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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 ("[email protected]" in emailsList) { ... }

if ("[email protected]" !in emailsList) { ... }

String interpolation

println("Name $name")

Learn the difference between Java and Kotlin string concatenation.

Read standard input safely

// 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.

The @JvmInline annotation is only needed for JVM backends.

{style="note"}

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 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)

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?