Lesson 3: Classes and Objects PDF

Lesson 3: Classes and objects This work is licensed under the Android Development with Kotlin Apache 2 license. 1 v1.0 About this lesson Lesson 3: Classes and objects ○ Classes ○ Inheritance ○ Exte...

Lesson 3: Classes and objects This work is licensed under the Android Development with Kotlin Apache 2 license. 1 v1.0 About this lesson Lesson 3: Classes and objects ○ Classes ○ Inheritance ○ Extension functions ○ Special classes ○ Organizing your code ○ Summary This work is licensed under the Android Development with Kotlin Apache 2 license. 2  Classes This work is licensed under the Android Development with Kotlin Apache 2 license. 3 Class Object Classes are blueprints for objects instances Classes define methods that operate on their object instances Class This work is licensed under the Android Development with Kotlin Apache 2 license. 4 Class versus object instance House Class Object Instances Data House color (String) Number of windows (Int) Is for sale (Boolean) Behavior updateColor() FOR putOnSale() SALE This work is licensed under the Android Development with Kotlin Apache 2 license. 5 Define and use a class Class Definition Create New Object Instance class House { val myHouse = House() val color: String = "white" println(myHouse) val numberOfWindows: Int = 2 val isForSale: Boolean = false fun updateColor(newColor: String) {...}... } This work is licensed under the Android Development with Kotlin Apache 2 license. 6 Constructors When a constructor is defined in the class header, it can contain: No parameters class A Parameters ○ Not marked with var or val → copy exists only within scope of the constructor class B(x: Int) ○ Marked var or val → copy exists in all instances of the class class C(val y: Int) This work is licensed under the Android Development with Kotlin Apache 2 license. 7 Constructor examples class A val aa = A() class B(x: Int) val bb = B(12) println(bb.x) => compiler error unresolved reference class C(val y: Int) val cc = C(42) println(cc.y) => 42 This work is licensed under the Android Development with Kotlin Apache 2 license. 8 Default parameters Class instances can have default values. Use default values to reduce the number of constructors needed Default parameters can be mixed with required parameters More concise (don’t need to have multiple constructor versions) class Box(val length: Int, val width:Int = 20, val height:Int = 40) val box1 = Box(100, 20, 40) val box2 = Box(length = 100) val box3 = Box(length = 100, width = 20, height = 40) This work is licensed under the Android Development with Kotlin Apache 2 license. 9 Primary constructor Declare the primary constructor within the class header. class Circle(i: Int) { init {... } } This is technically equivalent to: class Circle { constructor(i: Int) {... } } This work is licensed under the Android Development with Kotlin Apache 2 license. 10 Initializer block Any required initialization code is run in a special init block Multiple init blocks are allowed init blocks become the body of the primary constructor This work is licensed under the Android Development with Kotlin Apache 2 license. 11 Initializer block example Use the init keyword: class Square(val side: Int) { init { println(side * 2) } } val s = Square(10) => 20 This work is licensed under the Android Development with Kotlin Apache 2 license. 12 Multiple constructors Use the constructor keyword to define secondary constructors Secondary constructors must call: ○ The primary constructor using this keyword OR ○ Another secondary constructor that calls the primary constructor Secondary constructor body is not required This work is licensed under the Android Development with Kotlin Apache 2 license. 13 Multiple constructors example class Circle(val radius:Double) { constructor(name:String) : this(1.0) constructor(diameter:Int) : this(diameter / 2.0) { println("in diameter constructor") } init { println("Area: ${Math.PI * radius * radius}") } } val c = Circle(3) This work is licensed under the Android Development with Kotlin Apache 2 license. 14 Properties Define properties in a class using val or var Access these properties using dot. notation with property name Set these properties using dot. notation with property name (only if declared a var) This work is licensed under the Android Development with Kotlin Apache 2 license. 15 Person class with name property class Person(var name: String) fun main() { val person = Person("Alex") println(person.name) Access with. person.name = "Joey" Set with. println(person.name) } This work is licensed under the Android Development with Kotlin Apache 2 license. 16 Custom getters and setters If you don’t want the default get/set behavior: Override get() for a property Override set() for a property (if defined as a var) Format: var propertyName: DataType = initialValue get() =... set(value) {... } This work is licensed under the Android Development with Kotlin Apache 2 license. 17 Custom getter class Person(val firstName: String, val lastName:String) { val fullName:String get() { return "$firstName $lastName" } } val person = Person("John", "Doe") println(person.fullName) => John Doe This work is licensed under the Android Development with Kotlin Apache 2 license. 18 Custom setter var fullName:String = "" get() = "$firstName $lastName" set(value) { val components = value.split(" ") firstName = components lastName = components field = value } person.fullName = "Jane Smith" This work is licensed under the Android Development with Kotlin Apache 2 license. 19 Member functions Classes can also contain functions Declare functions as shown in Functions in Lesson 2 ○ fun keyword ○ Can have default or required parameters ○ Specify return type (if not Unit) This work is licensed under the Android Development with Kotlin Apache 2 license. 20  Inheritance This work is licensed under the Android Development with Kotlin Apache 2 license. 21 Inheritance Kotlin has single-parent class inheritance Each class has exactly one parent class, called a superclass Each subclass inherits all members of its superclass including ones that the superclass itself has inherited If you don't want to be limited by only inheriting a single class, you can define an interface since you can implement as many of those as you want. This work is licensed under the Android Development with Kotlin Apache 2 license. 22 Interfaces Provide a contract all implementing classes must adhere to Can contain method signatures and property names Can derive from other interfaces Format: interface NameOfInterface { interfaceBody } This work is licensed under the Android Development with Kotlin Apache 2 license. 23 Interface example interface Shape { fun computeArea() : Double } class Circle(val radius:Double) : Shape { override fun computeArea() = Math.PI * radius * radius } val c = Circle(3.0) println(c.computeArea()) => 28.274333882308138 This work is licensed under the Android Development with Kotlin Apache 2 license. 24 Extending classes To extend a class: Create a new class that uses an existing class as its core (subclass) Add functionality to a class without creating a new one (extension functions) This work is licensed under the Android Development with Kotlin Apache 2 license. 25 Creating a new class Kotlin classes by default are not subclassable Use open keyword to allow subclassing Properties and functions are redefined with the override keyword This work is licensed under the Android Development with Kotlin Apache 2 license. 26 Classes are final by default Declare a class class A Try to subclass A class B : A =>Error: A is final and cannot be inherited from This work is licensed under the Android Development with Kotlin Apache 2 license. 27 Use open keyword Use open to declare a class so that it can be subclassed. Declare a class open class C Subclass from C class D : C() This work is licensed under the Android Development with Kotlin Apache 2 license. 28 Overriding Must use open for properties and methods that can be overridden (otherwise you get compiler error) Must use override when overriding properties and methods Something marked override can be overridden in subclasses (unless marked final) This work is licensed under the Android Development with Kotlin Apache 2 license. 29 Abstract classes Class is marked as abstract Cannot be instantiated, must be subclassed Similar to an interface with the added the ability to store state Properties and functions marked with abstract must be overridden Can include non-abstract properties and functions This work is licensed under the Android Development with Kotlin Apache 2 license. 30 Example abstract classes abstract class Food { abstract val kcal : Int abstract val name : String fun consume() = println("I'm eating ${name}") } class Pizza() : Food() { override val kcal = 600 override val name = "Pizza" } fun main() { Pizza().consume() // "I'm eating Pizza" } This work is licensed under the Android Development with Kotlin Apache 2 license. 31 When to use each Defining a broad spectrum of behavior or types? Consider an interface. Will the behavior be specific to that type? Consider a class. Need to inherit from multiple classes? Consider refactoring code to see if some behavior can be isolated into an interface. Want to leave some properties / methods abstract to be defined by subclasses? Consider an abstract class. You can extend only one class, but implement one or more This work is licensed under the Android Development with Kotlin 32 interfaces. Apache 2 license. Extension functions This work is licensed under the Android Development with Kotlin Apache 2 license. 33 Extension functions Add functions to an existing class that you cannot modify directly. Appears as if the implementer added it Not actually modifying the existing class Cannot access private instance variables Format: fun ClassName.functionName( params ) { body } This work is licensed under the Android Development with Kotlin Apache 2 license. 34 Why use extension functions? Add functionality to classes that are not open Add functionality to classes you don’t own Separate out core API from helper methods for classes you own Define extension functions in an easily discoverable place such as in the same file as the class, or a well-named function. This work is licensed under the Android Development with Kotlin Apache 2 license. 35 Extension function example Add isOdd() to Int class: fun Int.isOdd(): Boolean { return this % 2 == 1 } Call isOdd() on an Int: 3.isOdd() Extension functions are very powerful in Kotlin! This work is licensed under the Android Development with Kotlin Apache 2 license. 36 Special classes This work is licensed under the Android Development with Kotlin Apache 2 license. 37 Data class Special class that exists just to store a set of data Mark the class with the data keyword Generates getters for each property (and setters for vars too) Generates toString(), equals(), hashCode(), copy() methods, and destructuring operators Format: data class This work is licensed under the ( parameterList Apache Android Development with Kotlin ) 2 license. 38 Data class example Define the data class: data class Player(val name: String, val score: Int) Use the data class: val firstPlayer = Player("Lauren", 10) println(firstPlayer) => Player(name=Lauren, score=10) Data classes make your code much more concise! This work is licensed under the Android Development with Kotlin Apache 2 license. 39 Pair and Triple Pair and Triple are predefined data classes that store 2 or 3 pieces of data respectively Access variables with.first,.second,.third respectively Usually named data classes are a better option (more meaningful names for your use case) This work is licensed under the Android Development with Kotlin Apache 2 license. 40 Pair and Triple examples val bookAuthor = Pair("Harry Potter", "J.K. Rowling") println(bookAuthor) => (Harry Potter, J.K. Rowling) val bookAuthorYear = Triple("Harry Potter", "J.K. Rowling", 1997) println(bookAuthorYear) println(bookAuthorYear.third) => (Harry Potter, J.K. Rowling, 1997) 1997 This work is licensed under the Android Development with Kotlin Apache 2 license. 41 Pair to Pair's special to variant lets you omit parentheses and periods (infix function). It allows for more readable code val bookAuth1 = "Harry Potter".to("J. K. Rowling") val bookAuth2 = "Harry Potter" to "J. K. Rowling" => bookAuth1 and bookAuth2 are Pair (Harry Potter, J. K. Rowling) Also used in collections like Map and HashMap val map = mapOf(1 to "x", 2 to "y", 3 to "zz") => map of Int to String {1=x, 2=y, 3=zz} This work is licensed under the Android Development with Kotlin Apache 2 license. 42 Enum class User-defined data type for a set of named values Use this to require instances be one of several constant values The constant value is, by default, not visible to you Use enum before the class keyword Format: enum class EnumName { NAME1, NAME2, … NAMEn } Referenced via EnumName. This work is licensed under the Android Development with Kotlin Apache 2 license. 43 Enum class example Define an enum with red, green, and blue colors. enum class Color(val r: Int, val g: Int, val b: Int) { RED(255, 0, 0), GREEN(0, 255, 0), BLUE(0, 0, 255) } println("" + Color.RED.r + " " + Color.RED.g + " " + Color.RED.b) => 255 0 0 This work is licensed under the Android Development with Kotlin Apache 2 license. 44 Object/singleton Sometimes you only want one instance of a class to ever exist Use the object keyword instead of the class keyword Accessed with NameOfObject. This work is licensed under the Android Development with Kotlin Apache 2 license. 45 Object/singleton example object Calculator { fun add(n1: Int, n2: Int): Int { return n1 + n2 } } println(Calculator.add(2,4)) => 6 This work is licensed under the Android Development with Kotlin Apache 2 license. 46 Companion objects Lets all instances of a class share a single instance of a set of variables or functions Use companion keyword Referenced via ClassName.PropertyOrFunction This work is licensed under the Android Development with Kotlin Apache 2 license. 47 Companion object example class PhysicsSystem { companion object WorldConstants { val gravity = 9.8 val unit = "metric" fun computeForce(mass: Double, accel: Double): Double { return mass * accel } } } println(PhysicsSystem.WorldConstants.gravity) println(PhysicsSystem.WorldConstants.computeForce(10.0, 10.0)) Android Development with Kotlin This work is licensed under the 48 Apache 2 license. Organizing your code This work is licensed under the Android Development with Kotlin Apache 2 license. 49 Single file, multiple entities Kotlin DOES NOT enforce a single entity (class/interface) per file convention You can and should group related structures in the same file Be mindful of file length and clutter This work is licensed under the Android Development with Kotlin Apache 2 license. 50 Packages Provide means for organization Identifiers are generally lower case words separated by periods Declared in the first non-comment line of code in a file following the package keyword package org.example.game This work is licensed under the Android Development with Kotlin Apache 2 license. 51 Example class hierarchy org.example.veh icle Vehicle org.example.vehicle.m org.example.vehicle.ca oped r Moped Car Moped50cc Sedan Moped100cc Hatchback This work is licensed under the Android Development with Kotlin Apache 2 license. 52 Visibility modifiers Use visibility modifiers to limit what information you expose. public means visible outside the class. Everything is public by default, including variables and methods of the class. private means it will only be visible in that class (or source file if you are working with functions). protected is the same as private, but it will also be visible to any subclasses. This work is licensed under the Android Development with Kotlin Apache 2 license. 53  Summary This work is licensed under the Android Development with Kotlin Apache 2 license. 54 Summary In Lesson 3, you learned about: Classes, constructors, and getters and setters Inheritance, interfaces, and how to extend classes Extension functions Special classes: data classes, enums, object/singletons, comp anion objects Packages Visibility modifiers This work is licensed under the Android Development with Kotlin Apache 2 license. 55 Pathway Practice what you’ve learned by completing the pathway: Lesson 3: Classes and objects This work is licensed under the Android Development with Kotlin Apache 2 license. 56