Idiomatic kotlin

@antonarhipov
Idiomatic Kotlin
from formatting to DSLs

Agenda
• Expressions

• Examples from standard library

• DSL

Anton Arhipov

@antonarhipov
Developer Advocate @ JetBrains

Idiomatic - using, containing, or denoting expressions
that are natural to a native speaker

Idiomatic - using, containing, or denoting expressions
that are natural to a native speaker
In case of a programming language:

•Conforms to a commonly accepted style

•E
ff
ectively uses features of the programming language

fun adjustSpeed(weather: Weather): Drive {

var result: Drive

if (weather is Rainy) {

result = Safe()

} else {

result = Calm()

}

return result

}


val result: Drive


result = Safe()

} else {

result = Calm()

}

return result

}


val result: Drive

result = if (weather is Rainy) {

Safe()

} else {

Calm()

}

return result

}


val result: Drive = if (weather is Rainy) {

Safe()

} else {

Calm()

}

return result

}


return if (weather is Rainy) {

Safe()

} else {

Calm()

}

}

fun adjustSpeed(weather: Weather): Drive = if (weather is Rainy) {

Safe()

} else {

Calm()

}

fun adjustSpeed(weather: Weather) = if (weather is Rainy) {

Safe()

} else {

Calm()

}

fun adjustSpeed(weather: Weather) = if (weather is Rainy) Safe() else Calm()

fun adjustSpeed(weather: Weather) = if (weather is Rainy) Safe() else Calm()


var result: Drive


result = Safe()

} else {

result = Calm()

}

return result

}

abstract class Weather

class Sunny : Weather()

class Rainy : Weather()

fun adjustSpeed(weather: Weather) = when (weather) {

is Rainy
-
>
Safe()

else
-
>
Calm()

}

sealed class Weather




is Rainy
-
>
Safe()

/
/
else
-
>
Calm()

}





is Rainy
-
>
Safe()

is Sunny
-
>
TODO()

}





is Rainy
-
>
Safe()

is Sunny
-
>
TODO()

}

Use expressions!

Use when as expression body

Use sealed classes with when

Use try as expression body
fun tryParse(number: String) : Int? {

try {

return Integer.parseInt(number)

} catch (e: NumberFormatException) {

return null

}

}

Use try as expression body
fun tryParse(number: String) = try {

Integer.parseInt(number)


null

}

Use try as expression
fun tryParse(number: String) : Int? {

val n = try {

Integer.parseInt(number)


null

}

println(n)

return n

}

Use elvis operator
class Person(val name: String?, val age: Int?)

val p = retrievePerson()
?
:
Person()

Use elvis operator as return and throw

fun processPerson(person: Person) {

val name = person.name

if (name
=
=
null)

throw IllegalArgumentException("Named required")

val age = person.age

if (age
=
=
null) return

println("$name: $age")

}

Use elvis operator as return and throw


val name = person.name
?
:

throw IllegalArgumentException("Named required")

val age = person.age
?
:
return

println("$name: $age")

}

Consider using null-safe call
val order = retrieveOrder()

if (order
=
=
null
|
|
order.customer
=
=
null
|
|
order.customer.address
=
=
null){

throw IllegalArgumentException("Invalid Order")

}

val city = order.customer.address.city


val city = order
?
.
customer
?
.
address
?
.
city


val city = order
?
.
customer
?
.
address
?
.
city

?
:
throw IllegalArgumentException("Invalid Order")

Avoid not-null assertions !!

val city = order
!
!
.customer
!
!
.address
!
!
.city

“You may notice that the double exclamation mark looks a bit rude:

it’s almost like you’re yelling at the compiler. This is intentional.” - Kotlin in Action

class MyTest {

class State(val data: String)

private var state: State? = null

@BeforeEach

fun setup() {

state = State("abc")

}

@Test

fun foo() {

assertEquals("abc", state
!
!
.data)

}

}

class MyTest {


private var state: State? = null

@BeforeEach

fun setup() {


}

@Test

fun foo() {

assertEquals("abc", state
!
!
.data)

}

}
class MyTest {


private lateinit var state: State

@BeforeEach

fun setup() {


}

@Test

fun foo() {

assertEquals("abc", state.data)

}

}
- use lateinit

Consider using ?.let for null-checks

if (order
!
=
null){

processCustomer(order.customer)

}


if (order
!
=
null){


}
retrieveOrder()
?
.
let {

processCustomer(it.customer)

}
retrieveOrder()
?
.
customer
?
.
let {
:
:
processCustomer }
or


if (order
!
=
null){


}
retrieveOrder()
?
.
let {


}
No need for an extra variable
retrieveOrder()
?
.
let {


}
retrieveOrder()
?
.
customer
?
.
let {
:
:
processCustomer }
or


if (order
!
=
null){


}
retrieveOrder()
?
.
let {


}
retrieveOrder()
?
.
let {


}
retrieveOrder()
?
.
customer
?
.
let {
:
:
processCustomer }
or

Consider using safe cast for type checking
override fun equals(other: Any?) : Boolean {

val command = other as Command

return command.id
=
=
id

}

Consider using safe cast for type checking

val command = other as Command

return command.id
=
=
id

}

return (other as? Command)
?
.
id
=
=
id

}

Use range checks instead of comparison pairs
fun isLatinUppercase(c: Char) =

c
>
=
'A'
&
&
c
<
=
'Z'

Use range checks instead of comparison pairs
fun isLatinUppercase(c: Char) =

c in 'A'
.
.
'Z'

Ranges in loops
fun main(args: Array<String>) {

for (i in 0
.
.
args.size - 1) {

println("$i: ${args[i]}")

}

}

Ranges in loops

for (i in 0
.
.
args.size - 1) {


}

}

for (i in 0 until args.size) {


}

Ranges in loops

for (i in 0
.
.
args.size - 1) {


}

}



}
for (i in args.indices) {


}

Ranges in loops

for (i in 0
.
.
args.size - 1) {


}

}



}
for (i in args.indices) {


}
for ((i, arg) in args.withIndex()) {

println("$i: $arg")

}

Don’t create classes just to hold functions
class StringUtils {

companion object {

fun isPhoneNumber(s: String) =

s.length
=
=
7
&
&
s.all { it.isDigit() }

}

}

class StringUtils {

companion object {


s.length
=
=
7
&
&

}

}

object StringUtils {


s.length
=
=
7
&
&

}

class StringUtils {

companion object {


s.length
=
=
7
&
&

}

}



s.length
=
=
7
&
&

}


s.length
=
=
7
&
&

Use extension functions
class StringUtils {

companion object {


s.length
=
=
7
&
&

}

}



s.length
=
=
7
&
&

}


s.length
=
=
7
&
&

fun String.isPhoneNumber() =

length
=
=
7
&
&
all { it.isDigit() }

Extension or a member?
https://kotlinlang.org/docs/coding-conventions.html#extension-functions
•Use extension functions liberally.

•If a function works primarily on an object, consider making it an
extension with that object as a receiver.

•Minimize API pollution, restrict the visibility.

•As necessary, use local extension functions, member extension
functions, or top-level extension functions with private visibility.

Use default values instead of overloading
class Phonebook {

fun print() {

print(",")

}

fun print(columnSeparator: String) {}

}


Phonebook().print("|")

}

Use default values instead of overloading
class Phonebook {

fun print() {

print(",")

}

fun print(columnSeparator: String) {}

}


Phonebook().print("|")

}

class Phonebook {

fun print(separator: String = ",") {}

fun someFun(x: Int) {}

}


Phonebook().print(separator = "|")

}

Return multiple values using data classes
fun namedNum(): Pair<Int, String> =

1 to "one"

/
/
same but shorter

fun namedNum2() = 1 to "one"


val pair = namedNum()

val number = pair.first

val name = pair.second

}

fun namedNum(): Pair<Int, String> =

1 to "one"

/
/
same but shorter

fun namedNum2() = 1 to "one"


val pair = namedNum()

val number = pair.first

val name = pair.second

}
data class GameResult(

val rank: Int,

val name: String

)

fun namedNum() =

GameResult(1, "Player 1")


val (rank, name) = namedNum()

println("$name, rank $rank")

}

data class GameResult(

val rank: Int,

val name: String

)

fun namedNum() =

GameResult(1, "Player 1")


val (rank, name) = namedNum()

println("$name, rank $rank")

}
GameResult var1 = namedNum();

int var2 = var1.component1();

String var3 = var1.component2();

Destructuring in loops
fun printMap(map: Map<String, String>) {

for (item in map.entries) {

println("${item.key}
-
>
${item.value}")

}

}

Destructuring in loops

for (item in map.entries) {

println("${item.key}
-
>
${item.value}")

}

}

for ((key, value) in map) {

println("$key
-
>
$value")

}

}

Destructuring in lists
data class NameExt(

val name: String,

val ext: String?

)

fun splitNameExt(filename: String): NameExt {

if ('.' in filename) {

val parts = filename.split('.', limit = 2)

return NameExt(parts[0], parts[1])

}

return NameExt(filename, null)

}

fun splitNameAndExtension(filename: String): NameExt {

if ('.' in filename) {

val (name, ext) = filename.split('.', limit = 2)

return NameExt(name, ext)

}

return NameExt(filename, null)

}

Use type aliases for functional types
class Event

class EventDispatcher {

fun addClickHandler(handler: (Event)
-
>
Unit) {}

fun removeClickHandler(handler: (Event)
-
>
Unit) {}

}

Use type aliases for functional types
class Event


fun addClickHandler(handler: (Event)
-
>
Unit) {}

fun removeClickHandler(handler: (Event)
-
>
Unit) {}

}
typealias ClickHandler = (Event)
-
>
Unit


fun addClickHandler(handler: ClickHandler) {

}

fun removeClickHandler(handler: ClickHandler) {

}

}

Verify parameters using require()
class Person(

val name: String?,

val age: Int

)


if (person.age < 18) {

throw IllegalArgumentException("Adult required")

}

}

Verify parameters using require()
class Person(

val name: String?,

val age: Int

)


if (person.age < 18) {

throw IllegalArgumentException("Adult required")

}

}


require(person.age
>
=
18) { "Adult required" }

}

Select objects by type with filterIsInstance
fun findAllStrings(objects: List<Any>) =

objects.filter { it is String }




objects.filterIsInstance<String>()

fun findAllStrings(objects: List<Any>) : List<Any> =


fun findAllStrings(objects: List<Any>) : List<String> =

objects.filterIsInstance<String>()

Apply operation to non-null elements mapNotNull
data class Result(

val data: Any?,

val error: String?

)

fun listErrors(results: List<Result>): List<String> =

results.map { it.error }.filterNotNull()

fun listErrors(results: List<Result>): List<String> =

results.mapNotNull { it.errorMessage }

compareBy compares by multiple keys
class Person(

val name: String,

val age: Int

)

fun sortPersons(persons: List<Person>) =

persons.sortedWith(Comparator<Person> { person1, person2
-
>

val rc = person1.name.compareTo(person2.name)

if (rc
!
=
0)

rc

else

person1.age - person2.age

})

compareBy compares by multiple keys
class Person(

val name: String,

val age: Int

)


persons.sortedWith(Comparator<Person> { person1, person2
-
>

val rc = person1.name.compareTo(person2.name)

if (rc
!
=
0)

rc

else

person1.age - person2.age

})


persons.sortedWith(compareBy(Person
:
:
name, Person
:
:
age))

groupBy to group elements
class Request(

val url: String,

val remoteIP: String,

val timestamp: Long

)

fun analyzeLog(log: List<Request>) {

val map = mutableMapOf<String, MutableList<Request
>
>
()

for (request in log) {

map.getOrPut(request.url) { mutableListOf() }

.add(request)

}

}

groupBy to group elements
class Request(

val url: String,

val remoteIP: String,

val timestamp: Long

)


val map = mutableMapOf<String, MutableList<Request
>
>
()

for (request in log) {

map.getOrPut(request.url) { mutableListOf() }

.add(request)

}

}


val map = log.groupBy(Request
:
:
url)

}

Use coerceIn to ensure numbers in range
fun updateProgress(value: Int) {

val actualValue = when {

value < 0
-
>
0

value > 100
-
>
100

else
-
>
value

}

}
fun updateProgress(value: Int) {

val actualValue = value.coerceIn(0, 100)

}

Initializing objects with apply
val dataSource = BasicDataSource(
)

dataSource.driverClassName = "com.mysql.jdbc.Driver"
dataSource.url = "jdbc:mysql://domain:3309/db"
dataSource.username = "username"
dataSource.password = "password"
dataSource.maxTotal = 40
dataSource.maxIdle = 40
dataSource.minIdle = 4
val dataSource = BasicDataSource().apply
{

driverClassName = "com.mysql.jdbc.Driver"
url = "jdbc:mysql://domain:3309/db"
username = "username"
password = "password"
maxTotal = 40
maxIdle = 40
minIdle = 4
}

final ClientBuilder builder = new ClientBuilder();

builder.setFirstName("Anton");

builder.setLastName("Arhipov");

final TwitterBuilder twitterBuilder = new TwitterBuilder();

twitterBuilder.setHandle("@antonarhipov");

builder.setTwitter(twitterBuilder.build());

final CompanyBuilder companyBuilder = new CompanyBuilder();

companyBuilder.setName("JetBrains");

companyBuilder.setCity("Tallinn");

builder.setCompany(companyBuilder.build());

final Client client = builder.build();

System.out.println("Created client is: " + client);

val builder = ClientBuilder()

builder.firstName = "Anton"

builder.lastName = "Arhipov"

val twitterBuilder = TwitterBuilder()

twitterBuilder.handle = "@antonarhipov"

builder.twitter = twitterBuilder.build()

val companyBuilder = CompanyBuilder()

companyBuilder.name = "JetBrains"

companyBuilder.city = "Tallinn"

builder.company = companyBuilder.build()

val client = builder.build()

println("Created client is: $client")


builder.firstName = "Anton"

builder.lastName = "Arhipov"

val twitterBuilder = TwitterBuilder()

twitterBuilder.handle = "@antonarhipov"

builder.twitter = twitterBuilder.build()

val companyBuilder = CompanyBuilder()

companyBuilder.name = "JetBrains"

companyBuilder.city = "Tallinn"

builder.company = companyBuilder.build()

val client = builder.build()


val client = ClientBuilder().apply {

firstName = "Anton"

lastName = "Arhipov"

twitter = TwitterBuilder().apply {

handle = "@antonarhipov"

}.build()

company = CompanyBuilder().apply {

name = "JetBrains"

city = "Tallinn"

}.build()

}.build()


“Domain Speci
fi
c”,
i.e. tailored for a speci
fi
c task

“Domain Speci
fi
c”,
i.e. tailored for a speci
fi
c task
Examples:

•Compose strings - stringBuilder

•Create HTML documents - kotlinx.html

•Con
fi
gure routing logic for a web app - ktor

•Generally, build any object graphs. See “type-safe builders”

buildString
//Java
String name = "Joe";
StringBuilder sb = new StringBuilder();
for (int i = 0; i < 5; i++) {
sb.append("Hello, ");
sb.append(name);
sb.append("!n");
}
System.out.println(sb);
//Kotlin
val name = "Joe"
val s = buildString {
repeat(5) {
append("Hello, ")
append(name)
appendLine("!")
}
}
println(s)

kotlinx.html
System.out.appendHTML().html {

body {

div {

a("http:
/
/
kotlinlang.org") {

target = ATarget.blank

+"Main site"

}

}

}

}

Ktor
fun main() {

embeddedServer(Netty, port = 8080, host = "0.0.0.0") {

routing {

get("/html-dsl") {

call.respondHtml {

body {

h1 { +"HTML" }

ul {

for (n in 1
.
.
10) {

li { +"$n" }

}

}

}

}

}

}

}.start(wait = true)

}

Ktor
fun main() {


routing {

get("/html-dsl") {

call.respondHtml {

body {

h1 { +"HTML" }

ul {

for (n in 1
.
.
10) {

li { +"$n" }

}

}

}

}

}

}


}

Ktor’s routing

Ktor
fun main() {


routing {

get("/html-dsl") {

call.respondHtml {

body {

h1 { +"HTML" }

ul {

for (n in 1
.
.
10) {

li { +"$n" }

}

}

}

}

}

}


}

kotlinx.html
Ktor’s routing

Lambda with receiver
T.() -> Unit

Build your vocabulary to abstract from scope functions

firstName = "Anton"




}.build()


name = "JetBrains"

city = "Tallinn"

}.build()

}.build()


fun client(c: ClientBuilder.()
-
>
Unit): Client {


c(builder)

return builder.build()

}

fun ClientBuilder.company(block: CompanyBuilder.()
-
>
Unit) {

company = CompanyBuilder().apply(block).build()

}

fun ClientBuilder.twitter(block: TwitterBuilder.()
-
>
Unit) {

twitter = TwitterBuilder().apply(block).build()

}

firstName = "Anton"




}.build()


name = "JetBrains"

city = "Tallinn"

}.build()

}.build()


val client = client {

firstName = "Anton"


twitter {


}

company {

name = "JetBrains"

city = "Tallinn"

}

}



firstName = "Anton"




}.build()


name = "JetBrains"

city = "Tallinn"

}.build()

}.build()


https://speakerdeck.com/antonarhipov
https://github.com/antonarhipov/idiomatic-kotlin
@antonarhipov

Idiomatic kotlin

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