Style Rule Set

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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.

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

Severity: Style

Debt: 5min

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 it should be carefully considered if merging the if statements actually does improve readability or if it hides some edge-cases from the reader.

Severity: Style

Debt: 5min

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.

Severity: Style

Debt: 20min

Configuration options:

• conversionFunctionPrefix (default: 'to')

  allowed conversion function names

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.

Severity: Style

Debt: 20min

Noncompliant Code:

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

Compliant Code:

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

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.

Severity: Style

Debt: 5min

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.

Severity: Style

Debt: 5min

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 https://kotlinlang.org/docs/reference/operator-overloading.html#indexed. Prefer the usage of the indexed access operator [] for map or list element access or insert methods.

Severity: Style

Debt: 5min

Noncompliant Code:

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

Compliant Code:

val map = Map<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 usecases, 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.

Severity: Style

Debt: 5min

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
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.

Severity: Style

Debt: 5min

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()
}

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.

Severity: Style

Debt: 10min

Configuration options:

• values (default: ['TODO:', 'FIXME:', 'STOPSHIP:'])

  forbidden comment strings

• allowedPatterns (default: '')

  ignores comments which match the specified regular expression. For example Ticket|Task.

Noncompliant Code:

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

ForbiddenImport

This rule allows to set a list of forbidden imports. This can be used to discourage the use of unstable, experimental or deprecated APIs. Detekt will then report all imports that are forbidden.

Severity: Style

Debt: 10min

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:

package foo

import kotlin.jvm.JvmField
import kotlin.SinceKotlin

ForbiddenMethodCall

This rule allows to set a list of forbidden methods. This can be used to discourage the use of unstable, experimental or deprecated methods, especially for methods imported from external libraries. Detekt will then report all methods invocation that are forbidden.

Requires Type Resolution

Severity: Style

Debt: 10min

Configuration options:

• methods (default: ['kotlin.io.println', 'kotlin.io.print'])

  Comma separated list of fully qualified method signatures which are forbidden

Noncompliant Code:

import java.lang.System
fun main() {
    System.gc()
}

ForbiddenPublicDataClass

The data classes are bad for the binary compatibility in public APIs. Avoid to use it.

This rule is aimed to library maintainers. If you are developing a final application you don’t need to care about this issue.

More info: https://jakewharton.com/public-api-challenges-in-kotlin/

Severity: Style

Debt: 20min

Configuration options:

• ignorePackages (default: ['*.internal', '*.internal.*'])

  ignores classes in the specified packages.

Noncompliant Code:

data class C(val a: String) // violation: public data class

Compliant Code:

internal data class C(val a: String)

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.

Requires Type Resolution

Severity: Style

Debt: 5min

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 to define the constant directly as a const val.

Severity: Style

Debt: 10min

Configuration options:

• ignoreOverridableFunction (default: true)

  if overriden functions should be ignored

• excludedFunctions (default: 'describeContents')

  excluded functions

• excludeAnnotatedFunction (default: ['dagger.Provides'])

  allows to provide a list of annotations that disable this check

Noncompliant Code:

fun functionReturningConstantString() = "1"

Compliant Code:

const val constantString = "1"

LibraryCodeMustSpecifyReturnType

Library functions/properties should have an explicit return type. Inferred return type can easily be changed by mistake which may lead to breaking changes.

Severity: Style

Debt: 5min

Noncompliant Code:

// code from a library
val strs = listOf("foo, bar")
fun bar() = 5
class Parser {
    fun parse() = ...
}

Compliant Code:

// code from a library
val strs: List<String> = listOf("foo, bar")
fun bar(): Int = 5

class Parser {
    fun parse(): ParsingResult = ...
}

LibraryEntitiesShouldNotBePublic

Library typealias and classes should be internal or private.

Severity: Style

Debt: 5min

Noncompliant Code:

// code from a library
class A

Compliant Code:

// code from a library
internal class A

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.

Severity: Style

Debt: 10min

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.

Severity: Style

Debt: 10min

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

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
    }
}

MandatoryBracesIfStatements

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

Severity: Style

Debt: 5min

Noncompliant Code:

val i = 1
if (i > 0)
    println(i)

Compliant Code:

val x = if (condition) 5 else 4

MandatoryBracesLoops

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

Severity: Style

Debt: 5min

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)

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.

Severity: Style

Debt: 5min

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

MayBeConst

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.

Severity: Style

Debt: 5min

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: http://kotlinlang.org/docs/reference/coding-conventions.html#modifiers

Severity: Style

Debt: 5min

Noncompliant Code:

lateinit internal private val str: String

Compliant Code:

private internal lateinit val str: String

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.

Severity: Style

Debt: 5min

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.

Severity: Style

Debt: 5min

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.

Severity: Style

Debt: 5min

OptionalAbstractKeyword

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

Severity: Style

Debt: 5min

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.

Severity: Style

Debt: 5min

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

OptionalWhenBraces

This rule reports unnecessary braces in when expressions. These optional braces should be removed.

Severity: Style

Debt: 5min

Noncompliant Code:

val i = 1
when (i) {
    1 -> { println("one") } // unnecessary curly braces since there is only one statement
    else -> println("else")
}

Compliant Code:

val i = 1
when (i) {
    1 -> println("one")
    else -> println("else")
}

PreferToOverPairSyntax

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

Using to is preferred.

Requires Type Resolution

Severity: Style

Debt: 5min

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.

Severity: Warning

Debt: 5min

Noncompliant Code:

class ProtectedMemberInFinalClass {
    protected var i = 0
}

Compliant Code:

class ProtectedMemberInFinalClass {
    private var i = 0
}

RedundantExplicitType

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

Severity: Style

Debt: 5min

Noncompliant Code:

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

Compliant Code:

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

RedundantVisibilityModifierRule

This rule checks for redundant visibility modifiers.

Severity: Style

Debt: 5min

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 return methods allowed in methods.

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

Severity: Style

Debt: 10min

Configuration options:

• max (default: 2)

  define the maximum number of return statements allowed per function

• excludedFunctions (default: 'equals')

  define functions 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.

Severity: Style

Debt: 5min

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.

Severity: Warning

Debt: 5min

Noncompliant Code:

class IncorrectSerializable : Serializable {

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

Compliant Code:

class CorrectSerializable : Serializable {

    companion object {
        const val serialVersionUID = 1L
    }
}

SpacingBetweenPackageAndImports

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

Severity: Style

Debt: 5min

Noncompliant Code:

package foo
import a.b
class Bar { }

Compliant Code:

package foo

import a.b

class Bar { }

ThrowsCount

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

Severity: Style

Debt: 10min

Configuration options:

• max (default: 2)

  maximum amount of throw statements in a method

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.

Severity: Style

Debt: 5min

UnderscoresInNumericLiterals

This rule detects and reports decimal base 10 numeric literals above a certain length that should be underscore separated for readability. Underscores that do not make groups of 3 digits are also reported even if their length is under the acceptableDecimalLength. For Serializable classes or objects, the field serialVersionUID is explicitly ignored. For floats and doubles, anything to the right of the decimal is ignored.

Severity: Style

Debt: 5min

Configuration options:

• acceptableDecimalLength (default: 5)

  Length under which decimal base 10 literals are not required to have underscores

Noncompliant Code:

object Money {
    const val DEFAULT_AMOUNT = 1000000
}

Compliant Code:

object Money {
    const val DEFAULT_AMOUNT = 1_000_000
}

UnnecessaryAbstractClass

This rule inspects abstract classes. In case an abstract class does not have any concrete members it should be refactored into an interface. Abstract classes which do not define any abstract members should instead be refactored into concrete classes.

Severity: Style

Debt: 5min

Configuration options:

• excludeAnnotatedClasses (default: ['dagger.Module'])

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

Noncompliant Code:

abstract class OnlyAbstractMembersInAbstractClass { // violation: no concrete members

    abstract val i: Int
    abstract fun f()
}

abstract class OnlyConcreteMembersInAbstractClass { // violation: no abstract members

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

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: https://kotlinlang.org/docs/reference/annotations.html#annotation-use-site-targets

Severity: Style

Debt: 5min

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)

UnnecessaryApply

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

Requires Type Resolution

Severity: Style

Debt: 5min

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"
}

UnnecessaryInheritance

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

Severity: Style

Debt: 5min

Noncompliant Code:

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

UnnecessaryLet

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

Requires Type Resolution

Severity: Style

Debt: 5min

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

Severity: Style

Debt: 5min

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) }
}

UntilInsteadOfRangeTo

Reports calls to ‘..’ operator instead of calls to ‘until’. ‘until’ is applicable in cases where the upper range value is described as some value subtracted by 1. ‘until’ helps to prevent off-by-one errors.

Severity: Style

Debt: 5min

Noncompliant Code:

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

Compliant Code:

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

UnusedImports

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).

Severity: Style

Debt: 5min

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.

Severity: Maintainability

Debt: 5min

Aliases: unused

UnusedPrivateMember

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

Severity: Maintainability

Debt: 5min

Aliases: UNUSED_VARIABLE, UNUSED_PARAMETER, unused

Configuration options:

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

  unused private member names matching this regex are ignored

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.

Severity: Style

Debt: 5min

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.

Requires Type Resolution

Severity: Style

Debt: 5min

Noncompliant Code:

check(x != null)

Compliant Code:

checkNotNull(x)

UseCheckOrError

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

Severity: Style

Debt: 5min

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.

Read more about data class: https://kotlinlang.org/docs/reference/data-classes.html

Severity: Style

Debt: 5min

Configuration options:

• excludeAnnotatedClasses (default: [])

  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.

Requires Type Resolution

Severity: Style

Debt: 5min

Noncompliant Code:

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

Compliant Code:

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

UseIfInsteadOfWhen

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

See https://kotlinlang.org/docs/reference/coding-conventions.html#if-versus-when

Severity: Style

Debt: 5min

Noncompliant Code:

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

Compliant Code:

if (x == null) true else false

UseRequire

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

Severity: Style

Debt: 5min

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.

Requires Type Resolution

Severity: Style

Debt: 5min

Noncompliant Code:

require(x != null)

Compliant Code:

requireNotNull(x)

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.

Rule adapted from Kotlin’s IntelliJ plugin: https://github.com/JetBrains/kotlin/blob/f5d0a38629e7d2e7017ee645dc4d4bee60614e93/idea/src/org/jetbrains/kotlin/idea/inspections/collections/UselessCallOnNotNullInspection.kt

Requires Type Resolution

Severity: Performance

Debt: 5min

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 an class with a non-public constructor. Furthermore, this rule reports utility classes which are not final.

Severity: Style

Debt: 5min

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 (locally-scoped variables) that could be val, as they are not re-assigned. Val declarations are assign-once (read-only), which makes understanding the current state easier.

Severity: Maintainability

Debt: 5min

Aliases: CanBeVal

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.

Severity: Style

Debt: 5min

Configuration options:

• excludeImports (default: ['java.util.*', 'kotlinx.android.synthetic.*'])

  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()
}