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 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 ("john@example.com" in emailsList) { ... }
if ("jane@example.com" !in emailsList) { ... }
// 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.
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()
}
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())
}
// 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.
Last modified: 25 September 2024