Kotlin is fun

Each year Kotlin has been gaining on popularity in the software community. Its modern syntax and mechanisms allow to write simpler, more reliable and testable code.

This is an introductory talk which covers basics of the language and shows how one can leverage power of the language in their daily coding.

Transcript

1. Kotlin is fun Michał Sikora
5. Kotlin Statically typed programming language for modern multiplatform applications.
6. Kotlin Statically typed programming language for modern multiplatform applications.
7. Kotlin Statically typed programming language for modern multiplatform applications.
8. Kotlin Statically typed programming language for modern multiplatform applications.
9. Kotlin Statically typed programming language for modern multiplatform applications.
10. Syntax Kotlin Java fun isPositive(a: Int): Boolean { return a > 0 } public boolean isPositive(final int a) { return a > 0; }
11. Syntax Kotlin Java fun isPositive(a: Int): Boolean { return a > 0 } public boolean isPositive(final int a) { return a > 0; }
12. Syntax Kotlin Java fun isPositive(a: Int): Boolean { return a > 0 } public boolean isPositive(final int a) { return a > 0; }
13. Syntax Kotlin Java fun isPositive(a: Int): Boolean { return a > 0 } public boolean isPositive(final int a) { return a > 0; }
14. Syntax Kotlin Java fun isPositive(a: Int): Boolean { return a > 0 } public boolean isPositive(final int a) { return a > 0; }
15. Syntax Kotlin Java fun isPositive(a: Int): Boolean { return a > 0 } public boolean isPositive(final int a) { return a > 0; }
16. Syntax Kotlin Java fun isPositive(a: Int): Boolean { return a > 0 } public boolean isPositive(final int a) { return a > 0; }
17. Syntax Kotlin Java fun isPositive(a: Int) = a > 0 public boolean isPositive(final int a) { return a > 0; }
18. val and var val name: String = "Reksio"
19. val and var val name = "Reksio"
20. val and var val name = "Reksio" name = "Burek"
21. val and var val name = "Reksio" name = "Burek" val cannot be reassigned
22. val and var var name = "Reksio" name = "Burek"
23. Default parameters fun highlightText(text: String, offset: Int, length: Int) val text = "Reksio" highlightText(text, 0, text.length)
24. Default parameters fun highlightText(text: String, offset: Int = 0, length: Int = text.length) val text = "Reksio" highlightText(text, 0, text.length)
25. Default parameters fun highlightText(text: String, offset: Int = 0, length: Int = text.length) val text = "Reksio" highlightText(text, 0, text.length)
26. Default parameters fun highlightText(text: String, offset: Int = 0, length: Int = text.length) val text = "Reksio" highlightText(text)
27. Named parameters fun someFancyMethod(isExternalCall: Boolean, isUserGoat: Boolean, isMyBankSafe: Boolean, isItHotOutside: Boolean, isItMonday: Boolean)
28. Named parameters fun someFancyMethod(isExternalCall: Boolean, isUserGoat: Boolean, isMyBankSafe: Boolean, isItHotOutside: Boolean, isItMonday: Boolean) someFancyMethod(false, true, true, false, true)
29. Named parameters fun someFancyMethod(isExternalCall: Boolean, isUserGoat: Boolean, isMyBankSafe: Boolean, isItHotOutside: Boolean, isItMonday: Boolean) someFancyMethod( isItHotOutside = false, isItMonday = true, isMyBankSafe = true, isUserGoat = true, isExternalCall = false)
30. Named parameters fun highlightText(text: String, offset: Int = 0, length: Int = text.length) val text = "Reksio" highlightText(text, length = 4)
31. when expression fun isZero(number: Int): Boolean { when (number) { 0 -> return true else -> return false } }
32. when expression fun isZero(number: Int): Boolean { when (number) { 0 -> return true else -> return false } }
33. when expression fun isZero(number: Int): Boolean { return when (number) { 0 -> true else -> false } }
34. when expression fun isZero(number: Int) = when (number) { 0 -> true else -> false }
35. when expression fun estimate(number: Int) = when (number) { 0 -> "Nothing" 1, 2, 3, 4, 5 -> "Low" in 6..10 -> "Medium" else -> if (number < 0) "Negative" else "High" }
36. if expression fun estimate(number: Int) = when (number) { 0 -> "Nothing" 1, 2, 3, 4, 5 -> "Low" in 6..10 -> "Medium" else -> if (number < 0) "Negative" else "High" }
37. Smart casting fun doSomething(item: Any) = when (item) { is String -> item.toUpperCase() is Int -> (item * 2).toString() is Boolean -> booleanAsString(item) else -> "" }
38. Properties Kotlin Java class UserKt(var name: String, var age: Int) public class UserJava { private String name; private int age; public UserJava(String name, int age) { this.name = name; this.age = age; } public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } }
39. Properties Kotlin Java class UserKt(var name: String, var age: Int) public class UserJava { private String name; private int age; public UserJava(String name, int age) { this.name = name; this.age = age; } public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } } val user = UserJava("Reksio", 50) user.age = 51 val age = user.age UserKt user = new UserKt("Reksio", 50); user.setAge(51); int user = user.getAge();
40. Data classes class User(val name: String, val age: Int) val user = User("Reksio", 50) user.toString() // com.example.User@6f2b958e
41. Data classes data class User(val name: String, val age: Int) val user = User("Reksio", 50) user.toString() // User(name=Reksio,age=50)
42. Data classes data class User(val name: String, val age: Int) val user = User("Reksio", 50) user.toString() // User(name=Reksio,age=50) val user2 = user.copy(age = 51)
43. Data classes public class User { @NonNull private final String name; private final int age; public User(@NonNull String name, int age) { this.name = name; this.age = age; } @NonNull public String getName() { return name; } public int getAge() { return age; } @Override public boolean equals(Object o) { if (this == o) return true; if (o == null || getClass() != o.getClass()) return false; User user = (User) o; if (age != user.age) return false; return name.equals(user.name); } @Override public int hashCode() { int result = name.hashCode(); result = 31 * result + age; return result; } @Override public String toString() { return "User{" + "name='" + name + '\'' + ", age=" + age + '}'; } } data class User(val name: String, val age: Int)
44. Sealed classes sealed class LoginUiModel { object Pending : LoginUiModel() data class Success(val username: String) : LoginUiModel() data class Error(val code: Int) : LoginUiModel() }
45. Sealed classes fun renderPending() fun renderSuccess(model: LoginUiModel.Success) fun renderUiModel(model: LoginUiModel) = when (model) { is LoginUiModel.Pending -> renderPending() is LoginUiModel.Success -> renderSuccess(model) }
46. Sealed classes fun renderUiModel(model: LoginUiModel) = when (model) { is LoginUiModel.Pending -> renderPending() is LoginUiModel.Success -> renderSuccess(model) } fun renderPending() fun renderSuccess(model: LoginUiModel.Success) when expresion must be exhaustive, add neccessary 'Error' branch or 'else' branch instead
47. Sealed classes fun renderUiModel(model: LoginUiModel) = when (model) { is LoginUiModel.Pending -> renderPending() is LoginUiModel.Success -> renderSuccess(model) else -> renderError(model as LoginUiModel.Error) } fun renderPending() fun renderSuccess(model: LoginUiModel.Success) fun renderError(model: LoginUiModel.Error)
48. Sealed classes fun renderPending() fun renderSuccess(model: LoginUiModel.Success) fun renderError(model: LoginUiModel.Error) fun renderUiModel(model: LoginUiModel) = when (model) { is LoginUiModel.Pending -> renderPending() is LoginUiModel.Success -> renderSuccess(model) is LoginUiModel.Error -> renderError(model) }
49. Null safety var name = "Reksio" name = null
50. Null safety var name = "Reksio" name = null null can not be a value of a non-null type String
51. Null safety var name: String = "Reksio" name = null
52. Null safety var name: String? = "Reksio" name = null
53. Null safety var name: String? = "Reksio" name = null
54. Null safety fun foo(text: String?): String { return text.toUpperCase() }
55. Null safety fun foo(text: String?): String { return text?.toUpperCase() }
56. Null safety fun foo(text: String?): String { return text?.toUpperCase() ?: "" }
57. Null safety fun foo(text: String?): String { return text!!.toUpperCase() }
58. Extension functions fun isPositive(a: Int) = a > 0 val a = 1 if (isPositive(a)) { // Do something }
59. Extension functions fun isPositive(a: Int) = a > 0 fun Int.isPositive() = this > 0
60. Extension functions fun isPositive(a: Int) = a > 0 fun Int.isPositive() = this > 0
61. Extension functions fun isPositive(a: Int) = a > 0 fun Int.isPositive() = this > 0
62. Extension functions fun Int.isPositive() = this > 0 val a = 1 if (a.isPositive()) { // Do something }
63. Lambdas val lambda1: (String) -> Unit val lambda2: (String) -> Boolean val lambda3: (Int, Int) -> Int
64. Lambdas val lambda1: (String) -> Unit = { text -> println(text) } val lambda2: (String) -> Boolean = { text -> text.isEmpty() } val lambda3: (Int, Int) -> Int = { x, y -> x + y }
65. Lambdas val lambda1: (String) -> Unit = { text -> println(text) } val lambda2: (String) -> Boolean = { text -> text.isEmpty() } val lambda3: (Int, Int) -> Int = { x, y -> x + y }
66. Lambdas val lambda1: (String) -> Unit = { text -> println(text) } val lambda2: (String) -> Boolean = { text -> text.isEmpty() } val lambda3: (Int, Int) -> Int = { x, y -> x + y }
67. Lambdas val lambda1: (String) -> Unit = { println(it) } val lambda2: (String) -> Boolean = { it.isEmpty() } val lambda3: (Int, Int) -> Int = { x, y -> x + y }
68. Lambdas val lambda1: (String) -> Unit = { println(it) } val lambda2: (String) -> Boolean = { it.isEmpty() } val lambda3: (Int, Int) -> Int = { x, y -> x + y } val a = lambda3(2, 5)
69. Higher-Order Functions fun isValid(text: String, validator: (String) -> Boolean) = validator(text)
70. Higher-Order Functions fun String.isValid(validator: (String) -> Boolean) = validator(this)
71. Higher-Order Functions fun String.isValid(validator: (String) -> Boolean) = validator(this) val noDigits: (String) -> Boolean = { !it.contains("\\d".toRegex()) } val allCapital: (String) -> Boolean = { it.matches("^[^a-z]*\$".toRegex()) } val peselNumber: (String) -> Boolean = { checkPesel(it) }
72. Higher-Order Functions fun String.isValid(validator: (String) -> Boolean) = validator(this) val noDigits: (String) -> Boolean = { !it.contains("\\d".toRegex()) } val allCapital: (String) -> Boolean = { it.matches("^[^a-z]*\$".toRegex()) } val peselNumber: (String) -> Boolean = { checkPesel(it) } "Reksio".isValid(noDigits) "PL".isValid(allCapital) "88061423471".isValid(peselNumber)
73. Higher-Order Functions fun String.isValid(validator: (String) -> Boolean) = validator(this) val noDigits: (String) -> Boolean = { !it.contains("\\d".toRegex()) } val allCapital: (String) -> Boolean = { it.matches("^[^a-z]*\$".toRegex()) } val peselNumber: (String) -> Boolean = { checkPesel(it) } "Reksio".isValid(noDigits) "PL".isValid(allCapital) "88061423471".isValid(peselNumber) "1945".isValid({ it.matches("\\d{4}".toRegex()) })
74. Higher-Order Functions fun String.isValid(validator: (String) -> Boolean) = validator(this) val noDigits: (String) -> Boolean = { !it.contains("\\d".toRegex()) } val allCapital: (String) -> Boolean = { it.matches("^[^a-z]*\$".toRegex()) } val peselNumber: (String) -> Boolean = { checkPesel(it) } "Reksio".isValid(noDigits) "PL".isValid(allCapital) "88061423471".isValid(peselNumber) "1945".isValid({ it.matches("\\d{4}".toRegex()) })
75. Higher-Order Functions fun String.isValid(validator: (String) -> Boolean) = validator(this) val noDigits: (String) -> Boolean = { !it.contains("\\d".toRegex()) } val allCapital: (String) -> Boolean = { it.matches("^[^a-z]*\$".toRegex()) } val peselNumber: (String) -> Boolean = { checkPesel(it) } "Reksio".isValid(noDigits) "PL".isValid(allCapital) "88061423471".isValid(peselNumber) "1945".isValid() { it.matches("\\d{4}".toRegex()) }
76. Higher-Order Functions fun String.isValid(validator: (String) -> Boolean) = validator(this) val noDigits: (String) -> Boolean = { !it.contains("\\d".toRegex()) } val allCapital: (String) -> Boolean = { it.matches("^[^a-z]*\$".toRegex()) } val peselNumber: (String) -> Boolean = { checkPesel(it) } "Reksio".isValid(noDigits) "PL".isValid(allCapital) "88061423471".isValid(peselNumber) "1945".isValid() { it.matches("\\d{4}".toRegex()) }
77. Higher-Order Functions fun String.isValid(validator: (String) -> Boolean) = validator(this) val noDigits: (String) -> Boolean = { !it.contains("\\d".toRegex()) } val allCapital: (String) -> Boolean = { it.matches("^[^a-z]*\$".toRegex()) } val peselNumber: (String) -> Boolean = { checkPesel(it) } "Reksio".isValid(noDigits) "PL".isValid(allCapital) "88061423471".isValid(peselNumber) "1945".isValid { it.matches("\\d{4}".toRegex()) }
78. Higher-Order Functions fun <T, R> List<T>.map(func: (T) -> R): List<R> { // Do something }
79. Higher-Order Functions fun <T, R> List<T>.map(func: (T) -> R): List<R> { // Do something }
80. Higher-Order Functions fun <T, R> List<T>.map(func: (T) -> R): List<R> { val result = ArrayList<R>() for (element in this) { result.add(func(element)) } return result }
81. Higher-Order Functions fun <T, R> List<T>.map(func: (T) -> R): List<R> { val result = ArrayList<R>() for (element in this) { result.add(func(element)) } return result } val names = listOf("Reksio", "Burek", "Falafel") val nameLengths = names.map { it.length } // [6, 5, 7]
82. Higher-Order Functions inline fun <T, R> List<T>.map(func: (T) -> R): List<R>
83. Higher-Order Functions inline fun <T, R> List<T>.map(func: (T) -> R): List<R> + val names = listOf("Reksio", "Burek", "Falafel") val nameLengths = names.map { it.length }
84. Higher-Order Functions inline fun <T, R> List<T>.map(func: (T) -> R): List<R> + val names = listOf("Reksio", "Burek", "Falafel") val nameLengths = names.map { it.length } = val names = listOf("Reksio", "Burek", "Falafel") val nameLengths: List<Int> val result = ArrayList<Int>() for (name in names) { result.add(name.length) } nameLengths = result
85. Higher-Order Functions val names = listOf("Reksio", "Burek", "Falafel", "") val twoBiggestNonEmptyNameLengths = names
86. Higher-Order Functions val names = listOf("Reksio", "Burek", "Falafel", "") val twoBiggestNonEmptyNameLengths = names .filter(String::isNotEmpty) // ["Reksio", "Burek", "Falafel"]
87. Higher-Order Functions val names = listOf("Reksio", "Burek", "Falafel", "") val twoBiggestNonEmptyNameLengths = names .filter(String::isNotEmpty) // ["Reksio", "Burek", "Falafel"] .map { it.length } // [6, 5, 7]
88. Higher-Order Functions val names = listOf("Reksio", "Burek", "Falafel", "") val twoBiggestNonEmptyNameLengths = names .filter(String::isNotEmpty) // ["Reksio", "Burek", "Falafel"] .map { it.length } // [6, 5, 7] .sorted() // [5, 6, 7]
89. Higher-Order Functions val names = listOf("Reksio", "Burek", "Falafel", "") val twoBiggestNonEmptyNameLengths = names .filter(String::isNotEmpty) // ["Reksio", "Burek", "Falafel"] .map { it.length } // [6, 5, 7] .sorted() // [5, 6, 7] .takeLast(2) // [6, 7]
90. Lambdas With Receivers val preferences: SharedPreferences preferences.edit() .putString("Dog", "Burek") .putBoolean("isGoodBoy", true) .remove("Owner") .apply() preferences.edit { putString("Dog", "Burek") putBoolean("isGoodBoy", true) remove("Owner") }
91. Lambdas With Receivers inline fun SharedPreferences.edit(action: (SharedPreferences.Editor) -> Unit) { val editor = edit() action(editor) editor.apply() }
92. Lambdas With Receivers inline fun SharedPreferences.edit(action: (SharedPreferences.Editor) -> Unit) { val editor = edit() action(editor) editor.apply() }
93. Lambdas With Receivers inline fun SharedPreferences.edit(action: (SharedPreferences.Editor) -> Unit) { val editor = edit() action(editor) editor.apply() }
94. Lambdas With Receivers inline fun SharedPreferences.edit(action: (SharedPreferences.Editor) -> Unit) { val editor = edit() action(editor) editor.apply() } preferences.edit { editor -> editor.putString("Dog", "Burek") .putBoolean("isGoodBoy", true) .remove("Owner") }
95. Lambdas With Receivers inline fun SharedPreferences.edit(action: (SharedPreferences.Editor) -> Unit) { val editor = edit() action(editor) editor.apply() } preferences.edit { it.putString("Dog", "Burek") .putBoolean("isGoodBoy", true) .remove("Owner") }
96. Lambdas With Receivers inline fun SharedPreferences.edit(action: (SharedPreferences.Editor) -> Unit) { val editor = edit() action(editor) editor.apply() } preferences.edit { it.putString("Dog", "Burek") .putBoolean("isGoodBoy", true) .remove("Owner") }
97. Lambdas With Receivers inline fun SharedPreferences.edit(action: SharedPreferences.Editor.() -> Unit) { val editor = edit() editor.action() editor.apply() }
98. Lambdas With Receivers inline fun SharedPreferences.edit(action: SharedPreferences.Editor.() -> Unit) { val editor = edit() editor.action() editor.apply() } preferences.edit { this.putString("Dog", "Burek") .putBoolean("isGoodBoy", true) .remove("Owner") }
99. Lambdas With Receivers inline fun SharedPreferences.edit(action: SharedPreferences.Editor.() -> Unit) { val editor = edit() editor.action() editor.apply() } preferences.edit { this.putString("Dog", "Burek") this.putBoolean("isGoodBoy", true) this.remove("Owner") }
100. Lambdas With Receivers inline fun SharedPreferences.edit(action: SharedPreferences.Editor.() -> Unit) { val editor = edit() editor.action() editor.apply() } preferences.edit { this.putString("Dog", "Burek") this.putBoolean("isGoodBoy", true) this.remove("Owner") }
101. Lambdas With Receivers inline fun SharedPreferences.edit(action: SharedPreferences.Editor.() -> Unit) { val editor = edit() editor.action() editor.apply() } preferences.edit { putString("Dog", "Burek") putBoolean("isGoodBoy", true) remove("Owner") }
102. Lambdas With Receivers html { head("Page title") { meta { httpEquiv = "Content-type" content = "text/html; charset=iso-8859-2" } meta { name = "Author" content = "Cygnus" } } body { h1("Heading") p("Paragraph") } }
103. Lambdas With Receivers login { username("Burek") password("") } submit { isInvalidPassword() } login { username("Burek") password("haslo") } submit { isWelcomeScreen() }
104. Resources • Website and documentation: https://kotlinlang.org/ • Online course: https://try.kotlinlang.org/ • Podcast: http://talkingkotlin.com/ • Slack: http://slack.kotlinlang.org/ • Reddit: https://www.reddit.com/r/Kotlin/ • Kotlin conference: https://kotlinconf.com/ • Twitter: @kotlin, @intelliyole, @hhariri, @JakeWharton, @RunChristinaRun • Plenty of materials on YouTube, Medium, etc.
105. Thank you!