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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.

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

Debt: 5min

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

Debt: 5min

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

Debt: 5min

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

Debt: 10min

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

Debt: 5min

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

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, 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

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.

Active by default: No

Debt: 20min

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

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?
)

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

Debt: 10min

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)

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

Debt: 5min

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

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.

Active by default: No

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 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

Debt: 5min

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

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.

Active by default: No

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()
}

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

Debt: 5min

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.

Active by default: Yes - Since v1.0.0

Debt: 10min

Configuration options:​

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

    forbidden comment strings

  • allowedPatterns (default: '')

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

  • customMessage (default: '')

    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

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:​

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

Debt: 10min

Configuration options:​

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

    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:​

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

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

Debt: 10min

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

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 declaring the constant as a const val.

Active by default: Yes - Since v1.2.0

Debt: 10min

Configuration options:​

  • ignoreOverridableFunction (default: true)

    if overriden functions should be ignored

  • ignoreActualFunction (default: true)

    if actual functions should be ignored

  • excludedFunctions (default: [])

    excluded functions

  • excludeAnnotatedFunction (default: [])

    Deprecated: Use ignoreAnnotated instead

    allows to provide a list of annotations that disable this check

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

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.

Active by default: Yes - Since v1.0.0

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

  • 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

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)

MaxChainedCallsOnSameLine​

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

Active by default: No

Requires Type Resolution

Debt: 5min

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

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

  • excludeRawStrings (default: true)

    if raw strings 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.

Active by default: Yes - Since v1.2.0

Debt: 5min

Aliases: MayBeConstant

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

Debt: 5min

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

Debt: 5min

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.

Active by default: No

Debt: 5min

Configuration options:​

  • indentSize (default: 4)

    indentation size

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

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.

Active by default: Yes - Since v1.0.0

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.

Active by default: No

Debt: 5min

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

Debt: 5min

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

Debt: 5min

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

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.

Active by default: No

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.

Active by default: No

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 <value1> to <value2> is preferred.

Active by default: No

Requires Type Resolution

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.

Active by default: Yes - Since v1.2.0

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.

Active by default: No

Requires Type Resolution

Debt: 5min

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

Debt: 5min

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()
}

RedundantVisibilityModifierRule​

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

Debt: 5min

Aliases: RedundantVisibilityModifier

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.

Active by default: Yes - Since v1.0.0

Debt: 10min

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

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 and declared as private.

More about SerialVersionUID

Active by default: Yes - Since v1.16.0

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.

Active by default: No

Debt: 5min

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

Debt: 5min

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

Debt: 10min

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.

Active by default: No

Debt: 5min

TrimMultilineRawString​

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

Active by default: No

Debt: 5min

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

Debt: 5min

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

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.

Active by default: Yes - Since v1.2.0

Requires Type Resolution

Debt: 5min

Configuration options:​

  • excludeAnnotatedClasses (default: [])

    Deprecated: Use ignoreAnnotated instead

    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() { }
}

Compliant Code:​

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

class OnlyConcreteMembersInClass {
    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: Annotation use-site targets

Active by default: No

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

Active by default: Yes - Since v1.16.0

Requires Type Resolution

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"
}

UnnecessaryBackticks​

This rule reports unnecessary backticks.

Active by default: No

Debt: 5min

Noncompliant Code:​

class `HelloWorld`

Compliant Code:​

class HelloWorld

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

Debt: 5min

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

Debt: 5min

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

Debt: 5min

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

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

Active by default: No

Debt: 5min

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) }
}

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.

Active by default: No

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).

Active by default: No

Debt: 5min

UnusedParameter​

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

Active by default: Yes - Since v1.23.0

Debt: 5min

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

Debt: 5min

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

Debt: 5min

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

Debt: 5min

Aliases: UNUSED_VARIABLE, 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)
}
}

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

Debt: 5min

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

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.

Active by default: Yes - Since v1.21.0

Requires Type Resolution

Debt: 5min

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

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 classes

Active by default: No

Debt: 5min

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

Debt: 5min

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

Debt: 5min

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

Debt: 5min

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

Debt: 5min

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

UseOrEmpty​

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

Active by default: Yes - Since v1.21.0

Requires Type Resolution

Debt: 5min

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

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.

Active by default: Yes - Since v1.21.0

Requires Type Resolution

Debt: 5min

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

Debt: 5min

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

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 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

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 (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

Debt: 5min

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

Debt: 5min

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()
}