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feat: add Kotlin intermediate tour objects
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‎docs/kr.tree

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<toc-element hidden="true" topic="kotlin-tour-intermediate-scope-functions.md"/>
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<toc-element hidden="true" topic="kotlin-tour-intermediate-lambdas-receiver.md"/>
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<toc-element hidden="true" topic="kotlin-tour-intermediate-classes-interfaces.md"/>
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<toc-element hidden="true" topic="kotlin-tour-intermediate-objects.md"/>
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</toc-element>
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<toc-element toc-title="Kotlin overview">
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<toc-element topic="multiplatform.topic"/>

‎docs/topics/tour/kotlin-tour-intermediate-objects.md

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[//]: # (title: Intermediate: Objects)
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<tldr>
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<p><img src="icon-1-done.svg" width="20" alt="First step" /> <a href="kotlin-tour-intermediate-extension-functions.md">Extension functions</a><br />
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<img src="icon-2-done.svg" width="20" alt="Second step" /> <a href="kotlin-tour-intermediate-scope-functions.md">Scope functions</a><br />
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<img src="icon-3-done.svg" width="20" alt="Third step" /> <a href="kotlin-tour-intermediate-lambdas-receiver.md">Lambda expressions with receiver</a><br />
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<img src="icon-4-done.svg" width="20" alt="Fourth step" /> <a href="kotlin-tour-intermediate-classes-interfaces.md">Classes and interfaces</a><br />
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<img src="icon-5.svg" width="20" alt="Fourth step" /> <strong>Objects</strong><br />
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<img src="icon-6-todo.svg" width="20" alt="Sixth step" /> <a href="kotlin-tour-intermediate-open-special-classes.md">Open and special classes</a><br />
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<img src="icon-7-todo.svg" width="20" alt="Seventh step" /> <a href="kotlin-tour-intermediate-properties.md">Properties</a><br />
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<img src="icon-8-todo.svg" width="20" alt="Eighth step" /> <a href="kotlin-tour-intermediate-null-safety.md">Null safety</a><br />
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<img src="icon-9-todo.svg" width="20" alt="Ninth step" /> <a href="kotlin-tour-intermediate-libraries-and-apis.md">Libraries and APIs</a></p>
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</tldr>
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In this chapter, you'll expand your understanding of classes by exploring object declarations. This knowledge will help
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you efficiently manage behavior across your projects.
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## Object declarations
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In Kotlin, you can use **object declarations** to declare a class with a single instance. In a sense, you declare the
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class and create the single instance _at the same time_. Object declarations are useful when you want to create a class to
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use as a single reference point for your program or to coordinate behavior across a system.
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> A class that has only one instance that is easily accessible is called a **singleton**.
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>
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{style="tip"}
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Objects in Kotlin are **lazy**, meaning they are created only when accessed. Kotlin also ensures that all
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objects are created in a thread-safe manner so that you don't have to check this manually.
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To create an object declaration, use the `object` keyword:
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```kotlin
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object DoAuth {}
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```
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Following the name of your `object`, add any properties or member functions within the object body defined by curly braces `{}`.
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> Objects can't have constructors, so they don't have headers like classes.
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>
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{style="note"}
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For example, let's say that you wanted to create an object called `DoAuth` that is responsible for authentication:
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```kotlin
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object DoAuth {
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fun takeParams(username: String, password: String) {
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println("input Auth parameters = $username:$password")
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}
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}
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fun main(){
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// The object is created when the takeParams() function is called
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DoAuth.takeParams("coding_ninja", "N1njaC0ding!")
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// input Auth parameters = coding_ninja:N1njaC0ding!
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}
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```
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{kotlin-runnable="true" id="kotlin-tour-object-declarations"}
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The object has a member function called `takeParams` that accepts `username` and `password` variables as parameters
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and returns a string to the console. The `DoAuth` object is only created when the function is called for the first time.
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> Objects can inherit from classes and interfaces. For example:
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>
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> ```kotlin
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> interface Auth {
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> fun takeParams(username: String, password: String)
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> }
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>
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> object DoAuth : Auth {
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> override fun takeParams(username: String, password: String) {
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> println("input Auth parameters = $username:$password")
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> }
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> }
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> ```
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>
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{style="note"}
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#### Data objects
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To make it easier to print the contents of an object declaration, Kotlin has **data** objects. Similar to data classes,
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which you learned about in the beginner tour, data objects automatically come with additional member functions:
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`toString()` and `equals()`.
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> Unlike data classes, data objects do not come automatically with the `copy()` member function because they only have
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> a single instance that can't be copied.
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>
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{type ="note"}
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To create a data object, use the same syntax as for object declarations but prefix it with the `data` keyword:
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```kotlin
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data object AppConfig {}
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```
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For example:
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```kotlin
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data object AppConfig {
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var appName: String = "My Application"
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var version: String = "1.0.0"
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}
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fun main() {
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println(AppConfig)
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// AppConfig
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println(AppConfig.appName)
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// My Application
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}
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```
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{kotlin-runnable="true" id="kotlin-tour-data-objects"}
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For more information about data objects, see [](object-declarations.md#data-objects).
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#### Companion objects
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In Kotlin, a class can have an object: a **companion** object. You can only have **one** companion object per class.
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A companion object is created only when its class is referenced for the first time.
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Any properties or functions declared inside a companion object are shared across all class instances.
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To create a companion object within a class, use the same syntax for an object declaration but prefix it with the `companion`
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keyword:
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```kotlin
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companion object Bonger {}
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```
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> A companion object doesn't have to have a name. If you don't define one, the default is `Companion`.
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>
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{style="note"}
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To access any properties or functions of the companion object, reference the class name. For example:
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```kotlin
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class BigBen {
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companion object Bonger {
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fun getBongs(nTimes: Int) {
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repeat(nTimes) { print("BONG ") }
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}
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}
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}
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fun main() {
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// Companion object is created when the class is referenced for the
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// first time.
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BigBen.getBongs(12)
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// BONG BONG BONG BONG BONG BONG BONG BONG BONG BONG BONG BONG
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}
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```
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{kotlin-runnable="true" kotlin-min-compiler-version="1.3" id="kotlin-tour-classes-companion-object"}
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This example creates a class called `BigBen` that contains a companion object called `Bonger`. The companion object
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has a member function called `getBongs()` that accepts an integer and prints `"BONG"` to the console the same number of times
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as the integer.
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In the `main()` function, the `getBongs()` function is called by referring to the class name. The companion object is created
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at this point. The `getBongs()` function is called with parameter `12`.
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For more information, see [](object-declarations.md#companion-objects).
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## Object declarations practice
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### Exercise 1 {initial-collapse-state="collapsed" collapsible="true" id="objects-exercise-1"}
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You run a coffee shop and have a system for tracking customer orders. Consider the code below and complete the declaration
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of the second data object so that the following code in the `main()` function runs successfully:
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|---|---|
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```kotlin
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interface Order {
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val orderId: String
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val customerName: String
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val orderTotal: Double
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}
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data object OrderOne: Order {
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override val orderId = "001"
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override val customerName = "Alice"
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override val orderTotal = 15.50
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}
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data object // Write your code here
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fun main() {
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// Print the contents of each data object
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println("Order One Details: $OrderOne")
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println("Order Two Details: $OrderTwo")
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// Check if the orders are identical
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println("Are the two orders identical? ${OrderOne == OrderTwo}")
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if (OrderOne == OrderTwo) {
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println("The orders are identical.")
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} else {
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println("The orders are unique.")
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}
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println("Do the orders have the same customer name? ${OrderOne.customerName == OrderTwo.customerName}")
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}
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```
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{validate="false" kotlin-runnable="true" kotlin-min-compiler-version="1.3" id="kotlin-tour-objects-exercise-1"}
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|---|---|
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```kotlin
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interface Order {
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val orderId: String
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val customerName: String
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val orderTotal: Double
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}
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data object OrderOne: Order {
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override val orderId = "001"
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override val customerName = "Alice"
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override val orderTotal = 15.50
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}
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data object OrderTwo: Order {
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override val orderId = "002"
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override val customerName = "Bob"
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override val orderTotal = 12.75
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}
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fun main() {
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// Print the contents of each data object
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println("Order One Details: $OrderOne")
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println("Order Two Details: $OrderTwo")
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// Check if the orders are identical
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println("Are the two orders identical? ${OrderOne == OrderTwo}")
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if (OrderOne == OrderTwo) {
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println("The orders are identical.")
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} else {
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println("The orders are unique.")
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}
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println("Do the orders have the same customer name? ${OrderOne.customerName == OrderTwo.customerName}")
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}
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```
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{initial-collapse-state="collapsed" collapsible="true" collapsed-title="Example solution" id="kotlin-tour-objects-solution-1"}
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### Exercise 2 {initial-collapse-state="collapsed" collapsible="true" id="objects-exercise-2"}
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Create an object declaration that inherits from the `Vehicle` interface to create a unique vehicle type: `FlyingSkateboard`.
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Implement the `name` property and the `move()` function in your object so that the following code in the `main()` function runs
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successfully:
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|---|---|
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```kotlin
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interface Vehicle {
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val name: String
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fun move(): String
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}
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object // Write your code here
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fun main() {
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println("${FlyingSkateboard.name}: ${FlyingSkateboard.move()}")
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println("${FlyingSkateboard.name}: ${FlyingSkateboard.fly()}")
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}
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```
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{validate="false" kotlin-runnable="true" kotlin-min-compiler-version="1.3" id="kotlin-tour-objects-exercise-2"}
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|---|---|
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```kotlin
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interface Vehicle {
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val name: String
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fun move(): String
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}
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object FlyingSkateboard : Vehicle {
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override val name = "Flying Skateboard"
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override fun move() = "Glides through the air with a hover engine"
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fun fly(): String = "Woooooooo"
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}
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fun main() {
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println("${FlyingSkateboard.name}: ${FlyingSkateboard.move()}")
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println("${FlyingSkateboard.name}: ${FlyingSkateboard.fly()}")
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}
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```
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{initial-collapse-state="collapsed" collapsible="true" collapsed-title="Example solution" id="kotlin-tour-objects-solution-2"}
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### Exercise 3 {initial-collapse-state="collapsed" collapsible="true" id="objects-exercise-3"}
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You have an app where you want to record temperatures. The class itself stores the information in Celsius, but
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you want to provide an easy way to create an instance in Fahrenheit as well. Complete the data class so that
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the following code in the `main()` function runs successfully:
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<deflist collapsible="true">
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<def title="Hint">
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Use a companion object.
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</def>
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</deflist>
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|---|---|
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```kotlin
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data class Temperature(val celsius: Double) {
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val fahrenheit: Double = celsius * 9 / 5 + 32
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// Write your code here
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}
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fun main() {
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val fahrenheit = 90.0
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val temp = Temperature.fromFahrenheit(fahrenheit)
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println("${temp.celsius}°C is $fahrenheit °F")
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}
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```
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{validate="false" kotlin-runnable="true" kotlin-min-compiler-version="1.3" id="kotlin-tour-objects-exercise-3"}
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|---|---|
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```kotlin
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data class Temperature(val celsius: Double) {
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val fahrenheit: Double = celsius * 9 / 5 + 32
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companion object {
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fun fromFahrenheit(fahrenheit: Double): Temperature = Temperature((fahrenheit - 32) * 5 / 9)
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}
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}
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fun main() {
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val fahrenheit = 90.0
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val temp = Temperature.fromFahrenheit(fahrenheit)
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println("${temp.celsius}°C is $fahrenheit °F")
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}
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```
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{initial-collapse-state="collapsed" collapsible="true" collapsed-title="Example solution" id="kotlin-tour-objects-solution-3"}
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## Next step
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[Intermediate: Open and special classes](kotlin-tour-intermediate-open-special-classes.md)

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