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@antonarhipov Idiomatic Kotlin

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Top-Level Functions & Extensions Scope Functions Default argument values and named parameters Expressions Null-safety Type-safe builders, a.k.a DSL

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Top-Level (Extension) Functions

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fun main() { doSomething() } fun doSomething() { doMoreStuff( : : finishWork) } fun doMoreStuff(callback: () -> Unit) { callback() } fun finishWork() { TODO("Not implemented yet") }

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fun main() { doSomething() } fun doSomething() { doMoreStuff( : : finishWork) } fun doMoreStuff(callback: () -> Unit) { callback() } fun finishWork() { TODO("Not implemented yet") }

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fun main() { doSomething() } fun doSomething() { doMoreStuff( : : finishWork) } fun doMoreStuff(callback: () -> Unit) { callback() } fun finishWork() { TODO("Not implemented yet") }

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fun main() { doSomething() } fun doSomething() { doMoreStuff( : : finishWork) } fun doMoreStuff(callback: () -> Unit) { callback() } fun finishWork() { TODO("Not implemented yet") } Just functions, no classes!

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Expressions & composition

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fun stopProducer() { if (this : : producer.isInitialized) { runBlocking { runCatching { producer.close() }.onFailure { println("failed to close queue producer: $it") } } } } fun stopBroker() { if (this : : broker.isInitialized) { runBlocking { runCatching { broker.close() }.onFailure { println("failed to close queue producer: $it") } } } } fun stopService() { if (this : : service.isInitialized) { runBlocking { runCatching { service.close() }.onFailure { println("failed to close queue producer: $it") } } } }

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fun stopProducer() { if (this : : producer.isInitialized) { runBlocking { runCatching { producer.close() }.onFailure { println("failed to close queue producer: $it") } } } } fun stopBroker() { if (this : : broker.isInitialized) { runBlocking { runCatching { broker.close() }.onFailure { println("failed to close queue producer: $it") } } } } fun stopService() { if (this : : service.isInitialized) { runBlocking { runCatching { service.close() }.onFailure { println("failed to close queue producer: $it") } } } } Duplicates

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fun stopProducer() { if (this : : producer.isInitialized) { runBlocking { runCatching { producer.close() }.onFailure { println("failed to close queue producer: $it") } } } } fun stopBroker() { if (this : : broker.isInitialized) { runBlocking { runCatching { broker.close() }.onFailure { println("failed to close queue producer: $it") } } } } fun stopService() { if (this : : service.isInitialized) { runBlocking { runCatching { service.close() }.onFailure { println("failed to close queue producer: $it") } } } } Di ff erent objects without common interface

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fun stopResource(predicate: () - > Boolean, close: suspend () -> Unit) { if (predicate()) { runBlocking { runCatching { close() }.onFailure { println("failed to close the resource: $it") } } } }

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fun stopResource(predicate: () - > Boolean, close: suspend () -> Unit) { if (predicate()) { runBlocking { runCatching { close() }.onFailure { println("failed to close the resource: $it") } } } }

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fun stopResource(predicate: () - > Boolean, close: suspend () -> Unit) { if (predicate()) { runBlocking { runCatching { close() }.onFailure { println("failed to close the resource: $it") } } } }

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Don’t create classes just to hold functions!

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class StringUtils { companion object { fun isPhoneNumber(s: String) = s.length = = 7 & & s.all { it.isDigit() } } }

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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() } }

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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() }

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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() }

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Extension or a member? https://kotlinlang.org/docs/coding-conventions.html#extension-functions Use extension functions liberally Restrict the visibility to minimize API pollution As necessary, use local extension functions, member extension functions, or top-level extension functions with private visibility

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+

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fun findMessageById(id: String) = db.query( "select * from messages where id = ?", RowMapper { rs, _ -> Message(rs.getString("id"), rs.getString("text")) }, id ) val db: JdbcTemplate = ...

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fun findMessageById(id: String) = db.query( "select * from messages where id = ?", RowMapper { rs, _ -> Message(rs.getString("id"), rs.getString("text")) }, id ) @Override public List query(String sql, RowMapper rowMapper, @Nullable Object . .. args) throws DataAccessException { return result(query(sql, args, new RowMapperResultSetExtractor < > (rowMapper))); }

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fun findMessageById(id: String) = db.query( "select * from messages where id = ?", RowMapper { rs, _ -> Message(rs.getString("id"), rs.getString("text")) }, id )

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fun findMessageById(id: String) = db.query( "select * from messages where id = ?", RowMapper { rs, _ -> Message(rs.getString("id"), rs.getString("text")) }, id )

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fun findMessageById(id: String) = db.query( "select * from messages where id = ?", id, RowMapper { rs, _ -> Message(rs.getString("id"), rs.getString("text")) } )

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fun findMessageById(id: String) = db.query( "select * from messages where id = ?", id, { rs, _ -> Message(rs.getString("id"), rs.getString("text")) } ) SAM conversion

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fun findMessageById(id: String) = db.query( "select * from messages where id = ?", id) { rs, _ -> Message(rs.getString("id"), rs.getString("text")) } Trailing lambda parameter

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fun findMessageById(id: String) = db.query("select * from messages where id = ?", id ) { rs, _ -> Message(rs.getString("id"), rs.getString("text")) }

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fun findMessageById(id: String) = db.query("select * from messages where id = ?", id ) { rs, _ -> Message(rs.getString("id"), rs.getString("text")) } fun JdbcOperations.query(sql: String, vararg args: Any, function: (ResultSet, Int) -> T): List = query(sql, RowMapper { rs, i -> function(rs, i) }, *args) Extension function!

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Functions, Extensions, Single-expression functions, Trailing lambda, Varargs, SAM conversion

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Scope functions

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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

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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 }

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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 } public inline fun T.apply(block: T.() -> Unit): T { block() return this }

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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 } public inline fun T.apply(block: T.() -> Unit): T { block() return this } Lambda with receiver

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? . let val order = retrieveOrder() if (order != null){ processCustomer(order.customer) }

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val order = retrieveOrder() if (order != null){ processCustomer(order.customer) } retrieveOrder() ?. let { processCustomer(it.customer) } retrieveOrder() ?. customer ?. let( :: processCustomer) or ? . let

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let() as a helper for a complex condition if (some.complex.expression.let { it is Type && it.has.some.property }) { . .. }

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let() as a helper for a complex condition if (some.complex.expression.let { it is Type && it.has.some.property }) { . .. }

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let() as a helper for a complex condition if (some.complex.expression.let { it is Type && it.has.some.property }) { . .. }

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if (retrieveOrder().let { it is Subscription && it.customer.name == "Anton"}) { . .. } let() as a helper for a complex condition if (some.complex.expression.let { it is Type && it.has.some.property }) { . .. }

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fun makeDir(path: String) = path.let { File(it) }.also { it.mkdirs() }

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fun makeDir(path: String) = path.let { File(it) }.also { it.mkdirs() } Don’t overuse the scope functions!

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fun makeDir(path: String) = path.let { File(it) }.also { it.mkdirs() } fun makeDir(path: String) : File { val file = File(path) file.mkdirs() return file } This is simpler! Don’t overuse the scope functions!

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fun makeDir(path: String) = path.let { File(it) }.also { it.mkdirs() } fun makeDir(path: String) : File { val file = File(path) file.mkdirs() return file } fun makeDir(path: String) = File(path).also { it.mkdirs() } OK, this one is actually fi ne :) This is simpler! Don’t overuse the scope functions!

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Default argument values and named parameters

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fun find(name: String){ find(name, true) } fun find(name: String, recursive: Boolean){ } Function overloading

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fun find(name: String){ find(name, true) } fun find(name: String, recursive: Boolean){ } fun find(name: String, recursive: Boolean = true){ } Default argument value Function overloading

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fun find(name: String){ find(name, true) } fun find(name: String, recursive: Boolean){ } fun find(name: String, recursive: Boolean = true){ } fun main() { find("myfile.txt") } Default argument value Function overloading

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class Figure( val width: Int = 1, val height: Int = 1, val depth: Int = 1, color: Color = Color.BLACK, description: String = "This is a 3d figure", ) Figure(Color.RED, "Red figure")

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class Figure( val width: Int = 1, val height: Int = 1, val depth: Int = 1, color: Color = Color.BLACK, description: String = "This is a 3d figure", ) Figure(Color.RED, "Red figure") Compilation error

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class Figure( val width: Int = 1, val height: Int = 1, val depth: Int = 1, color: Color = Color.BLACK, description: String = "This is a 3d figure", ) Figure(color = Color.RED, description = "Red figure")

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Default argument values diminish the need for overloading in most cases

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Named parameters is a necessary tool for working with default argument values Default argument values diminish the need for overloading in most cases

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Expressions if when try

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fun adjustSpeed(weather: Weather): Drive { val result: Drive if (weather is Rainy) { result = Safe() } else { result = Calm() } return result } Let’s transform this code using Alt+Enter

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fun adjustSpeed(weather: Weather): Drive { val result: Drive if (weather is Rainy) { result = Safe() } else { result = Calm() } return result }

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fun adjustSpeed(weather: Weather): Drive { val result: Drive = if (weather is Rainy) { Safe() } else { Calm() } return result }

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fun adjustSpeed(weather: Weather): Drive { val result: Drive = if (weather is Rainy) { Safe() } else { Calm() } return result }

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fun adjustSpeed(weather: Weather): Drive { return if (weather is Rainy) { Safe() } else { Calm() } }

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fun adjustSpeed(weather: Weather): Drive { return if (weather is Rainy) { Safe() } else { Calm() } }

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

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

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

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

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fun adjustSpeed(weather: Weather) = if (weather is Rainy) Safe() else Calm() Is it concise? Sure!

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fun adjustSpeed(weather: Weather) = if (weather is Rainy) Safe() else Calm() Is it readable? It depends!

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fun adjustSpeed(weather: Weather) = if (weather is Rainy) Safe() else Calm() What does the function return?

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fun adjustSpeed(weather: Weather): Drive = ... fun adjustSpeed(weather: Weather) = ... For public API, keep the return type in the signature For private API it is generally OK to use type inference

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

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abstract class Weather class Sunny : Weather() class Rainy : Weather() fun adjustSpeed(weather: Weather) = when (weather) { is Rainy -> Safe() else -> Calm() }

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sealed class Weather class Sunny : Weather() class Rainy : Weather() fun adjustSpeed(weather: Weather) = when (weather) { is Rainy -> Safe() / / else -> Calm() }

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sealed class Weather class Sunny : Weather() class Rainy : Weather() fun adjustSpeed(weather: Weather) = when (weather) { is Rainy -> Safe() / / else -> Calm() }

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sealed class Weather class Sunny : Weather() class Rainy : Weather() fun adjustSpeed(weather: Weather) = when (weather) { is Rainy -> Safe() is Sunny -> TODO() }

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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 expression

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The when expression is your friend!

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val condition: Boolean = expression() when(condition) { true -> doThis() false -> doThat() } if (condition) { doThis() } else { doThat() } if when

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Use try as expression body fun tryParse(number: String) : Int? { try { return Integer.parseInt(number) } catch (e: NumberFormatException) { return null } }

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fun tryParse(number: String) = try { Integer.parseInt(number) } catch (e: NumberFormatException) { null } Use try as expression body

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Null-safety

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class Nullable { fun someFunction(){} } fun createNullable(): Nullable? = null fun main() { val n: Nullable? = createNullable() n.someFunction() }

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class Nullable { fun someFunction(){} } fun createNullable(): Nullable? = null fun main() { val n: Nullable? = createNullable() n.someFunction() }

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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

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Consider using null-safe call val order = retrieveOrder() val city = order ?. customer ? . address ?. city

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val order = retrieveOrder() val city = order ?. customer ? . address ?. city ?: throw IllegalArgumentException("Invalid Order") Consider using null-safe call

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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

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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) } }

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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

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

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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") }

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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") }

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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") }

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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") }

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Consider using safe cast for type checking override fun equals(other: Any?) : Boolean { val command = other as Command return command.id == id }

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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 }

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Domain Specific Languages

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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)

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kotlinx.html System.out.appendHTML().html { body { div { a("http: // kotlinlang.org") { target = ATarget.blank +"Main site" } } } }

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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) }

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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

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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

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Lambda with receiver T.() -> Unit

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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);

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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")

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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")

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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")

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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")

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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") 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() }

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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 person = person {

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https://speakerdeck.com/antonarhipov @antonarhipov https://github.com/antonarhipov