println(used)
}

UnusedPrivateClass

Reports unused private classes. If private classes are unused they should be removed. Otherwise, this dead code can lead to confusion and potential bugs.

Active by default: Yes - Since v1.2.0

Aliases: unused

UnusedPrivateMember

Reports unused private functions.

If these private functions are unused they should be removed. Otherwise, this dead code can lead to confusion and potential bugs.

Active by default: Yes - Since v1.16.0

Requires Type Resolution

Aliases: UNUSED_VARIABLE, UNUSED_PARAMETER, unused

Configuration options:

allowedNames (default: '')

unused private function names matching this regex are ignored

UnusedPrivateProperty

An unused private property can be removed to simplify the source file.

This rule also detects unused constructor parameters since these can become properties of the class when they are declared with val or var.

Active by default: Yes - Since v1.23.0

Requires Type Resolution

Aliases: UNUSED_PARAMETER, unused, UnusedPrivateMember

Configuration options:

allowedNames (default: 'ignored|expected|serialVersionUID')

unused property names matching this regex are ignored

Noncompliant Code:

class Foo {
    private val unused = "unused"
}

Compliant Code:

class Foo {
    private val used = "used"

    fun greet() {
        println(used)
    }
}

UnusedVariable

An unused variable can be removed to simplify the source file.

Active by default: Yes - Since v2.0.0

Requires Type Resolution

Aliases: UNUSED_VARIABLE, unused

Configuration options:

allowedNames (default: 'ignored|_')

unused variables names matching this regex are ignored

Noncompliant Code:

fun foo() {
    val unused = "unused"
}

Compliant Code:

fun foo() {
    val used = "used"
    println(used)
}

UseAnyOrNoneInsteadOfFind

Turn on this rule to flag find calls for null check that can be replaced with a any or none call.

Active by default: Yes - Since v1.21.0

Requires Type Resolution

Noncompliant Code:

listOf(1, 2, 3).find { it == 4 } != null
listOf(1, 2, 3).find { it == 4 } == null

Compliant Code:

listOf(1, 2, 3).any { it == 4 }
listOf(1, 2, 3).none { it == 4 }

UseArrayLiteralsInAnnotations

This rule detects annotations which use the arrayOf(...) syntax instead of the array literal [...] syntax. The latter should be preferred as it is more readable.

Active by default: Yes - Since v1.21.0

Noncompliant Code:

@PositiveCase(arrayOf("..."))

Compliant Code:

@NegativeCase(["..."])

UseCheckNotNull

Turn on this rule to flag check calls for not-null check that can be replaced with a checkNotNull call.

Active by default: Yes - Since v1.21.0

Requires Type Resolution

Noncompliant Code:

check(x != null)

Compliant Code:

checkNotNull(x)

UseCheckOrError

Kotlin provides a concise way to check invariants as well as pre- and post-conditions. Prefer them instead of manually throwing an IllegalStateException.

Active by default: Yes - Since v1.21.0

Requires Type Resolution

Noncompliant Code:

if (value == null) throw IllegalStateException("value should not be null")
if (value < 0) throw IllegalStateException("value is $value but should be at least 0")
when(a) {
    1 -> doSomething()
    else -> throw IllegalStateException("Unexpected value")
}

Compliant Code:

checkNotNull(value) { "value should not be null" }
check(value >= 0) { "value is $value but should be at least 0" }
when(a) {
    1 -> doSomething()
    else -> error("Unexpected value")
}

UseDataClass

Classes that simply hold data should be refactored into a data class. Data classes are specialized to hold data and generate hashCode, equals and toString implementations as well.

Configuration options:

~~excludeAnnotatedClasses~~ (default: [])

Deprecated: Use ignoreAnnotated instead

allows to provide a list of annotations that disable this check
allowVars (default: false)

allows to relax this rule in order to exclude classes that contains one (or more) vars

Noncompliant Code:

class DataClassCandidate(val i: Int) {
    val i2: Int = 0
}

Compliant Code:

data class DataClass(val i: Int, val i2: Int)

// classes with delegating interfaces are compliant
interface I
class B() : I
class A(val b: B) : I by b

UseEmptyCounterpart

Instantiation of an object's "empty" state should use the object's "empty" initializer for clarity purposes.

Active by default: No

Requires Type Resolution

Noncompliant Code:

arrayOf()
listOf() // or listOfNotNull()
mapOf()
sequenceOf()
setOf()

Compliant Code:

emptyArray()
emptyList()
emptyMap()
emptySequence()
emptySet()

UseIfEmptyOrIfBlank

This rule detects isEmpty or isBlank calls to assign a default value. They can be replaced with ifEmpty or ifBlank calls.

Active by default: No

Requires Type Resolution

Noncompliant Code:

fun test(list: List<Int>, s: String) {
    val a = if (list.isEmpty()) listOf(1) else list
    val b = if (list.isNotEmpty()) list else listOf(2)
    val c = if (s.isBlank()) "foo" else s
    val d = if (s.isNotBlank()) s else "bar"
}

Compliant Code:

fun test(list: List<Int>, s: String) {
    val a = list.ifEmpty { listOf(1) }
    val b = list.ifEmpty { listOf(2) }
    val c = s.ifBlank { "foo" }
    val d = s.ifBlank { "bar" }
}

UseIfInsteadOfWhen

Binary expressions are better expressed using an if expression than a when expression.

See if versus when

Active by default: No

Configuration options:

ignoreWhenContainingVariableDeclaration (default: false)

ignores when statements with a variable declaration used in the subject

Noncompliant Code:

when (x) {
    null -> true
    else -> false
}

Compliant Code:

if (x == null) true else false

UseIsNullOrEmpty

This rule detects null or empty checks that can be replaced with isNullOrEmpty() call.

Active by default: Yes - Since v1.21.0

Requires Type Resolution

Noncompliant Code:

fun foo(x: List<Int>?) {
    if (x == null || x.isEmpty()) return
}
fun bar(x: List<Int>?) {
    if (x == null || x.count() == 0) return
}
fun baz(x: List<Int>?) {
    if (x == null || x.size == 0) return
}

Compliant Code:

if (x.isNullOrEmpty()) return

UseLet

if expressions that either check for not-null and return null in the false case or check for null and returns null in the truthy case are better represented as ?.let {} blocks.

Active by default: No

Noncompliant Code:

if (x != null) { transform(x) } else null
if (x == null) null else y

Compliant Code:

x?.let { transform(it) }
x?.let { y }

UseOrEmpty

This rule detects ?: emptyList() that can be replaced with orEmpty() call.

Active by default: Yes - Since v1.21.0

Requires Type Resolution

Noncompliant Code:

fun test(x: List<Int>?, s: String?) {
    val a = x ?: emptyList()
    val b = s ?: ""
}

Compliant Code:

fun test(x: List<Int>?, s: String?) {
    val a = x.orEmpty()
    val b = s.orEmpty()
}

UseRequire

Kotlin provides a much more concise way to check preconditions than to manually throw an IllegalArgumentException.

Active by default: Yes - Since v1.21.0

Requires Type Resolution

Noncompliant Code:

if (value == null) throw IllegalArgumentException("value should not be null")
if (value < 0) throw IllegalArgumentException("value is $value but should be at least 0")

Compliant Code:

requireNotNull(value) { "value should not be null" }
require(value >= 0) { "value is $value but should be at least 0" }

UseRequireNotNull

Turn on this rule to flag require calls for not-null check that can be replaced with a requireNotNull call.

Active by default: Yes - Since v1.21.0

Requires Type Resolution

Noncompliant Code:

require(x != null)

Compliant Code:

requireNotNull(x)

UseSumOfInsteadOfFlatMapSize

Turn on this rule to flag flatMap and size/count calls that can be replaced with a sumOf call.

Active by default: No

Requires Type Resolution

Noncompliant Code:

class Foo(val foo: List<Int>)
list.flatMap { it.foo }.size
list.flatMap { it.foo }.count()
list.flatMap { it.foo }.count { it > 2 }
listOf(listOf(1), listOf(2, 3)).flatten().size

Compliant Code:

list.sumOf { it.foo.size }
list.sumOf { it.foo.count() }
list.sumOf { it.foo.count { foo -> foo > 2 } }
listOf(listOf(1), listOf(2, 3)).sumOf { it.size }

UselessCallOnNotNull

The Kotlin stdlib provides some functions that are designed to operate on references that may be null. These functions can also be called on non-nullable references or on collections or sequences that are known to be empty - the calls are redundant in this case and can be removed or should be changed to a call that does not check whether the value is null or not.

Active by default: Yes - Since v1.2.0

Requires Type Resolution

Noncompliant Code:

val testList = listOf("string").orEmpty()
val testList2 = listOf("string").orEmpty().map { _ }
val testList3 = listOfNotNull("string")
val testString = ""?.isNullOrBlank()

Compliant Code:

val testList = listOf("string")
val testList2 = listOf("string").map { }
val testList3 = listOf("string")
val testString = ""?.isBlank()

UtilityClassWithPublicConstructor

A class which only contains utility variables and functions with no concrete implementation can be refactored into an object or a class with a non-public constructor. Furthermore, this rule reports utility classes which are not final.

Active by default: Yes - Since v1.2.0

Noncompliant Code:

class UtilityClassViolation {

    // public constructor here
    constructor() {
        // ...
    }

    companion object {
        val i = 0
    }
}

open class UtilityClassViolation private constructor() {

    // ...
}

Compliant Code:

class UtilityClass {

    private constructor() {
        // ...
    }

    companion object {
        val i = 0
    }
}
object UtilityClass {

    val i = 0
}

VarCouldBeVal

Reports var declarations (both local variables and private class properties) that could be val, as they are not re-assigned. Val declarations are assign-once (read-only), which makes understanding the current state easier.

Active by default: Yes - Since v1.16.0

Requires Type Resolution

Aliases: CanBeVal

Configuration options:

ignoreLateinitVar (default: false)

Whether to ignore uninitialized lateinit vars

Noncompliant Code:

fun example() {
    var i = 1 // violation: this variable is never re-assigned
    val j = i + 1
}

Compliant Code:

fun example() {
    val i = 1
    val j = i + 1
}

WildcardImport

Wildcard imports should be replaced with imports using fully qualified class names. This helps increase clarity of which classes are imported and helps prevent naming conflicts.

Library updates can introduce naming clashes with your own classes which might result in compilation errors.

NOTE: This rule has a twin implementation NoWildcardImports in the formatting rule set (a wrapped KtLint rule). When suppressing an issue of WildcardImport in the baseline file, make sure to suppress the corresponding NoWildcardImports issue.

Active by default: Yes - Since v1.0.0

Configuration options:

excludeImports (default: ['java.util.*'])

Define a list of package names that should be allowed to be imported with wildcard imports.

Noncompliant Code:

import io.gitlab.arturbosch.detekt.*

class DetektElements {
    val element1 = DetektElement1()
    val element2 = DetektElement2()
}

Compliant Code:

import io.gitlab.arturbosch.detekt.DetektElement1
import io.gitlab.arturbosch.detekt.DetektElement2

class DetektElements {
    val element1 = DetektElement1()
    val element2 = DetektElement2()
}

AbstractClassCanBeConcreteClass​

Configuration options:​

Noncompliant Code:​

Compliant Code:​

AbstractClassCanBeInterface​

Noncompliant Code:​

Compliant Code:​

AlsoCouldBeApply​

Noncompliant Code:​

Compliant Code:​

BracesOnIfStatements​

Configuration options:​

Noncompliant Code:​

Compliant Code:​

BracesOnWhenStatements​

Configuration options:​

Noncompliant Code:​

Compliant Code:​

CanBeNonNullable​

Noncompliant Code:​

Compliant Code:​

CascadingCallWrapping​

Configuration options:​

Noncompliant Code:​

Compliant Code:​

ClassOrdering​

Noncompliant Code:​

Compliant Code:​

CollapsibleIfStatements​

Noncompliant Code:​

Compliant Code:​

DataClassContainsFunctions​

Configuration options:​

Noncompliant Code:​

DataClassShouldBeImmutable​

Noncompliant Code:​

Compliant Code:​

DestructuringDeclarationWithTooManyEntries​

Configuration options:​

Noncompliant Code:​

Compliant Code:​

DoubleNegativeExpression​

Noncompliant Code:​

Compliant Code:​

DoubleNegativeLambda​

Configuration options:​

Noncompliant Code:​

Compliant Code:​

EqualsNullCall​

Noncompliant Code:​

Compliant Code:​

EqualsOnSignatureLine​

Noncompliant Code:​

Compliant Code:​

ExplicitCollectionElementAccessMethod​

Noncompliant Code:​

Compliant Code:​

ExplicitItLambdaParameter​

Noncompliant Code:​

Compliant Code:​

ExpressionBodySyntax​

Configuration options:​

Noncompliant Code:​

Compliant Code:​

ForbiddenAnnotation​

Configuration options:​

Noncompliant Code:​

Compliant Code:​

ForbiddenComment​

Configuration options:​

Noncompliant Code:​

ForbiddenImport​

Configuration options:​

Noncompliant Code:​

ForbiddenMethodCall​

Configuration options:​

Noncompliant Code:​

ForbiddenSuppress​

Configuration options:​

Noncompliant Code:​

AbstractClassCanBeConcreteClass

Configuration options:

Noncompliant Code:

Compliant Code:

AbstractClassCanBeInterface

Noncompliant Code:

Compliant Code:

AlsoCouldBeApply

Noncompliant Code:

Compliant Code:

BracesOnIfStatements

Configuration options:

Noncompliant Code:

Compliant Code:

BracesOnWhenStatements

Configuration options:

Noncompliant Code:

Compliant Code:

CanBeNonNullable

Noncompliant Code:

Compliant Code:

CascadingCallWrapping

Configuration options:

Noncompliant Code:

Compliant Code:

ClassOrdering

Noncompliant Code:

Compliant Code:

CollapsibleIfStatements

Noncompliant Code:

Compliant Code:

DataClassContainsFunctions

Configuration options:

Noncompliant Code:

DataClassShouldBeImmutable

Noncompliant Code:

Compliant Code:

DestructuringDeclarationWithTooManyEntries

Configuration options:

Noncompliant Code:

Compliant Code:

DoubleNegativeExpression

Noncompliant Code:

Compliant Code:

DoubleNegativeLambda

Configuration options:

Noncompliant Code:

Compliant Code:

EqualsNullCall

Noncompliant Code:

Compliant Code:

EqualsOnSignatureLine

Noncompliant Code:

Compliant Code:

ExplicitCollectionElementAccessMethod

Noncompliant Code:

Compliant Code:

ExplicitItLambdaParameter

Noncompliant Code:

Compliant Code:

ExpressionBodySyntax

Configuration options:

Noncompliant Code:

Compliant Code:

ForbiddenAnnotation

Configuration options:

Noncompliant Code:

Compliant Code:

ForbiddenComment

Configuration options:

Noncompliant Code:

ForbiddenImport

Configuration options:

Noncompliant Code:

ForbiddenMethodCall

Configuration options:

Noncompliant Code:

ForbiddenSuppress

Configuration options:

Noncompliant Code: