Idioms
A collection of random and frequently used idioms in Kotlin. If you have a favorite idiom, contribute it by sending a pull request.
Creating 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 = "") { ... }
Filtering a list
val positives = list.filter { x -> x > 0 }
Or alternatively, even shorter:
val positives = list.filter { it > 0 }
Checking element presence in a collection.
if ("[email protected]" in emailsList) { ... } if ("[email protected]" !in emailsList) { ... }
String Interpolation
println("Name $name")
Instance Checks
when (x) { is Foo -> ... is Bar -> ... else -> ... }
Traversing a map/list of pairs
for ((k, v) in map) { println("$k -> $v") }
k
, v
can be called anything.
Using ranges
for (i in 1..100) { ... } // closed range: includes 100 for (i in 1 until 100) { ... } // half-open range: does not include 100 for (x in 2..10 step 2) { ... } for (x in 10 downTo 1) { ... } if (x in 1..10) { ... }
Read-only list
val list = listOf("a", "b", "c")
Read-only map
val map = mapOf("a" to 1, "b" to 2, "c" to 3)
Accessing a map
println(map["key"]) map["key"] = value
Lazy property
val p: String by lazy { // compute the string }
Extension Functions
fun String.spaceToCamelCase() { ... } "Convert this to camelcase".spaceToCamelCase()
Creating a singleton
object Resource { val name = "Name" }
If not null shorthand
val files = File("Test").listFiles() println(files?.size)
If not null and else shorthand
val files = File("Test").listFiles() println(files?.size ?: "empty")
Executing a statement 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() ?: ""
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
fun foo(param: Int) { val result = if (param == 1) { "one" } else if (param == 2) { "two" } else { "three" } }
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. E.g. with the when-expression:
fun transform(color: String): Int = when (color) { "Red" -> 0 "Green" -> 1 "Blue" -> 2 else -> throw IllegalArgumentException("Invalid color param value") }
Calling 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() }
Configuring properties of an object (apply
)
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()) }
Convenient form for a generic function that requires the generic type information
// 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)
Consuming a nullable Boolean
val b: Boolean? = ... if (b == true) { ... } else { // `b` is false or null }
Swapping two variables
var a = 1 var b = 2 a = b.also { b = a }
TODO(): Marking code as incomplete
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 toolwindow.
© 2010–2020 JetBrains s.r.o. and Kotlin Programming Language contributors
Licensed under the Apache License, Version 2.0.
https://kotlinlang.org/docs/reference/idioms.html