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Idiomatic kotlin

Anton Arhipov, profile picture
Anton Arhipov

The document discusses idiomatic Kotlin programming. It covers several topics including: - Using expressions idiomatically in Kotlin like if/when expressions - Examples of idiomatic patterns from the Kotlin standard library - Best practices for null safety using safe calls, let, elvis operator - Idiomatic use of classes, functions, and data classes - Leveraging the Kotlin standard library

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@antonarhipov Idiomatic Kotlin from formatting to DSLs
Agenda • Expressions • Examples from standard library • DSL
Anton Arhipov @antonarhipov Developer Advocate @ JetBrains
Idiomatic kotlin
Idiomatic kotlin
Idiomatic kotlin
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
Idiomatic kotlin
Expressions try, if, when
fun adjustSpeed(weather: Weather): Drive { var result: Drive if (weather is Rainy) { result = Safe() } else { result = Calm() } return result }
fun adjustSpeed(weather: Weather): Drive { var result: Drive if (weather is Rainy) { result = Safe() } else { result = Calm() } return result }
fun adjustSpeed(weather: Weather): Drive { val result: Drive if (weather is Rainy) { result = Safe() } else { result = Calm() } return result }
fun adjustSpeed(weather: Weather): Drive { val result: Drive if (weather is Rainy) { result = Safe() } else { result = Calm() } return result }
fun adjustSpeed(weather: Weather): Drive { val result: Drive result = if (weather is Rainy) { Safe() } else { Calm() } return result }
fun adjustSpeed(weather: Weather): Drive { val result: Drive result = if (weather is Rainy) { Safe() } else { Calm() } return result }
fun adjustSpeed(weather: Weather): Drive { val result: Drive = if (weather is Rainy) { Safe() } else { Calm() } return result }
fun adjustSpeed(weather: Weather): Drive { val result: Drive = if (weather is Rainy) { Safe() } else { Calm() } return result }
fun adjustSpeed(weather: Weather): Drive { return if (weather is Rainy) { Safe() } else { Calm() } }
fun adjustSpeed(weather: Weather): Drive { return if (weather is Rainy) { Safe() } else { Calm() } }
fun adjustSpeed(weather: Weather): Drive = 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()
fun adjustSpeed(weather: Weather) = if (weather is Rainy) Safe() else Calm() fun adjustSpeed(weather: Weather): Drive { var result: Drive if (weather is Rainy) { result = Safe() } else { result = Calm() } return result }
fun adjustSpeed(weather: Weather) = if (weather is Rainy) Safe() else Calm()
fun adjustSpeed(weather: Weather) = if (weather is Rainy) Safe() else Calm()
abstract class Weather class Sunny : Weather() class Rainy : Weather() fun adjustSpeed(weather: Weather) = when (weather) { is Rainy - > Safe() else - > Calm() }
sealed class Weather class Sunny : Weather() class Rainy : Weather() fun adjustSpeed(weather: Weather) = when (weather) { is Rainy - > Safe() / / else - > Calm() }
sealed class Weather class Sunny : Weather() class Rainy : Weather() fun adjustSpeed(weather: Weather) = when (weather) { is Rainy - > Safe() / / else - > Calm() }
sealed class Weather class Sunny : Weather() class Rainy : Weather() fun adjustSpeed(weather: Weather) = when (weather) { is Rainy - > Safe() is Sunny - > TODO() }
sealed class Weather class Sunny : Weather() class Rainy : Weather() fun adjustSpeed(weather: Weather) = when (weather) { 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) } catch (e: NumberFormatException) { null }
Use try as expression fun tryParse(number: String) : Int? { val n = try { Integer.parseInt(number) } catch (e: NumberFormatException) { 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 class Person(val name: String?, val age: Int?) 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: $age") }
Use elvis operator as return and throw class Person(val name: String?, val age: Int?) fun processPerson(person: Person) { val name = person.name if (name = = null) throw IllegalArgumentException("Named required") val age = person.age if (age = = null) return println("$name: $age") }
Use elvis operator as return and throw class Person(val name: String?, val age: Int?) fun processPerson(person: Person) { val name = person.name if (name = = null) throw IllegalArgumentException("Named required") val age = person.age if (age = = null) return println("$name: $age") }
Use elvis operator as return and throw class Person(val name: String?, val age: Int?) fun processPerson(person: Person) { val name = person.name ? : throw IllegalArgumentException("Named required") val age = person.age ? : return println("$name: $age") }
Nullability
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
Consider using null-safe call val order = retrieveOrder() val city = order ? . customer ? . address ? . city
Consider using null-safe call val order = retrieveOrder() val city = order ? . customer ? . address ? . city ? : throw IllegalArgumentException("Invalid Order")
Avoid not-null assertions !! val order = retrieveOrder() 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
Avoid not-null assertions !! 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) } }
Avoid not-null assertions !! 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 { class State(val data: String) private lateinit var state: State @BeforeEach fun setup() { state = State("abc") } @Test fun foo() { assertEquals("abc", state.data) } } - use lateinit
Consider using ?.let for null-checks val order = retrieveOrder() if (order ! = null){ processCustomer(order.customer) }
Consider using ?.let for null-checks val order = retrieveOrder() if (order ! = null){ processCustomer(order.customer) } retrieveOrder() ? . let { processCustomer(it.customer) } retrieveOrder() ? . customer ? . let { : : processCustomer } or
Consider using ?.let for null-checks val order = retrieveOrder() if (order ! = null){ processCustomer(order.customer) } retrieveOrder() ? . let { processCustomer(it.customer) } No need for an extra variable retrieveOrder() ? . let { processCustomer(it.customer) } retrieveOrder() ? . customer ? . let { : : processCustomer } or
Consider using ?.let for null-checks val order = retrieveOrder() if (order ! = null){ processCustomer(order.customer) } retrieveOrder() ? . let { processCustomer(it.customer) } retrieveOrder() ? . let { processCustomer(it.customer) } 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 override fun equals(other: Any?) : Boolean { val command = other as Command return command.id = = id } override fun equals(other: Any?) : Boolean { 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 > = 'A' & & c < = 'Z'
Use range checks instead of comparison pairs fun isLatinUppercase(c: Char) = c in 'A' . . '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: ${args[i]}") } }
Ranges in loops fun main(args: Array<String>) { for (i in 0 . . args.size - 1) { println("$i: ${args[i]}") } }
Ranges in loops fun main(args: Array<String>) { for (i in 0 . . args.size - 1) { println("$i: ${args[i]}") } } for (i in 0 until args.size) { println("$i: ${args[i]}") }
Ranges in loops fun main(args: Array<String>) { for (i in 0 . . args.size - 1) { println("$i: ${args[i]}") } } for (i in 0 until args.size) { println("$i: ${args[i]}") } for (i in args.indices) { println("$i: ${args[i]}") }
Ranges in loops fun main(args: Array<String>) { for (i in 0 . . args.size - 1) { println("$i: ${args[i]}") } } for (i in 0 until args.size) { println("$i: ${args[i]}") } for (i in args.indices) { println("$i: ${args[i]}") } for ((i, arg) in args.withIndex()) { println("$i: $arg") }
Classes and Functions
Don’t create classes just to hold functions class StringUtils { companion object { fun isPhoneNumber(s: String) = s.length = = 7 & & s.all { it.isDigit() } } }
Don’t create classes just to hold functions class StringUtils { companion object { fun isPhoneNumber(s: String) = s.length = = 7 & & s.all { it.isDigit() } } } object StringUtils { fun isPhoneNumber(s: String) = s.length = = 7 & & s.all { it.isDigit() } }
Don’t create classes just to hold functions class StringUtils { companion object { fun isPhoneNumber(s: String) = s.length = = 7 & & s.all { it.isDigit() } } } object StringUtils { fun isPhoneNumber(s: String) = s.length = = 7 & & s.all { it.isDigit() } } fun isPhoneNumber(s: String) = s.length = = 7 & & s.all { it.isDigit() }
Use extension functions class StringUtils { companion object { fun isPhoneNumber(s: String) = s.length = = 7 & & s.all { it.isDigit() } } } object StringUtils { fun isPhoneNumber(s: String) = s.length = = 7 & & s.all { it.isDigit() } } fun isPhoneNumber(s: String) = s.length = = 7 & & s.all { it.isDigit() } 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) {} } fun main(args: Array<String>) { Phonebook().print("|") }
Use default values instead of overloading class Phonebook { fun print() { print(",") } fun print(columnSeparator: String) {} } fun main(args: Array<String>) { Phonebook().print("|") } class Phonebook { fun print(separator: String = ",") {} fun someFun(x: Int) {} } fun main(args: Array<String>) { 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" fun main(args: Array<String>) { val pair = namedNum() val number = pair.first val name = pair.second }
Return multiple values using data classes fun namedNum(): Pair<Int, String> = 1 to "one" / / same but shorter fun namedNum2() = 1 to "one" fun main(args: Array<String>) { 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") fun main(args: Array<String>) { val (rank, name) = namedNum() println("$name, rank $rank") }
Return multiple values using data classes data class GameResult( val rank: Int, val name: String ) fun namedNum() = GameResult(1, "Player 1") fun main(args: Array<String>) { val (rank, name) = namedNum() println("$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} - > ${item.value}") } }
Destructuring in loops fun printMap(map: Map<String, String>) { for (item in map.entries) { println("${item.key} - > ${item.value}") } } fun printMap(map: Map<String, String>) { for ((key, value) in map) { println("$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) }
Idiomatic kotlin
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 class EventDispatcher { fun addClickHandler(handler: (Event) - > Unit) {} fun removeClickHandler(handler: (Event) - > Unit) {} } typealias ClickHandler = (Event) - > Unit class EventDispatcher { fun addClickHandler(handler: ClickHandler) { } fun removeClickHandler(handler: ClickHandler) { } }
Standard Library
Verify parameters using require() class Person( val name: String?, val age: Int ) fun processPerson(person: Person) { if (person.age < 18) { throw IllegalArgumentException("Adult required") } }
Verify parameters using require() class Person( val name: String?, val age: Int ) fun processPerson(person: Person) { if (person.age < 18) { throw IllegalArgumentException("Adult required") } } fun processPerson(person: Person) { require(person.age > = 18) { "Adult required" } }
Select objects by type with filterIsInstance fun findAllStrings(objects: List<Any>) = objects.filter { it is String }
Select objects by type with filterIsInstance fun findAllStrings(objects: List<Any>) = objects.filter { it is String } fun findAllStrings(objects: List<Any>) = objects.filterIsInstance<String>()
Select objects by type with filterIsInstance fun findAllStrings(objects: List<Any>) : List<Any> = objects.filter { it is String } 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 ) 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 }) fun sortPersons(persons: List<Person>) = 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 ) fun analyzeLog(log: List<Request>) { val map = mutableMapOf<String, MutableList<Request > > () for (request in log) { map.getOrPut(request.url) { mutableListOf() } .add(request) } } fun analyzeLog(log: List<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 }
Initializing objects with apply 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);
Initializing objects with apply 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")
Initializing objects with apply 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") val client = ClientBuilder().apply { firstName = "Anton" lastName = "Arhipov" twitter = TwitterBuilder().apply { handle = "@antonarhipov" }.build() company = CompanyBuilder().apply { name = "JetBrains" city = "Tallinn" }.build() }.build() println("Created client is: $client")
Domain Specific Languages
“Domain Speci fi c”, i.e. tailored for a speci fi c task
“Domain Speci fi c”, i.e. tailored for a speci fi c task Examples: •Compose strings - stringBuilder •Create HTML documents - kotlinx.html •Con fi gure routing logic for a web app - ktor •Generally, build any object graphs. See “type-safe builders”
buildString //Java String name = "Joe"; StringBuilder sb = new StringBuilder(); for (int i = 0; i < 5; i++) { sb.append("Hello, "); sb.append(name); sb.append("!n"); } System.out.println(sb); //Kotlin val name = "Joe" val s = buildString { repeat(5) { append("Hello, ") append(name) appendLine("!") } } println(s)
kotlinx.html System.out.appendHTML().html { body { div { a("http: / / kotlinlang.org") { target = ATarget.blank +"Main site" } } } }
Ktor fun main() { embeddedServer(Netty, port = 8080, host = "0.0.0.0") { routing { get("/html-dsl") { call.respondHtml { body { h1 { +"HTML" } ul { for (n in 1 . . 10) { li { +"$n" } } } } } } } }.start(wait = true) }
Ktor fun main() { 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 { +"$n" } } } } } } } }.start(wait = true) } Ktor’s routing
Ktor fun main() { embeddedServer(Netty, port = 8080, host = "0.0.0.0") { routing { get("/html-dsl") { call.respondHtml { body { h1 { +"HTML" } ul { for (n in 1 . . 10) { li { +"$n" } } } } } } } }.start(wait = true) } kotlinx.html Ktor’s routing
Lambda with receiver T.() -> Unit
Build your vocabulary to abstract from scope functions val client = ClientBuilder().apply { firstName = "Anton" lastName = "Arhipov" twitter = TwitterBuilder().apply { handle = "@antonarhipov" }.build() company = CompanyBuilder().apply { name = "JetBrains" city = "Tallinn" }.build() }.build() println("Created client is: $client")
Build your vocabulary to abstract from scope functions fun client(c: ClientBuilder.() - > Unit): Client { val builder = ClientBuilder() 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() } val client = ClientBuilder().apply { firstName = "Anton" lastName = "Arhipov" twitter = TwitterBuilder().apply { handle = "@antonarhipov" }.build() company = CompanyBuilder().apply { name = "JetBrains" city = "Tallinn" }.build() }.build() println("Created client is: $client")
val client = client { firstName = "Anton" lastName = "Arhipov" twitter { handle = "@antonarhipov" } company { name = "JetBrains" city = "Tallinn" } } println("Created client is: $client") Build your vocabulary to abstract from scope functions val client = ClientBuilder().apply { firstName = "Anton" lastName = "Arhipov" twitter = TwitterBuilder().apply { handle = "@antonarhipov" }.build() company = CompanyBuilder().apply { name = "JetBrains" city = "Tallinn" }.build() }.build() println("Created client is: $client")
https://speakerdeck.com/antonarhipov https://github.com/antonarhipov/idiomatic-kotlin @antonarhipov

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Idiomatic kotlin

  • 2. Agenda • Expressions • Examples from standard library • DSL
  • 7. Idiomatic - using, containing, or denoting expressions that are natural to a native speaker
  • 8. 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
  • 11. fun adjustSpeed(weather: Weather): Drive { var result: Drive if (weather is Rainy) { result = Safe() } else { result = Calm() } return result }
  • 12. fun adjustSpeed(weather: Weather): Drive { var result: Drive if (weather is Rainy) { result = Safe() } else { result = Calm() } return result }
  • 13. fun adjustSpeed(weather: Weather): Drive { val result: Drive if (weather is Rainy) { result = Safe() } else { result = Calm() } return result }
  • 14. fun adjustSpeed(weather: Weather): Drive { val result: Drive if (weather is Rainy) { result = Safe() } else { result = Calm() } return result }
  • 15. fun adjustSpeed(weather: Weather): Drive { val result: Drive result = if (weather is Rainy) { Safe() } else { Calm() } return result }
  • 16. fun adjustSpeed(weather: Weather): Drive { val result: Drive result = if (weather is Rainy) { Safe() } else { Calm() } return result }
  • 17. fun adjustSpeed(weather: Weather): Drive { val result: Drive = if (weather is Rainy) { Safe() } else { Calm() } return result }
  • 18. fun adjustSpeed(weather: Weather): Drive { val result: Drive = if (weather is Rainy) { Safe() } else { Calm() } return result }
  • 19. fun adjustSpeed(weather: Weather): Drive { return if (weather is Rainy) { Safe() } else { Calm() } }
  • 20. fun adjustSpeed(weather: Weather): Drive { return if (weather is Rainy) { Safe() } else { Calm() } }
  • 21. fun adjustSpeed(weather: Weather): Drive = if (weather is Rainy) { Safe() } else { Calm() }
  • 22. fun adjustSpeed(weather: Weather): Drive = if (weather is Rainy) { Safe() } else { Calm() }
  • 23. fun adjustSpeed(weather: Weather) = if (weather is Rainy) { Safe() } else { Calm() }
  • 24. fun adjustSpeed(weather: Weather) = if (weather is Rainy) { Safe() } else { Calm() }
  • 25. fun adjustSpeed(weather: Weather) = if (weather is Rainy) Safe() else Calm()
  • 26. fun adjustSpeed(weather: Weather) = if (weather is Rainy) Safe() else Calm() fun adjustSpeed(weather: Weather): Drive { var result: Drive if (weather is Rainy) { result = Safe() } else { result = Calm() } return result }
  • 27. fun adjustSpeed(weather: Weather) = if (weather is Rainy) Safe() else Calm()
  • 28. fun adjustSpeed(weather: Weather) = if (weather is Rainy) Safe() else Calm()
  • 29. abstract class Weather class Sunny : Weather() class Rainy : Weather() fun adjustSpeed(weather: Weather) = when (weather) { is Rainy - > Safe() else - > Calm() }
  • 30. sealed class Weather class Sunny : Weather() class Rainy : Weather() fun adjustSpeed(weather: Weather) = when (weather) { is Rainy - > Safe() / / else - > Calm() }
  • 31. sealed class Weather class Sunny : Weather() class Rainy : Weather() fun adjustSpeed(weather: Weather) = when (weather) { is Rainy - > Safe() / / else - > Calm() }
  • 32. sealed class Weather class Sunny : Weather() class Rainy : Weather() fun adjustSpeed(weather: Weather) = when (weather) { is Rainy - > Safe() is Sunny - > TODO() }
  • 33. sealed class Weather class Sunny : Weather() class Rainy : Weather() fun adjustSpeed(weather: Weather) = when (weather) { is Rainy - > Safe() is Sunny - > TODO() } Use expressions! Use when as expression body Use sealed classes with when
  • 34. Use try as expression body fun tryParse(number: String) : Int? { try { return Integer.parseInt(number) } catch (e: NumberFormatException) { return null } }
  • 35. Use try as expression body fun tryParse(number: String) = try { Integer.parseInt(number) } catch (e: NumberFormatException) { null }
  • 36. Use try as expression fun tryParse(number: String) : Int? { val n = try { Integer.parseInt(number) } catch (e: NumberFormatException) { null } println(n) return n }
  • 37. Use elvis operator class Person(val name: String?, val age: Int?) val p = retrievePerson() ? : Person()
  • 38. Use elvis operator as return and throw class Person(val name: String?, val age: Int?) 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: $age") }
  • 39. Use elvis operator as return and throw class Person(val name: String?, val age: Int?) fun processPerson(person: Person) { val name = person.name if (name = = null) throw IllegalArgumentException("Named required") val age = person.age if (age = = null) return println("$name: $age") }
  • 40. Use elvis operator as return and throw class Person(val name: String?, val age: Int?) fun processPerson(person: Person) { val name = person.name if (name = = null) throw IllegalArgumentException("Named required") val age = person.age if (age = = null) return println("$name: $age") }
  • 41. Use elvis operator as return and throw class Person(val name: String?, val age: Int?) fun processPerson(person: Person) { val name = person.name ? : throw IllegalArgumentException("Named required") val age = person.age ? : return println("$name: $age") }
  • 43. 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
  • 44. Consider using null-safe call val order = retrieveOrder() val city = order ? . customer ? . address ? . city
  • 45. Consider using null-safe call val order = retrieveOrder() val city = order ? . customer ? . address ? . city ? : throw IllegalArgumentException("Invalid Order")
  • 46. Avoid not-null assertions !! val order = retrieveOrder() 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
  • 47. Avoid not-null assertions !! 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) } }
  • 48. Avoid not-null assertions !! 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 { class State(val data: String) private lateinit var state: State @BeforeEach fun setup() { state = State("abc") } @Test fun foo() { assertEquals("abc", state.data) } } - use lateinit
  • 49. Consider using ?.let for null-checks val order = retrieveOrder() if (order ! = null){ processCustomer(order.customer) }
  • 50. Consider using ?.let for null-checks val order = retrieveOrder() if (order ! = null){ processCustomer(order.customer) } retrieveOrder() ? . let { processCustomer(it.customer) } retrieveOrder() ? . customer ? . let { : : processCustomer } or
  • 51. Consider using ?.let for null-checks val order = retrieveOrder() if (order ! = null){ processCustomer(order.customer) } retrieveOrder() ? . let { processCustomer(it.customer) } No need for an extra variable retrieveOrder() ? . let { processCustomer(it.customer) } retrieveOrder() ? . customer ? . let { : : processCustomer } or
  • 52. Consider using ?.let for null-checks val order = retrieveOrder() if (order ! = null){ processCustomer(order.customer) } retrieveOrder() ? . let { processCustomer(it.customer) } retrieveOrder() ? . let { processCustomer(it.customer) } retrieveOrder() ? . customer ? . let { : : processCustomer } or
  • 53. Consider using safe cast for type checking override fun equals(other: Any?) : Boolean { val command = other as Command return command.id = = id }
  • 54. Consider using safe cast for type checking override fun equals(other: Any?) : Boolean { val command = other as Command return command.id = = id } override fun equals(other: Any?) : Boolean { return (other as? Command) ? . id = = id }
  • 55. Use range checks instead of comparison pairs fun isLatinUppercase(c: Char) = c > = 'A' & & c < = 'Z'
  • 56. Use range checks instead of comparison pairs fun isLatinUppercase(c: Char) = c > = 'A' & & c < = 'Z'
  • 57. Use range checks instead of comparison pairs fun isLatinUppercase(c: Char) = c in 'A' . . 'Z'
  • 58. Use range checks instead of comparison pairs fun isLatinUppercase(c: Char) = c in 'A' . . 'Z'
  • 59. Ranges in loops fun main(args: Array<String>) { for (i in 0 . . args.size - 1) { println("$i: ${args[i]}") } }
  • 60. Ranges in loops fun main(args: Array<String>) { for (i in 0 . . args.size - 1) { println("$i: ${args[i]}") } }
  • 61. Ranges in loops fun main(args: Array<String>) { for (i in 0 . . args.size - 1) { println("$i: ${args[i]}") } } for (i in 0 until args.size) { println("$i: ${args[i]}") }
  • 62. Ranges in loops fun main(args: Array<String>) { for (i in 0 . . args.size - 1) { println("$i: ${args[i]}") } } for (i in 0 until args.size) { println("$i: ${args[i]}") } for (i in args.indices) { println("$i: ${args[i]}") }
  • 63. Ranges in loops fun main(args: Array<String>) { for (i in 0 . . args.size - 1) { println("$i: ${args[i]}") } } for (i in 0 until args.size) { println("$i: ${args[i]}") } for (i in args.indices) { println("$i: ${args[i]}") } for ((i, arg) in args.withIndex()) { println("$i: $arg") }
  • 65. Don’t create classes just to hold functions class StringUtils { companion object { fun isPhoneNumber(s: String) = s.length = = 7 & & s.all { it.isDigit() } } }
  • 66. Don’t create classes just to hold functions class StringUtils { companion object { fun isPhoneNumber(s: String) = s.length = = 7 & & s.all { it.isDigit() } } } object StringUtils { fun isPhoneNumber(s: String) = s.length = = 7 & & s.all { it.isDigit() } }
  • 67. Don’t create classes just to hold functions class StringUtils { companion object { fun isPhoneNumber(s: String) = s.length = = 7 & & s.all { it.isDigit() } } } object StringUtils { fun isPhoneNumber(s: String) = s.length = = 7 & & s.all { it.isDigit() } } fun isPhoneNumber(s: String) = s.length = = 7 & & s.all { it.isDigit() }
  • 68. Use extension functions class StringUtils { companion object { fun isPhoneNumber(s: String) = s.length = = 7 & & s.all { it.isDigit() } } } object StringUtils { fun isPhoneNumber(s: String) = s.length = = 7 & & s.all { it.isDigit() } } fun isPhoneNumber(s: String) = s.length = = 7 & & s.all { it.isDigit() } fun String.isPhoneNumber() = length = = 7 & & all { it.isDigit() }
  • 69. 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.
  • 70. Use default values instead of overloading class Phonebook { fun print() { print(",") } fun print(columnSeparator: String) {} } fun main(args: Array<String>) { Phonebook().print("|") }
  • 71. Use default values instead of overloading class Phonebook { fun print() { print(",") } fun print(columnSeparator: String) {} } fun main(args: Array<String>) { Phonebook().print("|") } class Phonebook { fun print(separator: String = ",") {} fun someFun(x: Int) {} } fun main(args: Array<String>) { Phonebook().print(separator = "|") }
  • 72. Return multiple values using data classes fun namedNum(): Pair<Int, String> = 1 to "one" / / same but shorter fun namedNum2() = 1 to "one" fun main(args: Array<String>) { val pair = namedNum() val number = pair.first val name = pair.second }
  • 73. Return multiple values using data classes fun namedNum(): Pair<Int, String> = 1 to "one" / / same but shorter fun namedNum2() = 1 to "one" fun main(args: Array<String>) { 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") fun main(args: Array<String>) { val (rank, name) = namedNum() println("$name, rank $rank") }
  • 74. Return multiple values using data classes data class GameResult( val rank: Int, val name: String ) fun namedNum() = GameResult(1, "Player 1") fun main(args: Array<String>) { val (rank, name) = namedNum() println("$name, rank $rank") } GameResult var1 = namedNum(); int var2 = var1.component1(); String var3 = var1.component2();
  • 75. Destructuring in loops fun printMap(map: Map<String, String>) { for (item in map.entries) { println("${item.key} - > ${item.value}") } }
  • 76. Destructuring in loops fun printMap(map: Map<String, String>) { for (item in map.entries) { println("${item.key} - > ${item.value}") } } fun printMap(map: Map<String, String>) { for ((key, value) in map) { println("$key - > $value") } }
  • 77. 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) }
  • 79. Use type aliases for functional types class Event class EventDispatcher { fun addClickHandler(handler: (Event) - > Unit) {} fun removeClickHandler(handler: (Event) - > Unit) {} }
  • 80. Use type aliases for functional types class Event class EventDispatcher { fun addClickHandler(handler: (Event) - > Unit) {} fun removeClickHandler(handler: (Event) - > Unit) {} } typealias ClickHandler = (Event) - > Unit class EventDispatcher { fun addClickHandler(handler: ClickHandler) { } fun removeClickHandler(handler: ClickHandler) { } }
  • 82. Verify parameters using require() class Person( val name: String?, val age: Int ) fun processPerson(person: Person) { if (person.age < 18) { throw IllegalArgumentException("Adult required") } }
  • 83. Verify parameters using require() class Person( val name: String?, val age: Int ) fun processPerson(person: Person) { if (person.age < 18) { throw IllegalArgumentException("Adult required") } } fun processPerson(person: Person) { require(person.age > = 18) { "Adult required" } }
  • 84. Select objects by type with filterIsInstance fun findAllStrings(objects: List<Any>) = objects.filter { it is String }
  • 85. Select objects by type with filterIsInstance fun findAllStrings(objects: List<Any>) = objects.filter { it is String } fun findAllStrings(objects: List<Any>) = objects.filterIsInstance<String>()
  • 86. Select objects by type with filterIsInstance fun findAllStrings(objects: List<Any>) : List<Any> = objects.filter { it is String } fun findAllStrings(objects: List<Any>) : List<String> = objects.filterIsInstance<String>()
  • 87. 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 }
  • 88. 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 })
  • 89. 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 }) fun sortPersons(persons: List<Person>) = persons.sortedWith(compareBy(Person : : name, Person : : age))
  • 90. 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) } }
  • 91. 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) } } fun analyzeLog(log: List<Request>) { val map = log.groupBy(Request : : url) }
  • 92. 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) }
  • 93. 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 }
  • 94. Initializing objects with apply 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);
  • 95. Initializing objects with apply 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")
  • 96. Initializing objects with apply 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") val client = ClientBuilder().apply { firstName = "Anton" lastName = "Arhipov" twitter = TwitterBuilder().apply { handle = "@antonarhipov" }.build() company = CompanyBuilder().apply { name = "JetBrains" city = "Tallinn" }.build() }.build() println("Created client is: $client")
  • 98. “Domain Speci fi c”, i.e. tailored for a speci fi c task
  • 99. “Domain Speci fi c”, i.e. tailored for a speci fi c task Examples: •Compose strings - stringBuilder •Create HTML documents - kotlinx.html •Con fi gure routing logic for a web app - ktor •Generally, build any object graphs. See “type-safe builders”
  • 100. buildString //Java String name = "Joe"; StringBuilder sb = new StringBuilder(); for (int i = 0; i < 5; i++) { sb.append("Hello, "); sb.append(name); sb.append("!n"); } System.out.println(sb); //Kotlin val name = "Joe" val s = buildString { repeat(5) { append("Hello, ") append(name) appendLine("!") } } println(s)
  • 101. kotlinx.html System.out.appendHTML().html { body { div { a("http: / / kotlinlang.org") { target = ATarget.blank +"Main site" } } } }
  • 102. Ktor fun main() { embeddedServer(Netty, port = 8080, host = "0.0.0.0") { routing { get("/html-dsl") { call.respondHtml { body { h1 { +"HTML" } ul { for (n in 1 . . 10) { li { +"$n" } } } } } } } }.start(wait = true) }
  • 103. 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 { +"$n" } } } } } } } }.start(wait = true) } Ktor’s routing
  • 104. Ktor fun main() { embeddedServer(Netty, port = 8080, host = "0.0.0.0") { routing { get("/html-dsl") { call.respondHtml { body { h1 { +"HTML" } ul { for (n in 1 . . 10) { li { +"$n" } } } } } } } }.start(wait = true) } kotlinx.html Ktor’s routing
  • 106. Build your vocabulary to abstract from scope functions val client = ClientBuilder().apply { firstName = "Anton" lastName = "Arhipov" twitter = TwitterBuilder().apply { handle = "@antonarhipov" }.build() company = CompanyBuilder().apply { name = "JetBrains" city = "Tallinn" }.build() }.build() println("Created client is: $client")
  • 107. Build your vocabulary to abstract from scope functions fun client(c: ClientBuilder.() - > Unit): Client { val builder = ClientBuilder() 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() } val client = ClientBuilder().apply { firstName = "Anton" lastName = "Arhipov" twitter = TwitterBuilder().apply { handle = "@antonarhipov" }.build() company = CompanyBuilder().apply { name = "JetBrains" city = "Tallinn" }.build() }.build() println("Created client is: $client")
  • 108. val client = client { firstName = "Anton" lastName = "Arhipov" twitter { handle = "@antonarhipov" } company { name = "JetBrains" city = "Tallinn" } } println("Created client is: $client") Build your vocabulary to abstract from scope functions val client = ClientBuilder().apply { firstName = "Anton" lastName = "Arhipov" twitter = TwitterBuilder().apply { handle = "@antonarhipov" }.build() company = CompanyBuilder().apply { name = "JetBrains" city = "Tallinn" }.build() }.build() println("Created client is: $client")