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 ("john@example.com" in emailsList) { ... }
if ("jane@example.com" !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"
}

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)

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 tool window.