A collection of random and frequently used idioms in Kotlin. If you have a favorite idiom, contribute it by sending a pull request.
data class Customer(val name: String, val email: String)provides a Customer class with the following functionality:
- getters (and setters in case of var{: .keyword }s) for all properties
equals()hashCode()toString()copy()component1(),component2(), ..., for all properties (see Data classes)
fun foo(a: Int = 0, b: String = "") { ... }val positives = list.filter { x -> x > 0 }Or alternatively, even shorter:
val positives = list.filter { it > 0 }println("Name $name")when (x) { is Foo -> ... is Bar -> ... else -> ... }for ((k, v) in map) { println("$k -> $v") }k, v can be called anything.
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) { ... }val list = listOf("a", "b", "c")val map = mapOf("a" to 1, "b" to 2, "c" to 3)println(map["key"]) map["key"] = valueval p: String by lazy { // compute the string }fun String.spaceToCamelCase() { ... } "Convert this to camelcase".spaceToCamelCase()object Resource { val name = "Name" }val files = File("Test").listFiles() println(files?.size)val files = File("Test").listFiles() println(files?.size ?: "empty")val data = ... val email = data["email"] ?: throw IllegalStateException("Email is missing!")val data = ... data?.let { ... // execute this block if not null }val data = ... val mapped = data?.let { transformData(it) } ?: defaultValueIfDataIsNullfun transform(color: String): Int { return when (color) { "Red" -> 0 "Green" -> 1 "Blue" -> 2 else -> throw IllegalArgumentException("Invalid color param value") } }fun test() { val result = try { count() } catch (e: ArithmeticException) { throw IllegalStateException(e) } // Working with result }fun foo(param: Int) { val result = if (param == 1) { "one" } else if (param == 2) { "two" } else { "three" } }fun arrayOfMinusOnes(size: Int): IntArray { return IntArray(size).apply { fill(-1) } }fun theAnswer() = 42This 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{: .keyword }-expression:
fun transform(color: String): Int = when (color) { "Red" -> 0 "Green" -> 1 "Blue" -> 2 else -> throw IllegalArgumentException("Invalid color param value") }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 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)val b: Boolean? = ... if (b == true) { ... } else { // `b` is false or null }