3. Object-Oriented Programming in Kotlin [kotlin-workshop]

Part of https://github.com/jetBrains/kotlin-workshop.

Covers:
- Constructor syntax
- Properties
- Class modifiers (enum, data, inner, sealed)
- Objects, companion objects
- Constants
- Annotations

Transcript

1. Object-oriented programming in Kotlin
2. Feels the same …with some improvements
3. final, open, abstract public, private, internal The defaults are different
4. final, open, abstract public, private, /*package-private*/ The defaults are different internal explicit: non-final visible in a module
5. Different constructor syntax
6. Concise primary constructor class Person(val name: String, val age: Int)
7. Full primary constructor syntax (1) class Person(name: String) { init { } } constructor parameter constructor body
8. Full primary constructor syntax (2) class Person(name: String) { val name: String init { this.name = name } } constructor parameter constructor body
9. val/var on parameter creates a property class Person(name: String) { val name: String init { this.name = name } } class Person(val name: String) is the same as:
10. Secondary constructor class Rectangle(val height: Int, val width: Int) { constructor(side: Int) : this(side, side) { … } } secondary constructor primary constructor
11. Properties, not fields
12. property = field + accessor(s) read-only property = field + getter mutable property = field + getter + setter Property ( val / var )
13. 
 class Contact( val name: String, var address: String ) contact.getAddress(); contact.setAddress("..."); contact.address contact.address = "..."
14. property = field? + accessor(s) read-only property = field? + getter mutable property = field? + getter + setter Property ( val / var )
15. class Rectangle(val height: Int, val width: Int) { val isSquare: Boolean get() { return height == width } } Backing field might be absent
16. class StateLogger { var state = false set(value) { println("state has changed") field = value } } You can use field inside accessors (only)
17. val foo: Int val is read-only property (not immutable) get() = Random().nextInt()
18. Classes
19. Class modifiers enum, data, inner, sealed
20. Enum class import Color.* fun getDescription(color: Color) = when (color) { BLUE -> "cold" ORANGE -> "mild" RED -> "hot" } enum class Color { BLUE, ORANGE, RED } represents enumeration
21. Data modifier data class Contact(val name: String, val address: String) generates useful methods: equals, hashCode, copy, toString, and some others contact.copy(address = "new address")
22. Equals & reference equality val set1 = setOf(1, 2, 3) val set2 = setOf(1, 2, 3) set1 == set2 set1 === set2 checks reference equality calls equals true false
23. Sealed modifier sealed class Expr class Num(val value: Int) : Expr() class Sum(val left: Expr, val right: Expr) : Expr() restricts class hierarchy: all subclasses must be located in the same file fun eval(e: Expr): Int = when (e) { is Num -> e.value is Sum -> eval(e.left) + eval(e.right) }
24. Inner modifier adds reference to the outer class class A { class B inner class C { …this@A… } }
25. interface Repository { fun getById(id: Int): Customer fun getAll(): List<Customer> } interface Logger { fun logAll() } class Controller( repository: Repository, logger: Logger ) : Repository by repository, Logger by logger fun use(controller: Controller) { controller.logAll() } Class delegation
26. Objects & companion objects
27. object = singleton object KSingleton { fun foo() {} }
28. Object = singleton public class JSingleton { public final static JSingleton INSTANCE = new JSingleton(); private JSingleton() { } public void foo() {} } object KSingleton { fun foo() {} }
29. Using Kotlin object from Java public class UsingKotlinObjectFromJava { public static void main(String[] args) { JSingleton.INSTANCE.foo(); KSingleton.INSTANCE.foo(); } }
30. companion object class A { companion object { fun foo() = 1 } } fun main(args: Array<String>) { A.foo() } special object inside a class
31. companion object can implement an interface interface Factory<T> { fun create(): T } class A { private constructor() companion object : Factory<A> { override fun create(): A { return A() } }
32. No static keyword Declare “static” members: - at the top-level - inside objects - inside companion objects
33. @JvmStatic class C {
 companion object {
 @JvmStatic fun foo() {}
 fun bar() {}
 }
 } // Java
 C.foo();
 C.bar(); C.Companion.foo(); C.Companion.bar();
34. @JvmStatic object Obj {
 @JvmStatic fun foo() {}
 fun bar() {}
 } // Java Obj.foo();
 Obj.bar();
 Obj.INSTANCE.foo(); Obj.INSTANCE.bar();
35. Constants • const (for primitive types and String) • @JvmField
36. Compile-time constants for primitive types and String const val answer = 42 the value is inlined
37. @JvmField @JvmField val property = MyClass() // the same as // Java public static final MyClass property = new MyClass(); exposes a Kotlin property as a field in Java
38. @JvmField makes a property static if used inside object class A { @JvmField val property = MyClass() } object B { @JvmField val property = MyClass() } regular field generated static field generated
39. Annotations
40. Annotation syntax @file:JvmName("FooUtils") use-site target annotation name annotation arguments
41. Why use-site target? @Rule var folder @property:Rule @field:Rule @getter:Rule @setter:Rule might be:
42. @JvmName // Data.kt fun foo() {
 } public final class DataKt {
 public static void foo() {
 }
 } @file:JvmName("FooUtils") fun foo() {
 } public final class FooUtils {
 public static final void foo() {
 }
 }
43. @JvmName fun List<String>.filterValid(): List<String>
 fun List<Int>.filterValid(): List<Int> Error: platform declaration clash: declarations have the same JVM signature (filterValid(Ljava/util/List;)Ljava/util/List;)
44. @JvmName fun List<String>.filterValid(): List<String>
 @JvmName("filterValidInt")
 fun List<Int>.filterValid(): List<Int>
45. @JvmOverloads 
 fun f(a: String, b: Int = 0, c: String = "abc") {
 // ...
 } // Java
 void f(String a, int b, String c) { }

46. @JvmOverloads @JvmOverloads
 fun f(a: String, b: Int = 0, c: String = "abc") {
 // ...
 } // Java
 void f(String a, int b, String c) { }
 void f(String a, int b) { }
 void f(String a) { }
47. @Throws @Throws(IOException::class)
 fun foo() {
 throw IOException()
 }
48. Checked exceptions // Java
 try {
 DemoKt.foo();
 }
 catch (IOException e) {
 // ...
 } fun foo() { throw IOException() } Error: Exception java.io.IOException is never thrown in body of corresponding try statement
49. Checked exceptions // Java
 try {
 DemoKt.foo();
 }
 catch (IOException e) {
 // ...
 } @Throws(IOException::class) fun foo() { throw IOException() }
50. Copyright © 2017 https://github.com/JetBrains/kotlin-workshop