Idioms
A collection of random and frequently used idioms in Kotlin. If you have a favorite idiom, contribute it by sending a pull request.
Create DTOs (POJOs/POCOs)
data class Customer(val name: String, val email: String)
provides a Customer class with the following functionality:
getters (and setters in case of vars) for all properties
equals()
hashCode()
toString()
copy()
component1(), component2(), ..., for all properties (see Data classes)
Default values for function parameters
fun foo(a: Int = 0, b: String = "") { ... }
Filter a list
val positives = list.filter { x -> x > 0 }
Or alternatively, even shorter:
val positives = list.filter { it > 0 }
Learn the difference between Java and Kotlin filtering.
Check the presence of an element in a collection
if ("john@example.com" in emailsList) { ... } if ("jane@example.com" !in emailsList) { ... }
// Reads a string and returns null if the input can't be converted into an integer. For example: Hi there! val wrongInt = readln().toIntOrNull() println(wrongInt) // null // Reads a string that can be converted into an integer and returns an integer. For example: 13 val correctInt = readln().toIntOrNull() println(correctInt) // 13
For more information, see Read standard input.
Instance checks
when (x) { is Foo -> ... is Bar -> ... else -> ... }
Read-only list
val list = listOf("a", "b", "c")
Read-only map
val map = mapOf("a" to 1, "b" to 2, "c" to 3)
Access a map entry
println(map["key"]) map["key"] = value
Traverse a map or a list of pairs
for ((k, v) in map) { println("$k -> $v") }
k and v can be any convenient names, such as name and age.
Iterate over a range
for (i in 1..100) { ... } // closed-ended range: includes 100 for (i in 1..<100) { ... } // open-ended range: does not include 100 for (x in 2..10 step 2) { ... } for (x in 10 downTo 1) { ... } (1..10).forEach { ... }
Lazy property
val p: String by lazy { // the value is computed only on first access // compute the string }
Extension functions
fun String.spaceToCamelCase() { ... } "Convert this to camelcase".spaceToCamelCase()
Create a singleton
object Resource { val name = "Name" }
Use inline value classes for type-safe values
@JvmInline value class EmployeeId(private val id: String) @JvmInline value class CustomerId(private val id: String)
If you accidentally mix up EmployeeId and CustomerId, a compilation error is triggered.
Instantiate an abstract class
abstract class MyAbstractClass { abstract fun doSomething() abstract fun sleep() } fun main() { val myObject = object : MyAbstractClass() { override fun doSomething() { // ... } override fun sleep() { // ... } } myObject.doSomething() }
If-not-null shorthand
val files = File("Test").listFiles() println(files?.size) // size is printed if files is not null
If-not-null-else shorthand
val files = File("Test").listFiles() // For simple fallback values: println(files?.size ?: "empty") // if files is null, this prints "empty" // To calculate a more complicated fallback value in a code block, use `run` val filesSize = files?.size ?: run { val someSize = getSomeSize() someSize * 2 } println(filesSize)
Execute an expression if null
val values = ... val email = values["email"] ?: throw IllegalStateException("Email is missing!")
Get first item of a possibly empty collection
val emails = ... // might be empty val mainEmail = emails.firstOrNull() ?: ""
Learn the difference between Java and Kotlin first item getting.
Execute if not null
val value = ... value?.let { ... // execute this block if not null }
Map nullable value if not null
val value = ... val mapped = value?.let { transformValue(it) } ?: defaultValue // defaultValue is returned if the value or the transform result is null.
Return on when statement
fun transform(color: String): Int { return when (color) { "Red" -> 0 "Green" -> 1 "Blue" -> 2 else -> throw IllegalArgumentException("Invalid color param value") } }
try-catch expression
fun test() { val result = try { count() } catch (e: ArithmeticException) { throw IllegalStateException(e) } // Working with result }
if expression
val y = if (x == 1) { "one" } else if (x == 2) { "two" } else { "other" }
Builder-style usage of methods that return Unit
fun arrayOfMinusOnes(size: Int): IntArray { return IntArray(size).apply { fill(-1) } }
Single-expression functions
fun theAnswer() = 42
This is equivalent to
fun theAnswer(): Int { return 42 }
This can be effectively combined with other idioms, leading to shorter code. For example, with the when expression:
fun transform(color: String): Int = when (color) { "Red" -> 0 "Green" -> 1 "Blue" -> 2 else -> throw IllegalArgumentException("Invalid color param value") }
Call multiple methods on an object instance (with)
class Turtle { fun penDown() fun penUp() fun turn(degrees: Double) fun forward(pixels: Double) } val myTurtle = Turtle() with(myTurtle) { //draw a 100 pix square penDown() for (i in 1..4) { forward(100.0) turn(90.0) } penUp() }
val myRectangle = Rectangle().apply { length = 4 breadth = 5 color = 0xFAFAFA }
This is useful for configuring properties that aren't present in the object constructor.
Java 7's try-with-resources
val stream = Files.newInputStream(Paths.get("/some/file.txt")) stream.buffered().reader().use { reader -> println(reader.readText()) }
// public final class Gson { // ... // public <T> T fromJson(JsonElement json, Class<T> classOfT) throws JsonSyntaxException { // ... inline fun <reified T: Any> Gson.fromJson(json: JsonElement): T = this.fromJson(json, T::class.java)
Swap two variables
var a = 1 var b = 2 a = b.also { b = a }
Mark code as incomplete (TODO)
Kotlin's standard library has a TODO() function that will always throw a NotImplementedError. Its return type is Nothing so it can be used regardless of expected type. There's also an overload that accepts a reason parameter:
fun calcTaxes(): BigDecimal = TODO("Waiting for feedback from accounting")
IntelliJ IDEA's kotlin plugin understands the semantics of TODO() and automatically adds a code pointer in the TODO tool window.
07 July 2025