website/versioned_docs/version-1.21.0/rules/style.md
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.
This rule inspects variables marked as nullable and reports which could be declared as non-nullable instead.
Active by default: No
Requires Type Resolution
Debt: 10min
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)
}
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)
}
Requires that all chained calls are placed on a new line if a preceding one is.
Active by default: No
Debt: 5min
includeElvis (default: true)
require trailing elvis expressions to be wrapped on a new line
foo()
.bar().baz()
foo().bar().baz()
foo()
.bar()
.baz()
This rule ensures class contents are ordered as follows as recommended by the Kotlin Coding Conventions:
Active by default: No
Debt: 5min
class OutOfOrder {
companion object {
const val IMPORTANT_VALUE = 3
}
fun returnX(): Int {
return x
}
private val x = 2
}
class InOrder {
private val x = 2
fun returnX(): Int {
return x
}
companion object {
const val IMPORTANT_VALUE = 3
}
}
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
val i = 1
if (i > 0) {
if (i < 5) {
println(i)
}
}
val i = 1
if (i > 0 && i < 5) {
println(i)
}
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
conversionFunctionPrefix (default: 'to')
allowed conversion function names
data class DataClassWithFunctions(val i: Int) {
fun foo() { }
}
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
data class MutableDataClass(var i: Int) {
var s: String? = null
}
data class ImmutableDataClass(
val i: Int,
val s: String?
)
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
maxDestructuringEntries (default: 3)
maximum allowed elements in a destructuring declaration
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)
data class FewerElements(val a: Int, val b: Int, val c: Int)
val (a, b, c) = TooManyElements(1, 2, 3)
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
fun isNull(str: String) = str.equals(null)
fun isNull(str: String) = str == null
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
fun stuff(): Int
= 5
fun <V> foo(): Int where V : Int
= 5
fun stuff() = 5
fun stuff() =
foo.bar()
fun <V> foo(): Int where V : Int = 5
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
val map = mutableMapOf<String, String>()
map.put("key", "value")
val value = map.get("key")
val map = mutableMapOf<String, String>()
map["key"] = "value"
val value = map["key"]
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.
Active by default: Yes - Since v1.21.0
Debt: 5min
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) }
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)
}
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
includeLineWrapping (default: false)
include return statements with line wraps in it
fun stuff(): Int {
return 5
}
fun stuff() = 5
fun stuff() {
return
moreStuff()
.getStuff()
.stuffStuff()
}
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
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
val a = "" // TODO: remove please
// FIXME: this is a hack
fun foo() { }
// STOPSHIP:
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.
Active by default: No
Debt: 10min
imports (default: [])
imports which should not be used
forbiddenPatterns (default: '')
reports imports which match the specified regular expression. For example net.*R.
package foo
import kotlin.jvm.JvmField
import kotlin.SinceKotlin
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 method invocations that are forbidden.
Active by default: No
Requires Type Resolution
Debt: 10min
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 likefun String.hello(a: Int) you should add the receiver parameter as the first parameter like this: hello(kotlin.String, kotlin.Int)
fun main() {
println()
val myPrintln : () -> Unit = ::println
kotlin.io.print("Hello, World!")
}
Data classes are bad for binary compatibility in public APIs. Avoid using them.
This rule is aimed at library maintainers. If you are developing a final application you can ignore this issue.
More info: Public API challenges in Kotlin
Active by default: Yes - Since v1.16.0
Debt: 20min
ignorePackages (default: ['*.internal', '*.internal.*'])
ignores classes in the specified packages.
data class C(val a: String) // violation: public data class
internal data class C(val a: String)
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.
Detekt will report suppression of all forbidden rules.
This rule is not capable of reporting suppression of itself, as that's a language feature with precedence.
Active by default: No
Debt: 10min
rules (default: [])
Rules whose suppression is forbidden.
package foo
// When the rule "MaximumLineLength" is forbidden
@Suppress("MaximumLineLength", "UNCHECKED_CAST")
class Bar
package foo
// When the rule "MaximumLineLength" is forbidden
@Suppress("UNCHECKED_CAST")
class Bar
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
ignoreOverridden (default: false)
ignores void types in signatures of overridden functions
ignoreUsageInGenerics (default: false)
ignore void types as generic arguments
runnable: () -> Void
var aVoid: Void? = null
runnable: () -> Unit
Void::class
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
ignoreOverridableFunction (default: true)
if overridden functions should be ignored
ignoreActualFunction (default: true)
if actual functions should be ignored
excludedFunctions (default: '')
excluded functions
excludeAnnotatedFunction[])
Deprecated: Use ignoreAnnotated instead
allows to provide a list of annotations that disable this check
fun functionReturningConstantString() = "1"
const val constantString = "1"
Functions/properties exposed as public APIs of a library should have an explicit return type. Inferred return type can easily be changed by mistake which may lead to breaking changes.
See also: Kotlin 1.4 Explicit API
Active by default: Yes - Since v1.2.0
Requires Type Resolution
Debt: 5min
// code from a library
val strs = listOf("foo, bar")
fun bar() = 5
class Parser {
fun parse() = ...
}
// code from a library
val strs: List<String> = listOf("foo, bar")
fun bar(): Int = 5
class Parser {
fun parse(): ParsingResult = ...
}
Library typealias and classes should be internal or private.
Active by default: Yes - Since v1.16.0
Debt: 5min
// code from a library
class A
// code from a library
internal class A
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
maxJumpCount (default: 1)
maximum allowed jumps in a loop
val strs = listOf("foo, bar")
for (str in strs) {
if (str == "bar") {
break
} else {
continue
}
}
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
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
class User {
fun checkName(name: String) {
if (name.length > 42) {
throw IllegalArgumentException("username is too long")
}
// ...
}
}
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
}
}
This rule detects multi-line if statements which do not have braces.
Adding braces would improve readability and avoid possible errors.
Active by default: No
Debt: 5min
val i = 1
if (i > 0)
println(i)
val x = if (condition) 5 else 4
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
for (i in 0..10)
println(i)
while (true)
println("Hello, world")
do
println("Hello, world")
while (true)
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)
Limits the number of chained calls which can be placed on a single line.
Active by default: No
Requires Type Resolution
Debt: 5min
maxChainedCalls (default: 5)
maximum chained calls allowed on a single line
a().b().c().d().e().f()
a().b().c()
.d().e().f()
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
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
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
val myConstant = "abc"
const val MY_CONSTANT = "abc"
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
lateinit internal val str: String
internal lateinit val str: String
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
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
}
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)
}
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
internal class Outer {
// explicit public modifier still results in an internal nested class
public class Nested
}
internal class Outer {
class Nested1
internal class Nested2
}
This rule reports files which do not end with a line separator.
Active by default: Yes - Since v1.0.0
Debt: 5min
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
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
value ?: true
value ?: false
value != false
value == true
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
object : Foo {
override fun bar() {
}
}
Foo {
}
This rule reports abstract modifiers which are unnecessary and can be removed.
Active by default: Yes - Since v1.0.0
Debt: 5min
abstract interface Foo { // abstract keyword not needed
abstract fun x() // abstract keyword not needed
abstract var y: Int // abstract keyword not needed
}
interface Foo {
fun x()
var y: Int
}
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
fun foo(): Unit {
return Unit
}
fun foo() = Unit
fun doesNothing() {
Unit
}
fun foo() { }
// overridden no-op functions are allowed
override fun foo() = Unit
This rule reports unnecessary braces in when expressions. These optional braces should be removed.
Active by default: No
Debt: 5min
val i = 1
when (i) {
1 -> { println("one") } // unnecessary curly braces since there is only one statement
else -> println("else")
}
val i = 1
when (i) {
1 -> println("one")
else -> println("else")
}
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
val pair = Pair(1, 2)
val pair = 1 to 2
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
class ProtectedMemberInFinalClass {
protected var i = 0
}
class ProtectedMemberInFinalClass {
private var i = 0
}
Local properties do not need their type to be explicitly provided when the inferred type matches the explicit type.
Active by default: No
Debt: 5min
fun function() {
val x: String = "string"
}
fun function() {
val x = "string"
}
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
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 }
}
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()
}
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
public interface Foo { // public per default
public fun bar() // public per default
}
interface Foo {
fun bar()
}
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
max (default: 2)
define the maximum number of return statements allowed per function
excludedFunctions (default: 'equals')
define a free-form comma separated 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
fun foo(i: Int): String {
when (i) {
1 -> return "one"
2 -> return "two"
else -> return "other"
}
}
fun foo(i: Int): String {
return when (i) {
1 -> "one"
2 -> "two"
else -> "other"
}
}
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
fun numberMagic(number: Number) {
val i = if (number is Int) number else null
// ...
}
fun numberMagic(number: Number) {
val i = number as? Int
// ...
}
Classes which implement the Serializable interface should also correctly declare a serialVersionUID.
This rule verifies that a serialVersionUID was correctly defined.
Active by default: Yes - Since v1.16.0
Debt: 5min
class IncorrectSerializable : Serializable {
companion object {
val serialVersionUID = 1 // wrong declaration for UID
}
}
class CorrectSerializable : Serializable {
companion object {
const val serialVersionUID = 1L
}
}
This rule verifies spacing between package and import statements as well as between import statements and class declarations.
Active by default: No
Debt: 5min
package foo
import a.b
class Bar { }
package foo
import a.b
class Bar { }
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
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
fun foo(i: Int) {
when (i) {
1 -> throw IllegalArgumentException()
2 -> throw IllegalArgumentException()
3 -> throw IllegalArgumentException()
}
}
fun foo(i: Int) {
when (i) {
1,2,3 -> throw IllegalArgumentException()
}
}
This rule reports lines that end with a whitespace.
Active by default: No
Debt: 5min
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
acceptableDecimalLength5)
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.
const val DEFAULT_AMOUNT = 1000000
const val DEFAULT_AMOUNT = 1_000_000
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
Debt: 5min
excludeAnnotatedClasses[])
Deprecated: Use ignoreAnnotated instead
Allows you to provide a list of annotations that disable this check.
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() { }
}
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
@property:Inject private val foo: String = "bar" // violation: unnecessary @property:
class Module(@param:Inject private val foo: String) // violation: unnecessary @param:
class Module(@Inject private val foo: String)
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
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`
config.apply {
version = "1.2"
environment = "test"
}
This rule reports unnecessary backticks.
Active by default: No
Debt: 5min
class `HelloWorld`
class HelloWorld
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
val x = listOf(1, 2, 3)
.filter { it > 1 }
.count()
val x = listOf(1, 2, 3)
.filter { it > 1 }
.isEmpty()
val x = listOf(1, 2, 3)
.count { it > 2 }
}
val x = listOf(1, 2, 3)
.none { it > 1 }
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
class A : Any()
class B : Object()
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
class A {
val foo = "BAR"
inner class B {
val fizz = "BUZZ"
fun printFizz() {
println(fizz)
}
}
}
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
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)`
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) }
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
val local = (5 + 3)
if ((local == 8)) { }
fun foo() {
function({ input -> println(input) })
}
val local = 5 + 3
if (local == 8) { }
fun foo() {
function { input -> println(input) }
}
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
for (i in 0 .. 10 - 1) {}
val range = 0 .. 10 - 1
for (i in 0 until 10) {}
val range = 0 until 10
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
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
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.
Active by default: Yes - Since v1.16.0
Debt: 5min
Aliases: UNUSED_VARIABLE, UNUSED_PARAMETER, unused
allowedNames (default: '(_|ignored|expected|serialVersionUID)')
unused private member names matching this regex are ignored
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
listOf(1, 2, 3).find { it == 4 } != null
listOf(1, 2, 3).find { it == 4 } == null
listOf(1, 2, 3).any { it == 4 }
listOf(1, 2, 3).none { it == 4 }
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
@PositiveCase(arrayOf("..."))
@NegativeCase(["..."])
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
check(x != null)
checkNotNull(x)
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
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")
}
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")
}
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
excludeAnnotatedClasses[])
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
class DataClassCandidate(val i: Int) {
val i2: Int = 0
}
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
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
arrayOf()
listOf() // or listOfNotNull()
mapOf()
sequenceOf()
setOf()
emptyArray()
emptyList()
emptyMap()
emptySequence()
emptySet()
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
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"
}
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" }
}
Binary expressions are better expressed using an if expression than a when expression.
See if versus when
Active by default: No
Debt: 5min
when (x) {
null -> true
else -> false
}
if (x == null) true else false
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
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
}
if (x.isNullOrEmpty()) return
This rule detects ?: emptyList() that can be replaced with orEmpty() call.
Active by default: Yes - Since v1.21.0
Requires Type Resolution
Debt: 5min
fun test(x: List<Int>?, s: String?) {
val a = x ?: emptyList()
val b = s ?: ""
}
fun test(x: List<Int>?, s: String?) {
val a = x.orEmpty()
val b = s.orEmpty()
}
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
if (value == null) throw IllegalArgumentException("value should not be null")
if (value < 0) throw IllegalArgumentException("value is $value but should be at least 0")
requireNotNull(value) { "value should not be null" }
require(value >= 0) { "value is $value but should be at least 0" }
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
require(x != null)
requireNotNull(x)
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
val testList = listOf("string").orEmpty()
val testList2 = listOf("string").orEmpty().map { _ }
val testList3 = listOfNotNull("string")
val testString = ""?.isNullOrBlank()
val testList = listOf("string")
val testList2 = listOf("string").map { }
val testList3 = listOf("string")
val testString = ""?.isBlank()
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
class UtilityClassViolation {
// public constructor here
constructor() {
// ...
}
companion object {
val i = 0
}
}
open class UtilityClassViolation private constructor() {
// ...
}
class UtilityClass {
private constructor() {
// ...
}
companion object {
val i = 0
}
}
object UtilityClass {
val i = 0
}
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
ignoreLateinitVar (default: false)
Whether to ignore uninitialized lateinit vars
fun example() {
var i = 1 // violation: this variable is never re-assigned
val j = i + 1
}
fun example() {
val i = 1
val j = i + 1
}
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
excludeImports (default: ['java.util.*'])
Define a list of package names that should be allowed to be imported with wildcard imports.
import io.gitlab.arturbosch.detekt.*
class DetektElements {
val element1 = DetektElement1()
val element2 = DetektElement2()
}
import io.gitlab.arturbosch.detekt.DetektElement1
import io.gitlab.arturbosch.detekt.DetektElement2
class DetektElements {
val element1 = DetektElement1()
val element2 = DetektElement2()
}