Kotlin Standard Library Gems

Kotlin's standard library has a lot of useful functionality to offer. In this presentation, we will unearth some hidden gems that will come in handy the next time you write Kotlin code. Because if you know how to wield it, the Kotlin standard library is a powerful tool that can help you be more productive in solving your problems and be more expressive in your code.

Transcript

1. Kotlin Standard Library Gems @antonarhipov
2. Concise & modern Coroutines Multiplatform Nice stdlib Null-safety
3. Collections Scope functions Builders eXtras
4. Collections
6. listOf("#1", "#2", "#3") [#1, #2, #3]
7. listOf("#1", "#2", "#3") List(3) { "#${it + 1}" } [#1, #2, #3] [#1, #2, #3]
8. listOf("#1", "#2", "#3") List(3) { "#${it + 1}" } val ml = MutableList(5) { "?" } ml[0] = "a" ml[2] = "c" [#1, #2, #3] [#1, #2, #3] [a, ?, c, ?, ?]
9. listOf("#1", "#2", "#3") List(3) { "#${it + 1}" } val ml = MutableList(5) { "?" } ml[0] = "a" ml[2] = "c" [#1, #2, #3] [#1, #2, #3] [a, ?, c, ?, ?] ml.joinToString("") a?c ? ?
10. listOf("#1", "#2", "#3") List(3) { "#${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 ? ?
11. val students = listOf("Ann", "Joe", "Max", "Anton", "John", "Bo", "Ken").shuffled() println(students.chunked(2)) println(students.windowed(3))
12. 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]]
13. 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]
14. 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)]
15. 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)]
16. 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)]
17. youtube.com/kotlin
18. abcd bcde cdef defg xya xyb xyc Count the total of characters that are present on each line in every group of strings
19. 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
20. 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
21. 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
22. 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
23. 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
24. val input = """ abcd bcde cdef defg xya xyb xyc """.trimIndent()
25. val input = """ abcd bcde cdef defg xya xyb xyc """.trimIndent() val groups: List<String> = input.split("\n\n")
26. val input = """ abcd bcde cdef defg xya xyb xyc """.trimIndent() val groups: List<String> = input.split("\n\n")
27. val input = """ abcd bcde cdef defg xya xyb xyc """.trimIndent() val groups: List<String> = input.split("\n\n") var total = 0 for (group in groups) { val listOfSets: List<Set<Char >> = group.split("\n").map(String :: toSet) var result = listOfSets.first() for (set in listOfSets) { result = result intersect set } total += result.count() }
28. val input = """ abcd bcde cdef defg xya xyb xyc """.trimIndent() val groups: List<String> = input.split("\n\n") var total = 0 for (group in groups) { val listOfSets: List<Set<Char >> = group.split("\n").map(String :: toSet) var result = listOfSets.first() for (set in listOfSets) { result = result intersect set } total += result.count() }
29. val input = """ abcd bcde cdef defg xya xyb xyc """.trimIndent() val groups: List<String> = input.split("\n\n") var total = 0 for (group in groups) { val listOfSets: List<Set<Char >> = group.split("\n").map(String :: toSet) var result = listOfSets.first() for (set in listOfSets) { result = result intersect set } total += result.count() }
30. val input = """ abcd bcde cdef defg xya xyb xyc """.trimIndent() val groups: List<String> = input.split("\n\n") var total = 0 for (group in groups) { val listOfSets: List<Set<Char >> = group.split("\n").map(String :: toSet) var result = listOfSets.first() for (set in listOfSets) { result = result intersect set } total += result.count() }
31. val input = """ abcd bcde cdef defg xya xyb xyc """.trimIndent() val groups: List<String> = input.split("\n\n") var total = 0 for (group in groups) { val listOfSets: List<Set<Char >> = group.split("\n").map(String :: toSet) var result = listOfSets.first() for (set in listOfSets) { result = result intersect set } total += result.count() } Transforming data
32. val input = """ abcd bcde cdef defg xya xyb xyc """.trimIndent() val groups: List<String> = input.split("\n\n") var total = 0 for (group in groups) { val listOfSets: List<Set<Char >> = 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
33. val groups: List<String> = input.split("\n\n")
34. val groups: List<String> = input.split("\n\n") // List<List<Set<Char >>> val step1 = groups.map { it.split("\n").map(String :: toSet) } Transforming data
35. val groups: List<String> = input.split("\n\n") // List<List<Set<Char >>> val step1 = groups.map { it.split("\n").map(String :: toSet) } Transforming data
36. val groups: List<String> = input.split("\n\n") // List<List<Set<Char >>> val step1 = groups.map { it.split("\n").map(String :: toSet) } Transforming data
37. val groups: List<String> = input.split("\n\n") // List<List<Set<Char >>> val step1 = groups.map { it.split("\n").map(String :: toSet) } Transforming data
38. val groups: List<String> = input.split("\n\n") // List<List<Set<Char >>> val step1 = groups.map { it.split("\n").map(String :: toSet) } Transforming data
39. val groups: List<String> = input.split("\n\n") // List<List<Set<Char >>> 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
40. val groups: List<String> = input.split("\n\n") // List<List<Set<Char >>> 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
41. val groups: List<String> = input.split("\n\n") // List<List<Set<Char >>> 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
42. val groups: List<String> = input.split("\n\n") // List<List<Set<Char >>> 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
43. val groups: List<String> = input.split("\n\n") // List<List<Set<Char >>> 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
44. val groups: List<String> = input.split("\n\n") // List<List<Set<Char >>> 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() }
45. fold & reduce
46. 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
47. 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?
48. listOf(1, 2, 3, 4, 5).fold(10) { a, b - > a + b } / / 25 runningFold runningReduce
49. 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]
50. Scope functions
52. val result: = x.foo { } Return type Original object (x) Result of lambda Access to X it this
53. 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
54. 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 }
55. 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> T.apply(block: T.() -> Unit): T { block() return this }
56. 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> T.apply(block: T.() -> Unit): T { block() return this } Lambda with receiver
57. ? . let val order = retrieveOrder() if (order != null){ processCustomer(order.customer) }
58. val order = retrieveOrder() if (order != null){ processCustomer(order.customer) } retrieveOrder() ?. let { processCustomer(it.customer) } retrieveOrder() ?. customer ?. let( :: processCustomer) or ? . let
59. let() as a helper for a complex condition if (some.complex.expression.let { it is Type && it.has.some.property }) { . .. }
60. let() as a helper for a complex condition if (some.complex.expression.let { it is Type && it.has.some.property }) { . .. }
61. let() as a helper for a complex condition if (some.complex.expression.let { it is Type && it.has.some.property }) { . .. }
62. 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 }) { . .. }
63. fun makeDir(path: String) = path.let { File(it) }.also { it.mkdirs() }
64. fun makeDir(path: String) = path.let { File(it) }.also { it.mkdirs() } Don’t overuse the scope functions!
65. 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!
66. 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!
67. Builders
68. 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)
69. kotlinx.html System.out.appendHTML().html { body { div { a("http: // kotlinlang.org") { target = ATarget.blank +"Main site" } } } }
70. 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) }
71. 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 { +"$n" } } } } } } } }.start(wait = true) } Ktor’s routing
72. Ktor fun main() { embeddedServer(Netty, port = 8080, host = "0.0.0.0") { routing { get("/html-dsl") { call.respondHtml { body { h1 { +"HTML" } ul { for (n in 1 .. 10) { li { +"$n" } } } } } } } }.start(wait = true) } kotlinx.html Ktor’s routing
73. Lambda with receiver T.() -> Unit The essential language feature for the type-safe builders, a.k.a. DSL
74. 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);
75. 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")
76. 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")
77. 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")
78. 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")
79. 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() }
80. 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!")
81. eXtras
82. Select objects by type with filterIsInstance fun findAllStrings(objects: List<Any>) = objects.filter { it is String }
83. Select objects by type with filterIsInstance fun findAllStrings(objects: List<Any>) = objects.filter { it is String } fun findAllStrings(objects: List<Any>) = objects.filterIsInstance<String>()
84. 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>()
85. 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 })
86. 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)) compareBy compares by multiple keys
87. 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) } }
88. 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) } groupBy to group elements
89. 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) }
90. https://speakerdeck.com/antonarhipov @antonarhipov https://github.com/antonarhipov