Kotlin - Not you grandfather's Java

Introduction to Kotlin, for Java developers.

Presented on March 30th 2017 for the Portuguese Java User Group, at Talkdesk's Lisbon Office.

1. Kotlin: Not your grandfather’s Java João Carvalho - @johnkarva - PT.JUG
5. What is Kotlin?
8. https://try.kotlinlang.org
9. Philosophy
10. Pragmatic
11. Concise
12. Safe
13. Interoperable
14. Tooling
15. Lightweight 868KB runtime
17. public class HelloWorld { public static void main(String... args) { System.out .println("Hello world " + args[0]); } }
18. fun main(args: Array<String>) { println("Hello world ${args[0]}") }
19. Types
20. public void printName(Person person) { System.out.println(person.getName()); }
21. public void printName(Person person) { System.out.println(person.getName()); } null
23. fun printName(person: Person) { println(person.name) }
24. printName(null) ERROR: Null cannot be a value of non-null type Person.
25. fun printName(person: Person?) { if (person != null) { println(person.name) } }
26. Primitive Types
27. Arrays
28. Variables
29. final String name = "Kotlin"; int age = 5; // A few moments later age = 6;
30. val name: String = "Kotlin" var age: Int = 5 // A few moments later age = 6
31. val name = "Kotlin" var age = 5 // A few moments later age = 6
32. Functions/Expressions
33. fun max(one: Int, other: Int): Int { if (one > other) { return one } else { return other } }
34. fun max(one: Int, other: Int): Int { return if (one > other) { one } else { other } }
35. fun max(one: Int, other: Int): Int = if (one > other) { one } else { other }
36. fun max(one: Int, other: Int) = if (one > other) { one } else { other }
37. fun max(one: Int, other: Int) = if (one > other) one else other
38. Operators
39. when (x) { 1 -> print("x == 1") 2 -> print("x == 2") else -> { print("x is neither 1 nor 2") } }
40. Safe Navigation
41. private String safeGetName(Person person) { return person == null ? null : person.getName(); }
42. fun safeGetName(person: Person?): String? { return person?.name }
43. Elvis Operator
44. fun printName(person: Person?) { println("Name is ${person?.name ?: "Unknown"}") }
45. Smart Casts
46. fun printStringLength(any: Any) { if (any is String) { println("Length is ${any.length}") } else { return println("Oops, not a String") } }
47. fun printStringLength(any: Any) { val asString = any as String println("Length is ${asString.length}") }
48. fun printStringLength(any: Any) { val asString = any as? String println("Length is ${asString?.length ?: "Unknown"}") }
49. Classes
50. class Person { }
51. Final/Public by default
52. Constructors
53. class Fooer(name: String) { init { println("Setting up to foo $name") } }
54. class Fooer(name: String) { init { println("Setting up to foo $name") } constructor(name: String, option: String) : this(name) { println("\tUsing option $option") } }
55. Inheritance
56. open class Bar class Foo : Bar() { }
57. Properties
58. class Fooer(name: String) { private val name: String init { this.name = name } }
59. class Fooer(private val name: String)
60. private var name: String = "" get() { println("Returning $name") return name }
61. var name: String = "" private set
62. Delegated Properties
63. class Example { var p: String by Delegate() }
64. val lazyValue: String by lazy { println("computed!") "Hello" }
65. var name: String by Delegates.observable("<Unknown>") { _, old, new -> println("$old -> $new") }
66. Extension Functions
67. fun String.helloize() = "Hello $this" "World".helloize()
68. Data classes
69. public class Person { private String name; private int age; (...) }
70. public Person(String name, int age) { this.name = name; this.age = age; }
71. public String getName() { return name; } public void setName(String name) { this.name = name; }
72. @Override public boolean equals(Object o) { (Stuff generated by your IDE) } @Override public int hashCode() { int result = name.hashCode(); result = 31 * result + age; return result; }
73. @Override public String toString() { return "Person{" + "name='" + name + '\'' + ", age=" + age + '}'; }
75. data class Person(var name: String, var age: Int)
76. val john = Person("John", 21)
77. Immutable?
78. private final String name; public Person withName(String name) { return new Person(name, this.age); }
79. data class Person(var name: String, var age: Int)
80. data class Person(val name: String, val age: Int)
81. val john = Person("John", 21) val olderJohn = john.copy(age = 78)
82. Named Arguments
83. public class Person { public Person(String name, String lastName, int age, Instant birthDate, (...)) }
84. new Person("John", "Doe", 26, Instant.now(), (...));
86. Person(name = "John", lastName = "Doe", age = 26, birthDate = Instant.now())
87. Default Values
89. class BCryptPasswordEncoder( private val strength: Int = -1, private val random: SecureRandom = SecureRandom() )
90. Collections
91. Mutable vs Immutable
92. emptyList() listOf(1, 2, 3) mutableListOf(1, 2, 3) emptySet() setOf("John", "Jane") mutableSetOf("John", "Jane") emptyMap() mapOf("John" to "Doe") mutableMapOf("John" to "Doe")
93. Utilities
94. // apply BaseClientDetails().apply { clientId = "foo" } // takeIf userRepository.findOne("foo")?.takeIf(User::active) // use inputStream.use { it.read() } // let clientId?.let { loadClient(it) }
95. Going Functional
96. Higher-Order Functions
97. fun <T> doWithinLock(lock: Lock, body: () -> T): T { lock.lock() try { return body() } finally { lock.unlock() } }
98. doWithinLock(lock) { println("Hello lock") }
99. map.map { (key, value) -> "$key$value" }
100. Functions with Receiver
101. class HTML { var value = "" fun body(value: String) { this.value = value } } fun html(init: HTML.() -> Unit) = HTML().apply { init() } html { body("Hello world") }
102. Questions?