Kotlin, What Is It? ! Cody Engel 

Cody Engel NIU Computer Science Alum 

Blogging, It’s A Helluva Drug (Also got into programming through editing Wordpress themes) 

Android 2.2 Started Android Development when this was the dominant version. 

Started Learning Kotlin End of 2016 

Convinced Team to Use Kotlin in 2017 

Joined a Kotlin-only Android Team 2018 

All aboard the Kotlin hype-train. BreadLord 

Kotlin, A Brief History 

–Dmitry Jemerov, August 2, 2011 “We (JetBrains) want to become more productive by switching to a more expressive language.” 

Kotlin Named after the Russian island, Kotlin. 

Java Named after the Indonesian island of Java. 

Kotlin v1.0 released on February 15, 2016. 

Cody decides to start learning Kotlin in December 2016. 

Google I/O 2017 Kotlin officially supported by Google for Android development. 

Kotlin v1.2 released on November 28, 2017. Multiplatform Projects pick up steam. 

Kotlin 1.3 Coroutines will graduate from experimental to stable. 

Kotlin Basics (Less is More) 

Functions 

fun sum(a: Int, b: Int): Int { return a + b } Functions 

fun sum(a: Int, b: Int) = a + b Functions 

fun printSum(a: Int, b: Int): Unit { println("sum of $a and $b is ${a + b}") } Functions 

fun printSum(a: Int, b: Int) { println("sum of $a and $b is ${a + b}") } Functions 

Variables 

val immutableNumber: Int = 1 Variables 

val immutableNumber = 1 Variables 

var mutableNumber = 5 mutableNumber = 54 Variables 

String Templates 

var a = 1 val s1 = "a is $a" // a is 1 String Templates 

a = 2 val s2 = "${s1.replace("is", "was")}, but now is $a” // a was 1 but now is 2 String Templates 

Conditional Expressions 

fun maxOf(a: Int, b: Int): Int { if (a > b) { return a } else { return b } } Conditional Expressions 

fun maxOf(a: Int, b: Int) = if (a > b) a else b Conditional Expressions 

Handling Null 

val maybeNull: String? = null val notNull: String = if (maybeNull != null) maybeNull else "" Handling Null 

val maybeNull: String? = null val notNull: String = maybeNull ?: "" Handling Null 

val maybeNull: String? = null val length: Int = maybeNull?.length ?: 0 Handling Null 

val listWithNulls: List = listOf("Kotlin", null) for (item in listWithNulls) { item?.let { println(it) } //prints Kotlin and ignores null } Handling Null 

Type Casting 

if (x instanceof Integer) { System.out.print((Integer) x + 1); } else if (x instanceof String) { System.out.print(((String) x).length() + 1); } else if (x instanceof List) { Integer total = 0; for (Integer i : (List) x) { total += i; } System.out.print(total); } Type Casting 

if (x is Int) { print(x + 1) } else if (x is String) { print(x.length + 1) } else if (x is IntArray) { print(x.sum()) } Type Casting 

when (x) { is Int -> print(x + 1) is String -> print(x.length + 1) is IntArray -> print(x.sum()) } Type Casting 

Classes in Kotlin (Easier to Represent Data) 

–KotlinLang.org - Data Classes Documentation “We frequently create classes whose main purpose is to hold data. In such a class some standard functionality and utility functions are often mechanically derivable from the data. In Kotlin, this is called a data class.” 

public class Person { private final String name; private final int age; public Person(String name, int age) { this.name = name; this.age = age; } public String getName() { return name; } public int getAge() { return age; } } Data Classes 

public class Person { private final String name; private final int age; public Person(String name, int age) { this.name = name; this.age = age; } public String getName() { return name; } public int getAge() { return age; } // but wait… there’s more! @Override public boolean equals(Object o) { if (this == o) return true; if (o == null || getClass() != o.getClass()) return false; Person person = (Person) o; return age == person.age && Objects.equals(name, person.name); } @Override public int hashCode() { return Objects.hash(name, age); } @Override public String toString() { return "Person{" + "name='" + name + '\'' + ", age=" + age + '}'; } } Data Classes 

data class Person(val name: String, val age: Int) Data Classes 

val person = Person("Cody", "56") person.age person.name person.equals(anotherPerson) person.hashCode() person.toString() person.copy(name = "Bob") (name, age) = person Data Classes 

–KotlinLang.org - Sealed Classes Documentation “Sealed classes are used for representing restricted class hierarchies, when a value can have one of the types from a limited set, but cannot have any other type. They are, in a sense, an extension of enum classes: the set of values for an enum type is also restricted, but each enum constant exists only as a single instance, whereas a subclass of a sealed class can have multiple instances which can contain state.” 

–Cody Engel - Kotlin Lunch & Learn “Sealed classes are used for representing restricted class hierarchies. They are similar to enums, except they can have multiple instances of the same type.” 

sealed class Expr data class Const(val number: Double) : Expr() data class Sum(val firstNum: Expr, val secondNum: Expr) : Expr() object NotANumber : Expr() Sealed Classes 

fun eval(expr: Expr): Double = when(expr) { is Const -> expr.number is Sum -> eval(expr.firstNum) + eval(expr.secondNum) NotANumber -> Double.NaN // an `else` clause is not required because we've covered all the cases } Sealed Classes 

Default Values 

class ImageCache( val timeRetrieved: Long, val wasSuccessful: Boolean, val cacheExpirationMs: Long = 300000 ) Default Values 

class ImageCache( val timeRetrieved: Long, val wasSuccessful: Boolean, val cacheExpirationMs: Long = 300000 ) val imageCache = ImageCache( timeRetrieved = 40, wasSuccessful = false ) val imageCache = ImageCache( timeRetrieved = 40, wasSuccessful = false, cacheExpirationMs = 5000 ) Default Values 

Extensions (Less Utils and Better Null Safety) 

Totally Fictitious API Response { "p_admin": "1", "pg_list_add": "1", "pg_list_edit": “0", "pg_list_delete": "1", "pg_list_headers": "0", "pg_list_emailaccount": "1", "pg_list_bounce": "1", "pg_message_add": "1", "pg_message_edit": null, "pg_message_delete": "0" } 

Extension Property 

val String.asBoolean: Boolean get() = this != “0" //… val pAdmin: String = "1" pAdmin.asBoolean Extension Property 

val String?.asBoolean: Boolean get() = this != null && this != "0" //… val pAdmin: String? = null pAdmin.asBoolean Extension Property 

Extension Function 

fun MutableList.swap(index1: Int, index2: Int) { val tmp = this[index1] this[index1] = this[index2] this[index2] = tmp } //… val myList = mutableListOf(55, 32, 45) myList.swap(0, 2) Extension Function 

Delegation (Composition Over Inheritance) 

Property Delegation 

private ExpensiveObjectToCreate _expensiveObject = null; ExpensiveObjectToCreate getExpensiveObject() { if (_expensiveObject == null) { _expensiveObject = new ExpensiveObjectToCreate(); } return _expensiveObject; } Property Delegation 

val expensiveObject by lazy { ExpensiveObjectToCreate() } Property Delegation 

private class SynchronizedLazyImpl(initializer: () -> T, lock: Any? = null) : Lazy, Serializable { private var initializer: (() -> T)? = initializer @Volatile private var _value: Any? = UNINITIALIZED_VALUE // final field is required to enable safe publication of constructed instance private val lock = lock ?: this override val value: T get() { val _v1 = _value if (_v1 !== UNINITIALIZED_VALUE) { @Suppress("UNCHECKED_CAST") return _v1 as T } return synchronized(lock) { val _v2 = _value if (_v2 !== UNINITIALIZED_VALUE) { @Suppress("UNCHECKED_CAST") (_v2 as T) } else { val typedValue = initializer!!() _value = typedValue initializer = null typedValue } } } override fun isInitialized(): Boolean = _value !== UNINITIALIZED_VALUE override fun toString(): String = if (isInitialized()) value.toString() else "Lazy value not initialized yet." private fun writeReplace(): Any = InitializedLazyImpl(value) } Property Delegation 

Cody’s Animal App …Or the best example he could think of for class delegation. 

interface Communicate { fun communicate() } class CatCommunicator() : Communicate { override fun communicate() { println("Meow") } } class DogCommunicator() : Communicate { override fun communicate() { println("Woof") } } class Animal(communicate: Communicate) : Communicate by communicate fun main(args: Array) { val catCommunicator = CatCommunicator() val dogCommunicator = DogCommunicator() Animal(catCommunicator).communicate() // Meow Animal(dogCommunicator).communicate() // Woof } Class Delegation 

interface Communicate { fun communicate() } class CatCommunicator() : Communicate { override fun communicate() { println("Meow") } } class DogCommunicator() : Communicate { override fun communicate() { println("Woof") } } class Animal(communicate: Communicate) : Communicate by communicate fun main(args: Array) { val catCommunicator = CatCommunicator() val dogCommunicator = DogCommunicator() Animal(catCommunicator).communicate() // Meow Animal(dogCommunicator).communicate() // Woof } Class Delegation 

interface Communicate { fun communicate() } class CatCommunicator() : Communicate { override fun communicate() { println("Meow") } } class DogCommunicator() : Communicate { override fun communicate() { println("Woof") } } class Animal(communicate: Communicate) : Communicate by communicate fun main(args: Array) { val catCommunicator = CatCommunicator() val dogCommunicator = DogCommunicator() Animal(catCommunicator).communicate() // Meow Animal(dogCommunicator).communicate() // Woof } Class Delegation 

interface Communicate { fun communicate() } class CatCommunicator() : Communicate { override fun communicate() { println("Meow") } } class DogCommunicator() : Communicate { override fun communicate() { println("Woof") } } class Animal(communicate: Communicate) : Communicate by communicate fun main(args: Array) { val catCommunicator = CatCommunicator() val dogCommunicator = DogCommunicator() Animal(catCommunicator).communicate() // Meow Animal(dogCommunicator).communicate() // Woof } Class Delegation 

interface Communicate { fun communicate() } class CatCommunicator() : Communicate { override fun communicate() { println("Meow") } } class DogCommunicator() : Communicate { override fun communicate() { println("Woof") } } class Animal(communicate: Communicate) : Communicate by communicate fun main(args: Array) { val catCommunicator = CatCommunicator() val dogCommunicator = DogCommunicator() Animal(catCommunicator).communicate() // Meow Animal(dogCommunicator).communicate() // Woof } Class Delegation 

interface Communicate { fun communicate() } class CatCommunicator() : Communicate { override fun communicate() { println("Meow") } } class DogCommunicator() : Communicate { override fun communicate() { println("Woof") } } class Animal(communicate: Communicate) : Communicate by communicate fun main(args: Array) { val catCommunicator = CatCommunicator() val dogCommunicator = DogCommunicator() Animal(catCommunicator).communicate() // Meow Animal(dogCommunicator).communicate() // Woof } Class Delegation 

Higher Order Functions (Because Functional is Quite Functional) 

–Wikipedia “In mathematics and computer science, a higher-order function (also functional, functional form or functor) is a function that does at least one of the following: takes one or more functions as arguments (i.e. procedural parameters), returns a function as its result.” 

Sandwich Code 

Sandwich Code 

val start = System.currentTimeMillis() BreadLord.whatShouldMobileWorkOnThisWeek() val executionTime = System.currentTimeMillis() - start Sandwich Code 

val executionTime = measureTimeMillis { BreadLord.whatShouldMobileWorkOnThisWeek() } Sandwich Code 

public fun measureTimeMillis(block: () -> Unit) : Long { val start = System.currentTimeMillis() block() return System.currentTimeMillis() - start } Sandwich Code 

public inline fun measureTimeMillis(block: () -> Unit) : Long { val start = System.currentTimeMillis() block() return System.currentTimeMillis() - start } Sandwich Code 

Delayed Execution 

fun printNumbers(oneMillionNumbers: List) { oneMillionNumbers.forEach { number -> println(number) } } Delayed Execution 

val oneMillionNumbers = (0..1000000).toList() printNumbers(oneMillionNumbers) Delayed Execution 

fun oneMillionNumbers(): () -> List = { (0..1000000).toList() } printNumbers(oneMillionNumbers()) Delayed Execution 

Kotlin Core Libraries (Because Free is Usually Better) 

Collections 

(0..1000000) .filter { it % 2 == 0 } .map { it.toString() } .map { Pair(it, it.length) } Collections 

(0..1000000) .filter { it % 2 == 0 } .map { it.toString() } .map { Pair(it, it.length) } // 3 Lists, Expensive! Collections 

Sequence 

(0..1000000) .asSequence() .filter { it % 2 == 0 } .map { it.toString() } .map { Pair(it, it.length) } .toList() Sequence 

(0..1000000) .asSequence() .filter { it % 2 == 0 } .map { it.toString() } .map { Pair(it, it.length) } .toList() // 1 List, Sorta Cheap ¯\_(ツ)_/¯ Sequence 

Kotlin Core Libraries kotlin Core functions and types, available on all supported platforms. kotlin.annotation Library support for the Kotlin annotation facility. kotlin.browser - JS Access to top-level properties (document, window etc.) in the browser environment. kotlin.collections Collection types, such as Iterable, Collection, List, Set, Map and related top-level and extension functions. kotlin.comparisons Helper functions for creating Comparator instances. kotlin.concurrent - JVM Utility functions for concurrent programming. kotlin.coroutines.experimental Library support for coroutines, including support for lazy sequences. kotlin.coroutines.experimental.intrinsics Low-level building blocks for libraries that provide coroutine-based APIs. kotlin.dom - JS Utility functions for working with the browser DOM. kotlin.experimental Experimental APIs, subject to change in future versions of Kotlin. kotlin.io IO API for working with files and streams. kotlin.js - JS Functions and other APIs specific to the JavaScript platform. kotlin.jvm - JVM Functions and annotations specific to the Java platform. kotlin.math Mathematical functions and constants. kotlin.properties Standard implementations of delegates for delegated properties and helper functions for implementing custom delegates. kotlin.ranges Ranges, Progressions and related top-level and extension functions. kotlin.reflect Runtime API for Kotlin reflection kotlin.reflect.full - JVM Extensions for Kotlin reflection provided by kotlin-reflect library. kotlin.reflect.jvm - JVM Runtime API for interoperability between Kotlin reflection and Java reflection provided by kotlin-reflect library. kotlin.sequences Sequence type that represents lazily evaluated collections. Top-level functions for instantiating sequences and extension functions for sequences. kotlin.streams - JVM Utility functions for working with Java 8 streams. kotlin.system - JVM System-related utility functions. kotlin.text Functions for working with text and regular expressions. org.khronos.webgl - JS Kotlin JavaScript wrappers for the WebGL API. org.w3c.dom - JS Kotlin JavaScript wrappers for the DOM API. org.w3c.dom.css - JS Kotlin JavaScript wrappers for the DOM CSS API. org.w3c.dom.events - JS Kotlin JavaScript wrappers for the DOM events API. org.w3c.dom.parsing - JS Kotlin JavaScript wrappers for the DOM parsing API. org.w3c.dom.svg - JS Kotlin JavaScript wrappers for the DOM SVG API. org.w3c.dom.url - JS Kotlin JavaScript wrappers for the DOM URL API. org.w3c.fetch - JS Kotlin JavaScript wrappers for the W3C fetch API. org.w3c.files - JS Kotlin JavaScript wrappers for the W3C file API. org.w3c.notifications - JS Kotlin JavaScript wrappers for the Web Notifications API. org.w3c.performance - JS Kotlin JavaScript wrappers for the Navigation Timing API. org.w3c.workers - JS Kotlin JavaScript wrappers for the Web Workers API. org.w3c.xhr - JS Kotlin JavaScript wrappers for the XMLHttpRequest API. https://kotlinlang.org/api/latest/jvm/stdlib/index.html 

Why Kotlin? 

Less is More The language was designed to be concise. 

Kotlin & Java The interoperability is 

IDE Support JetBrains provides added functionality for Kotlin in IntelliJ. 

Kotlin/Native is Coming Write once, run anywhere. 

Kotlin Core Library Expanding The core library will continue to add features like Coroutines and Contracts. 

Resources • Official Documentation - https://kotlinlang.org/ • Try Online - https://try.kotlinlang.org/ • Why JetBrains needs Kotlin - https://blog.jetbrains.com/kotlin/2011/08/ why-jetbrains-needs-kotlin/ • What #android-dev is doing - https://github.com/ActiveCampaign/ android-crm