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Kotlin Standard Library Gems @antonarhipov

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Concise & modern Coroutines Multiplatform Nice stdlib Null-safety

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Collections Scope functions Builders eXtras

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Collections

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listOf("#1", "#2", "#3") [#1, #2, #3]

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listOf("#1", "#2", "#3") List(3) { "#${it + 1}" } [#1, #2, #3] [#1, #2, #3]

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listOf("#1", "#2", "#3") List(3) { "#${it + 1}" } val ml = MutableList(5) { "?" } ml[0] = "a" ml[2] = "c" [#1, #2, #3] [#1, #2, #3] [a, ?, c, ?, ?]

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listOf("#1", "#2", "#3") List(3) { "#${it + 1}" } val ml = MutableList(5) { "?" } ml[0] = "a" ml[2] = "c" [#1, #2, #3] [#1, #2, #3] [a, ?, c, ?, ?] ml.joinToString("") a?c ? ?

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listOf("#1", "#2", "#3") List(3) { "#${it + 1}" } val ml = MutableList(5) { "?" } ml[0] = "a" ml[2] = "c" ml.joinToString( separator = "-", prefix = " << ", postfix = " >> ", limit = 3, truncated = "etc." ){ "#$it" } [#1, #2, #3] [#1, #2, #3] [a, ?, c, ?, ?] < < #a- #? -#c-etc. >> ml.joinToString("") a?c ? ?

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val students = listOf("Ann", "Joe", "Max", "Anton", "John", "Bo", "Ken").shuffled() println(students.chunked(2)) println(students.windowed(3))

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val students = listOf("Ann", "Joe", "Max", "Anton", "John", "Bo", "Ken").shuffled() println(students.chunked(2)) println(students.windowed(3)) [[John, Ann], [Joe, Bo], [Max, Anton], [Ken]] [[John, Ann, Joe], [Ann, Joe, Bo], [Joe, Bo, Max], [Bo, Max, Anton], [Max, Anton, Ken]]

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val students = listOf("Ann", "Joe", "Max", "Anton", "John", "Bo", "Ken").shuffled() println(students.chunked(2)) println(students.windowed(3)) [[John, Ann], [Joe, Bo], [Max, Anton], [Ken]] [[John, Ann, Joe], [Ann, Joe, Bo], [Joe, Bo, Max], [Bo, Max, Anton], [Max, Anton, Ken]] listOf(11, 10, 12, 9, 10, 15).windowed(3) { it.average() } [11.0, 12.0, 12.0, 13.0]

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val cities = listOf("Amsterdam", "Paris", "New York") val countries = listOf("NL", "FR", "US") val citiesToCountries = cities zip countries [(Amsterdam, NL), (Paris, FR), (New York, US)]

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val cities = listOf("Amsterdam", "Paris", "New York") val countries = listOf("NL", "FR", "US") val citiesToCountries = cities zip countries val (first, second) = citiesToCountries.unzip() [(Amsterdam, NL), (Paris, FR), (New York, US)]

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val cities = listOf("Amsterdam", "Paris", "New York") val countries = listOf("NL", "FR", "US") val citiesToCountries = cities zip countries val (first, second) = citiesToCountries.unzip() [(Amsterdam, NL), (Paris, FR), (New York, US)] listOf(1, 2, 3, 4, 5).zipWithNext { a, b -> a + b } [3, 5, 7, 9] listOf(1, 2, 3, 4, 5).zipWithNext() [(1, 2), (2, 3), (3, 4), (4, 4)]

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youtube.com/kotlin

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abcd bcde cdef defg xya xyb xyc Count the total of characters that are present on each line in every group of strings

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abcd bcde cdef defg xya xyb xyc Group 1: Group 2: Count the total of characters that are present on each line in every group of strings

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abcd bcde cdef defg xya xyb xyc Group 1: Group 2: Count the total of characters that are present on each line in every group of strings

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abcd bcde cdef defg xya xyb xyc Group 1: Group 2: {d} {xy} Count the total of characters that are present on each line in every group of strings

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abcd bcde cdef defg xya xyb xyc Group 1: Group 2: {d} {xy} count = 1 count = 2 Count the total of characters that are present on each line in every group of strings

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abcd bcde cdef defg xya xyb xyc Group 1: Group 2: {d} {xy} count = 1 count = 2 Total = 3 Count the total of characters that are present on each line in every group of strings

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val input = """ abcd bcde cdef defg xya xyb xyc """.trimIndent()

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val input = """ abcd bcde cdef defg xya xyb xyc """.trimIndent() val groups: List = input.split("\n\n")

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val input = """ abcd bcde cdef defg xya xyb xyc """.trimIndent() val groups: List = input.split("\n\n")

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val input = """ abcd bcde cdef defg xya xyb xyc """.trimIndent() val groups: List = input.split("\n\n") var total = 0 for (group in groups) { val listOfSets: List> = group.split("\n").map(String :: toSet) var result = listOfSets.first() for (set in listOfSets) { result = result intersect set } total += result.count() }

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val input = """ abcd bcde cdef defg xya xyb xyc """.trimIndent() val groups: List = input.split("\n\n") var total = 0 for (group in groups) { val listOfSets: List> = group.split("\n").map(String :: toSet) var result = listOfSets.first() for (set in listOfSets) { result = result intersect set } total += result.count() }

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val input = """ abcd bcde cdef defg xya xyb xyc """.trimIndent() val groups: List = input.split("\n\n") var total = 0 for (group in groups) { val listOfSets: List> = group.split("\n").map(String :: toSet) var result = listOfSets.first() for (set in listOfSets) { result = result intersect set } total += result.count() }

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val input = """ abcd bcde cdef defg xya xyb xyc """.trimIndent() val groups: List = input.split("\n\n") var total = 0 for (group in groups) { val listOfSets: List> = group.split("\n").map(String :: toSet) var result = listOfSets.first() for (set in listOfSets) { result = result intersect set } total += result.count() }

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val input = """ abcd bcde cdef defg xya xyb xyc """.trimIndent() val groups: List = input.split("\n\n") var total = 0 for (group in groups) { val listOfSets: List> = group.split("\n").map(String :: toSet) var result = listOfSets.first() for (set in listOfSets) { result = result intersect set } total += result.count() } Transforming data

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val input = """ abcd bcde cdef defg xya xyb xyc """.trimIndent() val groups: List = input.split("\n\n") var total = 0 for (group in groups) { val listOfSets: List> = group.split("\n").map(String :: toSet) var result = listOfSets.first() for (set in listOfSets) { result = result intersect set } total += result.count() } Transforming data Calculating the result

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val groups: List = input.split("\n\n")

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val groups: List = input.split("\n\n") // List>> val step1 = groups.map { it.split("\n").map(String :: toSet) } Transforming data

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val groups: List = input.split("\n\n") // List>> val step1 = groups.map { it.split("\n").map(String :: toSet) } Transforming data

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val groups: List = input.split("\n\n") // List>> val step1 = groups.map { it.split("\n").map(String :: toSet) } Transforming data

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val groups: List = input.split("\n\n") // List>> val step1 = groups.map { it.split("\n").map(String :: toSet) } Transforming data

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val groups: List = input.split("\n\n") // List>> val step1 = groups.map { it.split("\n").map(String :: toSet) } Transforming data

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val groups: List = input.split("\n\n") // List>> val step1 = groups.map { it.split("\n").map(String :: toSet) } val step2 = step1.sumOf { it.reduce { a, b - > a intersect b }.count() } Transforming data Calculating the result

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val groups: List = input.split("\n\n") // List>> val step1 = groups.map { it.split("\n").map(String :: toSet) } val step2 = step1.sumOf { it.reduce { a, b - > a intersect b }.count() } Transforming data Calculating the result

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val groups: List = input.split("\n\n") // List>> val step1 = groups.map { it.split("\n").map(String :: toSet) } val step2 = step1.sumOf { it.reduce { a, b - > a intersect b }.count() } Transforming data Calculating the result

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val groups: List = input.split("\n\n") // List>> val step1 = groups.map { it.split("\n").map(String :: toSet) } val step2 = step1.sumOf { it.reduce { a, b - > a intersect b }.count() } Transforming data Calculating the result

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val groups: List = input.split("\n\n") // List>> val step1 = groups.map { it.split("\n").map(String :: toSet) } val step2 = step1.sumOf { it.reduce { a, b - > a intersect b }.count() } Transforming data Calculating the result

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val groups: List = input.split("\n\n") // List>> val step1 = groups.map { it.split("\n").map(String :: toSet) } val step2 = step1.sumOf { it.reduce { a, b - > a intersect b }.count() } Transforming data Calculating the result groups.map { group -> group.split(nl).map(String :: toSet) }.sumOf { answerSets -> answerSets.reduce { a, b -> a intersect b }.count() }

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fold & reduce

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listOf(1, 2, 3, 4, 5).fold(10) { a, b - > a + b } listOf(1, 2, 3, 4, 5).reduce { a, b - > a + b } listOf("#1", "#2", "#3").fold("#0") { a, b - > a + b } listOf("#1", "#2", "#3").reduce { a, b -> a + b } / / 25 / / 15 / / #0#1#2#3 / / #1#2#3

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listOf(1, 2, 3, 4, 5).fold(10) { a, b - > a + b } / / 25 How does this work? How do I trace the logic of this operation? What are the results intermediate steps?

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listOf(1, 2, 3, 4, 5).fold(10) { a, b - > a + b } / / 25 runningFold runningReduce

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listOf(1, 2, 3, 4, 5).runningFold(10) { a, b -> a + b } / / [10, 11, 13, 16, 20, 25] listOf(1, 2, 3, 4, 5).runningReduce { a, b - > a + b } / / [1, 3, 6, 10, 15] listOf("#1", "#2", "#3").runningReduce { a, b -> a + b } / / [#1, #1#2, #1#2#3]

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Scope functions

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val result: = x.foo { } Return type Original object (x) Result of lambda Access to X it this

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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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Builders

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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 The essential language feature for the type-safe builders, a.k.a. DSL

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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 { firstName = "Anton" lastName = "Arhipov" twitter { handle = "@antonarhipov" } company { name = "JetBrains" city = "Tallinn" } } println("Hello, $person!")

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eXtras

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Select objects by type with filterIsInstance fun findAllStrings(objects: List) = objects.filter { it is String }

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Select objects by type with filterIsInstance fun findAllStrings(objects: List) = objects.filter { it is String } fun findAllStrings(objects: List) = objects.filterIsInstance()

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Select objects by type with filterIsInstance fun findAllStrings(objects: List): List = objects.filter { it is String } fun findAllStrings(objects: List): List = objects.filterIsInstance()

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compareBy compares by multiple keys class Person( val name: String, val age: Int ) fun sortPersons(persons: List) = persons.sortedWith(Comparator { person1, person2 -> val rc = person1.name.compareTo(person2.name) if (rc != 0) rc else person1.age - person2.age })

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class Person( val name: String, val age: Int ) fun sortPersons(persons: List) = persons.sortedWith(Comparator { person1, person2 -> val rc = person1.name.compareTo(person2.name) if (rc != 0) rc else person1.age - person2.age }) fun sortPersons(persons: List) = persons.sortedWith(compareBy(Person :: name, Person :: age)) compareBy compares by multiple keys

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groupBy to group elements class Request( val url: String, val remoteIP: String, val timestamp: Long ) fun analyzeLog(log: List) { val map = mutableMapOf> () for (request in log) { map.getOrPut(request.url) { mutableListOf() } .add(request) } }

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class Request( val url: String, val remoteIP: String, val timestamp: Long ) fun analyzeLog(log: List) { val map = mutableMapOf> () for (request in log) { map.getOrPut(request.url) { mutableListOf() } .add(request) } } fun analyzeLog(log: List) { val map = log.groupBy(Request :: url) } groupBy to group elements

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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) }

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