Kotlin Help

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 var s) 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) { ... }

String interpolation

println("Name $name")

Learn the difference between Java and Kotlin string concatenation.

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 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) { ... }

Lazy property

val p: String by lazy { // compute the string }

Extension functions

fun String.spaceToCamelCase() { ... } "Convert this to camelcase".spaceToCamelCase()

Create a singleton

object Resource { val name = "Name" }

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() println(files?.size ?: "empty") // if files is null, this prints "empty"

Execute 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() ?: ""

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() }

Configure 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()) }

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)

Nullable Boolean

val b: Boolean? = ... if (b == true) { ... } else { // `b` is false or null }

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.

What’s next?

Last modified: 12 November 2021