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Potential-bugs Rule Set

The potential-bugs rule set provides rules that detect potential bugs.

AvoidReferentialEquality​

Kotlin supports two types of equality: structural equality and referential equality. While there are use cases for both, checking for referential equality for some types (such as String or List) is likely not intentional and may cause unexpected results.

Active by default: Yes - Since v1.21.0

Requires Type Resolution

Debt: 5min

Configuration options:​

  • forbiddenTypePatterns (default: ['kotlin.String'])

    Specifies those types for which referential equality checks are considered a rule violation. The types are defined by a list of simple glob patterns (supporting * and ? wildcards) that match the fully qualified type name.

Noncompliant Code:​

    val areEqual = "aString" === otherString
    val areNotEqual = "aString" !== otherString

Compliant Code:​

    val areEqual = "aString" == otherString
    val areNotEqual = "aString" != otherString

CastNullableToNonNullableType​

Reports cast of nullable variable to non-null type. Cast like this can hide null problems in your code. The compliant code would be that which will correctly check for two things (nullability and type) and not just one (cast).

Active by default: No

Requires Type Resolution

Debt: 5min

Configuration options:​

  • ignorePlatformTypes (default: true)

    Whether platform types should be considered as non-nullable and ignored by this rule

Noncompliant Code:​

fun foo(bar: Any?) {
    val x = bar as String
}

Compliant Code:​

fun foo(bar: Any?) {
    val x = checkNotNull(bar) as String
}

// Alternative
fun foo(bar: Any?) {
    val x = (bar ?: error("null assertion message")) as String
}

CastToNullableType​

Reports unsafe cast to nullable types. as String? is unsafed and may be misused as safe cast (as? String).

Active by default: No

Debt: 5min

Noncompliant Code:​

fun foo(a: Any?) {
    val x: String? = a as String? // If 'a' is not String, ClassCastException will be thrown.
}

Compliant Code:​

fun foo(a: Any?) {
    val x: String? = a as? String
}

CharArrayToStringCall​

Reports CharArray.toString() calls that do not return the expected result.

Active by default: No

Requires Type Resolution

Debt: 5min

Noncompliant Code:​

val s = ""
val charArray = "hello😅".toCharArray()

println("$s$charArray") // [C@4f023edb
println(charArray.toString()) // [C@4f023edb
println(s + charArray) // [C@4f023edb

Compliant Code:​

println("$s${charArray.concatToString()}") // hello😅
println(charArray.concatToString()) // hello😅
println(s + charArray.concatToString()) // hello😅

Deprecation​

Deprecated elements are expected to be removed in the future. Alternatives should be found if possible.

Active by default: No

Requires Type Resolution

Debt: 20min

Aliases: DEPRECATION

DontDowncastCollectionTypes​

Down-casting immutable types from kotlin.collections should be discouraged. The result of the downcast is platform specific and can lead to unexpected crashes. Prefer to use instead the toMutable<Type>() functions.

Active by default: No

Requires Type Resolution

Debt: 10min

Noncompliant Code:​

val list : List<Int> = getAList()
if (list is MutableList) {
    list.add(42)
}

(list as MutableList).add(42)

Compliant Code:​

val list : List<Int> = getAList()
list.toMutableList().add(42)

DoubleMutabilityForCollection​

Using var when declaring a mutable collection or value holder leads to double mutability. Consider instead declaring your variable with val or switching your declaration to use an immutable type.

By default, the rule triggers on standard mutable collections, however it can be configured to trigger on other types of mutable value types, such as MutableState from Jetpack Compose.

Active by default: Yes - Since v1.21.0

Requires Type Resolution

Debt: 5min

Aliases: DoubleMutability

Configuration options:​

  • mutableTypes (default: ['kotlin.collections.MutableList', 'kotlin.collections.MutableMap', 'kotlin.collections.MutableSet', 'java.util.ArrayList', 'java.util.LinkedHashSet', 'java.util.HashSet', 'java.util.LinkedHashMap', 'java.util.HashMap'])

    Define a list of mutable types to trigger on when defined with var.

Noncompliant Code:​

var myList = mutableListOf(1,2,3)
var mySet = mutableSetOf(1,2,3)
var myMap = mutableMapOf("answer" to 42)

Compliant Code:​

// Use val
val myList = mutableListOf(1,2,3)
val mySet = mutableSetOf(1,2,3)
val myMap = mutableMapOf("answer" to 42)

// Use immutable types
var myList = listOf(1,2,3)
var mySet = setOf(1,2,3)
var myMap = mapOf("answer" to 42)

DuplicateCaseInWhenExpression​

Rule deprecated as compiler performs this check by default

Flags duplicate case statements in when expressions.

If a when expression contains the same case statement multiple times they should be merged. Otherwise, it might be easy to miss one of the cases when reading the code, leading to unwanted side effects.

Active by default: Yes - Since v1.0.0

Debt: 10min

Noncompliant Code:​

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

Compliant Code:​

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

ElseCaseInsteadOfExhaustiveWhen​

This rule reports when expressions that contain an else case even though they have an exhaustive set of cases.

This occurs when the subject of the when expression is either an enum class, sealed class or of type boolean. Using else cases for these expressions can lead to unintended behavior when adding new enum types, sealed subtypes or changing the nullability of a boolean, since this will be implicitly handled by the else case.

Active by default: No

Requires Type Resolution

Debt: 5min

Configuration options:​

  • ignoredSubjectTypes (default: [])

    List of fully qualified types which should be ignored for when expressions with a subject. Example kotlinx.serialization.json.JsonObject

Noncompliant Code:​

enum class Color {
    RED,
    GREEN,
    BLUE
}

when(c) {
    Color.RED -> {}
    Color.GREEN -> {}
    else -> {}
}

Compliant Code:​

enum class Color {
    RED,
    GREEN,
    BLUE
}

when(c) {
    Color.RED -> {}
    Color.GREEN -> {}
    Color.BLUE -> {}
}

EqualsAlwaysReturnsTrueOrFalse​

Reports equals() methods which will always return true or false.

Equals methods should always report if some other object is equal to the current object. See the Kotlin documentation for Any.equals(other: Any?): https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-any/equals.html

Active by default: Yes - Since v1.2.0

Debt: 20min

Noncompliant Code:​

override fun equals(other: Any?): Boolean {
    return true
}

Compliant Code:​

override fun equals(other: Any?): Boolean {
    return this === other
}

EqualsWithHashCodeExist​

When a class overrides the equals() method it should also override the hashCode() method.

All hash-based collections depend on objects meeting the equals-contract. Two equal objects must produce the same hashcode. When inheriting equals or hashcode, override the inherited and call the super method for clarification.

Active by default: Yes - Since v1.0.0

Debt: 5min

Noncompliant Code:​

class Foo {

    override fun equals(other: Any?): Boolean {
        return super.equals(other)
    }
}

Compliant Code:​

class Foo {

    override fun equals(other: Any?): Boolean {
        return super.equals(other)
    }

    override fun hashCode(): Int {
        return super.hashCode()
    }
}

ExitOutsideMain​

Reports the usage of System.exit(), Runtime.exit(), Runtime.halt() and Kotlin's exitProcess() when used outside the main function. This makes code more difficult to test, causes unexpected behaviour on Android, and is a poor way to signal a failure in the program. In almost all cases it is more appropriate to throw an exception.

Active by default: No

Requires Type Resolution

Debt: 10min

Noncompliant Code:​

fun randomFunction() {
    val result = doWork()
    if (result == FAILURE) {
        exitProcess(2)
    } else {
        exitProcess(0)
    }
}

Compliant Code:​

fun main() {
    val result = doWork()
    if (result == FAILURE) {
        exitProcess(2)
    } else {
        exitProcess(0)
    }
}

ExplicitGarbageCollectionCall​

Reports all calls to explicitly trigger the Garbage Collector. Code should work independently of the garbage collector and should not require the GC to be triggered in certain points in time.

Active by default: Yes - Since v1.0.0

Debt: 20min

Noncompliant Code:​

System.gc()
Runtime.getRuntime().gc()
System.runFinalization()

HasPlatformType​

Platform types must be declared explicitly in public APIs to prevent unexpected errors.

Active by default: Yes - Since v1.21.0

Requires Type Resolution

Debt: 5min

Noncompliant Code:​

class Person {
    fun apiCall() = System.getProperty("propertyName")
}

Compliant Code:​

class Person {
    fun apiCall(): String = System.getProperty("propertyName")
}

IgnoredReturnValue​

This rule warns on instances where a function, annotated with either @CheckReturnValue or @CheckResult, returns a value but that value is not used in any way. The Kotlin compiler gives no warning for this scenario normally so that's the rationale behind this rule.

fun returnsValue() = 42 fun returnsNoValue() {}

Active by default: Yes - Since v1.21.0

Requires Type Resolution

Debt: 20min

Configuration options:​

  • restrictToAnnotatedMethods (default: true)

    Deprecated: Use restrictToConfig instead

    if the rule should check only annotated methods

  • restrictToConfig (default: true)

    If the rule should check only methods matching to configuration, or all methods

  • returnValueAnnotations (default: ['CheckResult', '*.CheckResult', 'CheckReturnValue', '*.CheckReturnValue'])

    List of glob patterns to be used as inspection annotation

  • ignoreReturnValueAnnotations (default: ['CanIgnoreReturnValue', '*.CanIgnoreReturnValue'])

    Annotations to skip this inspection

  • returnValueTypes (default: ['kotlin.sequences.Sequence', 'kotlinx.coroutines.flow.*Flow', 'java.util.stream.*Stream'])

    List of return types that should not be ignored

  • ignoreFunctionCall (default: [])

    List of function signatures which should be ignored by this rule. Specifying fully-qualified function signature with name only (i.e. java.time.LocalDate.now) will ignore all function calls matching the name. Specifying fully-qualified function signature with parameters (i.e. java.time.LocalDate.now(java.time.Clock)) will ignore only function calls matching the name and parameters exactly.

Noncompliant Code:​

returnsValue()

Compliant Code:​

if (42 == returnsValue()) {}
val x = returnsValue()

ImplicitDefaultLocale​

Prefer passing [java.util.Locale] explicitly than using implicit default value when formatting strings or performing a case conversion.

The default locale is almost always inappropriate for machine-readable text like HTTP headers. For example, if locale with tag ar-SA-u-nu-arab is a current default then %d placeholders will be evaluated to a number consisting of Eastern-Arabic (non-ASCII) digits. [java.util.Locale.US] is recommended for machine-readable output.

Active by default: Yes - Since v1.16.0

Requires Type Resolution

Debt: 5min

Noncompliant Code:​

String.format("Timestamp: %d", System.currentTimeMillis())
"Timestamp: %d".format(System.currentTimeMillis())

val str: String = getString()
str.toUpperCase()
str.toLowerCase()

Compliant Code:​

String.format(Locale.US, "Timestamp: %d", System.currentTimeMillis())
"Timestamp: %d".format(Locale.US, System.currentTimeMillis())

val str: String = getString()
str.toUpperCase(Locale.US)
str.toLowerCase(Locale.US)

ImplicitUnitReturnType​

Functions using expression statements have an implicit return type. Changing the type of the expression accidentally, changes the functions return type. This may lead to backward incompatibility. Use a block statement to make clear this function will never return a value.

Active by default: No

Requires Type Resolution

Debt: 5min

Configuration options:​

  • allowExplicitReturnType (default: true)

    if functions with explicit Unit return type should be allowed

Noncompliant Code:​

fun errorProneUnit() = println("Hello Unit")
fun errorProneUnitWithParam(param: String) = param.run { println(this) }
fun String.errorProneUnitWithReceiver() = run { println(this) }

Compliant Code:​

fun blockStatementUnit() {
    // code
}

// explicit Unit is compliant by default; can be configured to enforce block statement
fun safeUnitReturn(): Unit = println("Hello Unit")

InvalidRange​

Reports ranges which are empty. This might be a bug if it is used for instance as a loop condition. This loop will never be triggered then. This might be due to invalid ranges like (10..9) which will cause the loop to never be entered.

Active by default: Yes - Since v1.2.0

Debt: 10min

Noncompliant Code:​

for (i in 2..1) {}
for (i in 1 downTo 2) {}

val range1 = 2 until 1
val range2 = 2 until 2

Compliant Code:​

for (i in 2..2) {}
for (i in 2 downTo 2) {}

val range = 2 until 3

IteratorHasNextCallsNextMethod​

Verifies implementations of the Iterator interface. The hasNext() method of an Iterator implementation should not have any side effects. This rule reports implementations that call the next() method of the Iterator inside the hasNext() method.

Active by default: Yes - Since v1.2.0

Debt: 10min

Noncompliant Code:​

class MyIterator : Iterator<String> {

    override fun hasNext(): Boolean {
        return next() != null
    }
}

IteratorNotThrowingNoSuchElementException​

Reports implementations of the Iterator interface which do not throw a NoSuchElementException in the implementation of the next() method. When there are no more elements to return an Iterator should throw a NoSuchElementException.

See: https://docs.oracle.com/javase/7/docs/api/java/util/Iterator.html#next()

Active by default: Yes - Since v1.2.0

Debt: 10min

Noncompliant Code:​

class MyIterator : Iterator<String> {

    override fun next(): String {
        return ""
    }
}

Compliant Code:​

class MyIterator : Iterator<String> {

    override fun next(): String {
        if (!this.hasNext()) {
            throw NoSuchElementException()
        }
        // ...
    }
}

LateinitUsage​

Reports usages of the lateinit modifier.

Using lateinit for property initialization can be error-prone and the actual initialization is not guaranteed. Try using constructor injection or delegation to initialize properties.

Active by default: No

Debt: 20min

Configuration options:​

  • ignoreOnClassesPattern (default: '')

    Allows you to disable the rule for a list of classes

Noncompliant Code:​

class Foo {
    private lateinit var i1: Int
    lateinit var i2: Int
}

MapGetWithNotNullAssertionOperator​

Reports calls of the map access methods map[] or map.get() with a not-null assertion operator !!. This may result in a NullPointerException. Preferred access methods are map[] without !!, map.getValue(), map.getOrDefault() or map.getOrElse().

Based on an IntelliJ IDEA inspection MapGetWithNotNullAssertionOperatorInspection.

Active by default: Yes - Since v1.21.0

Requires Type Resolution

Debt: 5min

Noncompliant Code:​

val map = emptyMap<String, String>()
map["key"]!!

val map = emptyMap<String, String>()
map.get("key")!!

Compliant Code:​

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

val map = emptyMap<String, String>()
map.getValue("key")

val map = emptyMap<String, String>()
map.getOrDefault("key", "")

val map = emptyMap<String, String>()
map.getOrElse("key", { "" })

MissingPackageDeclaration​

Reports when the package declaration is missing.

Active by default: No

Debt: 5min

MissingWhenCase​

Rule deprecated as compiler performs this check by default

Turn on this rule to flag when expressions that do not check that all cases are covered when the subject is an enum or sealed class and the when expression is used as a statement.

When this happens it's unclear what was intended when an unhandled case is reached. It is better to be explicit and either handle all cases or use a default else statement to cover the unhandled cases.

Active by default: Yes - Since v1.2.0

Requires Type Resolution

Debt: 20min

Configuration options:​

  • allowElseExpression (default: true)

    whether else can be treated as a valid case for enums and sealed classes

Noncompliant Code:​

enum class Color {
    RED,
    GREEN,
    BLUE
}

fun whenOnEnumFail(c: Color) {
    when(c) {
        Color.BLUE -> {}
        Color.GREEN -> {}
    }
}

Compliant Code:​

enum class Color {
    RED,
    GREEN,
    BLUE
}

fun whenOnEnumCompliant(c: Color) {
    when(c) {
        Color.BLUE -> {}
        Color.GREEN -> {}
        Color.RED -> {}
    }
}

fun whenOnEnumCompliant2(c: Color) {
    when(c) {
        Color.BLUE -> {}
        else -> {}
    }
}

NullCheckOnMutableProperty​

Reports null-checks on mutable properties, as these properties' value can be changed - and thus make the null-check invalid - after the execution of the if-statement.

Active by default: No

Requires Type Resolution

Debt: 10min

Noncompliant Code:​

class A(private var a: Int?) {
fun foo() {
    if (a != null) {
      println(2 + a!!)
    }
}
}

Compliant Code:​

class A(private val a: Int?) {
fun foo() {
    if (a != null) {
      println(2 + a)
    }
}
}

NullableToStringCall​

Reports toString() calls with a nullable receiver that may return the string "null".

Active by default: No

Requires Type Resolution

Debt: 5min

Noncompliant Code:​

fun foo(a: Any?): String {
    return a.toString()
}

fun bar(a: Any?): String {
    return "$a"
}

Compliant Code:​

fun foo(a: Any?): String {
    return a?.toString() ?: "-"
}

fun bar(a: Any?): String {
    return "${a ?: "-"}"
}

PropertyUsedBeforeDeclaration​

Reports properties that are used before declaration.

Active by default: No

Requires Type Resolution

Debt: 5min

Noncompliant Code:​

class C {
    private val number
        get() = if (isValid) 1 else 0

    val list = listOf(number)

    private val isValid = true
}

fun main() {
    println(C().list) // [0]
}

Compliant Code:​

class C {
    private val isValid = true

    private val number
        get() = if (isValid) 1 else 0

    val list = listOf(number)
}

fun main() {
    println(C().list) // [1]
}

RedundantElseInWhen​

Rule deprecated as compiler performs this check by default

Reports when expressions that contain a redundant else case. This occurs when it can be verified that all cases are already covered when checking cases on an enum or sealed class.

Active by default: Yes - Since v1.2.0

Requires Type Resolution

Debt: 5min

Noncompliant Code:​

enum class Color {
    RED,
    GREEN,
    BLUE
}

fun whenOnEnumFail(c: Color) {
    when(c) {
        Color.BLUE -> {}
        Color.GREEN -> {}
        Color.RED -> {}
        else -> {}
    }
}

Compliant Code:​

enum class Color {
    RED,
    GREEN,
    BLUE
}

fun whenOnEnumCompliant(c: Color) {
    when(c) {
        Color.BLUE -> {}
        Color.GREEN -> {}
        else -> {}
    }
}

fun whenOnEnumCompliant2(c: Color) {
    when(c) {
        Color.BLUE -> {}
        Color.GREEN -> {}
        Color.RED -> {}
    }
}

UnconditionalJumpStatementInLoop​

Reports loops which contain jump statements that jump regardless of any conditions. This implies that the loop is only executed once and thus could be rewritten without a loop altogether.

Active by default: No

Debt: 10min

Noncompliant Code:​

for (i in 1..2) break

Compliant Code:​

for (i in 1..2) {
    if (i == 1) break
}

UnnecessaryNotNullCheck​

Reports unnecessary not-null checks with requireNotNull or checkNotNull that can be removed by the user.

Active by default: No

Requires Type Resolution

Debt: 5min

Noncompliant Code:​

var string = "foo"
println(requireNotNull(string))

Compliant Code:​

var string : String? = "foo"
println(requireNotNull(string))

UnnecessaryNotNullOperator​

Reports unnecessary not-null operator usage (!!) that can be removed by the user.

Active by default: Yes - Since v1.16.0

Requires Type Resolution

Debt: 5min

Noncompliant Code:​

val a = 1
val b = a!!

Compliant Code:​

val a = 1
val b = a

UnnecessarySafeCall​

Reports unnecessary safe call operators (?.) that can be removed by the user.

Active by default: Yes - Since v1.16.0

Requires Type Resolution

Debt: 5min

Noncompliant Code:​

val a: String = ""
val b = a?.length

Compliant Code:​

val a: String? = null
val b = a?.length

UnreachableCatchBlock​

Reports unreachable catch blocks. Catch blocks can be unreachable if the exception has already been caught in the block above.

Active by default: Yes - Since v1.21.0

Requires Type Resolution

Debt: 5min

Noncompliant Code:​

fun test() {
    try {
        foo()
    } catch (t: Throwable) {
        bar()
    } catch (e: Exception) {
        // Unreachable
        baz()
    }
}

Compliant Code:​

fun test() {
    try {
        foo()
    } catch (e: Exception) {
        baz()
    } catch (t: Throwable) {
        bar()
    }
}

UnreachableCode​

Reports unreachable code. Code can be unreachable because it is behind return, throw, continue or break expressions. This unreachable code should be removed as it serves no purpose.

Active by default: Yes - Since v1.0.0

Requires Type Resolution

Debt: 10min

Noncompliant Code:​

for (i in 1..2) {
    break
    println() // unreachable
}

throw IllegalArgumentException()
println() // unreachable

fun f() {
    return
    println() // unreachable
}

UnsafeCallOnNullableType​

Reports unsafe calls on nullable types. These calls will throw a NullPointerException in case the nullable value is null. Kotlin provides many ways to work with nullable types to increase null safety. Guard the code appropriately to prevent NullPointerExceptions.

Active by default: Yes - Since v1.2.0

Requires Type Resolution

Debt: 20min

Noncompliant Code:​

fun foo(str: String?) {
    println(str!!.length)
}

Compliant Code:​

fun foo(str: String?) {
    println(str?.length)
}

UnsafeCast​

Reports casts that will never succeed.

Active by default: Yes - Since v1.16.0

Requires Type Resolution

Debt: 20min

Aliases: UNCHECKED_CAST

Noncompliant Code:​

fun foo(s: String) {
    println(s as Int)
}

fun bar(s: String) {
    println(s as? Int)
}

Compliant Code:​

fun foo(s: Any) {
    println(s as Int)
}

UnusedUnaryOperator​

Detects unused unary operators.

Active by default: Yes - Since v1.21.0

Requires Type Resolution

Debt: 5min

Noncompliant Code:​

val x = 1 + 2
    + 3 + 4
println(x) // 3

Compliant Code:​

val x = 1 + 2 + 3 + 4
println(x) // 10

UselessPostfixExpression​

Reports postfix expressions (++, --) which are unused and thus unnecessary. This leads to confusion as a reader of the code might think the value will be incremented/decremented. However, the value is replaced with the original value which might lead to bugs.

Active by default: Yes - Since v1.21.0

Debt: 20min

Noncompliant Code:​

var i = 0
i = i--
i = 1 + i++
i = i++ + 1

fun foo(): Int {
    var i = 0
    // ...
    return i++
}

Compliant Code:​

var i = 0
i--
i = i + 2
i = i + 2

fun foo(): Int {
    var i = 0
    // ...
    i++
    return i
}

WrongEqualsTypeParameter​

Reports equals() methods which take in a wrongly typed parameter. Correct implementations of the equals() method should only take in a parameter of type Any? See: https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-any/equals.html

Active by default: Yes - Since v1.2.0

Debt: 10min

Noncompliant Code:​

class Foo {

    fun equals(other: String): Boolean {
        return super.equals(other)
    }
}

Compliant Code:​

class Foo {

    fun equals(other: Any?): Boolean {
        return super.equals(other)
    }
}