KotlinIN2000

Gaute Berge

Slides made by: Erlend Stenlund & Steffen Almås

Kotlin - Intro

● Replacement for Java● Multiplatform● .kt

Kotlin - Hello World

● Top level declaration● No class● println moved to stdlib● Semicolon● fun● Unit vs void● Optional args in main● public is default

public class Hey { public static void main(String[] args) { System.out.println("Hello World!"); }}

fun main() { println("Hello World!")}

Kotlin - Interoperability

● 100% interoperable with Java

● Calling Java directly from Kotlin

○ and vice versa

● Can have both in the same project

(but not in the same file)

Kotlin - Variables

val x = 5var y = "Foo"

x = 6y = "Bar"

Val cannot be reassigned

val x: Int = 5var y: String = "Foo"

Kotlin - Functions

fun hei() { println("Hello World")}

fun hei(): Unit { println("Hello World")}

Kotlin - Functions (cont.)

fun add(x: Int, y: Int): Int { return x + y}

fun add(x: Int, y: Int) = x + y

Standard function declaration

Expression function

Kotlin - Functions: Extension functions

public class StringUtils { public static char middle(String s) { return s.charAt(s.length() / 2); }}

String s = "hello";char mid = StringUtils.middle(s);

fun String.middle() = get(length / 2)

val s = "hello"val mid = s.middle()

Extends the functionality of a class

Kotlin - Jetpack

● Android Jetpack

● Android KTX

Kotlin - Types

● Int

● Double

● Float

● String

● Array<T>

● …

● No primitive types

Kotlin - String Templates

● String Templates are expressions that is evaluated

and concatenated in a String

val s = "abc"println("$s.length is ${s.length}")

>> abc.length is 3

Basert på

https://developer.android.com/training/basics/firstapp/creating-project

Control Flow

Kotlin - if

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

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

Expression Function

Kotlin - when

when(x) { 0 -> print("Zero") 1 -> print("One") 2 -> print("Two") 3 -> print("Three") 4 -> print("Four") 5 -> print("Five") 6 -> print("Six") 7 -> print("Seven") 8 -> print("Eight") 9 -> print("Nine") else -> { print("What?") }}

● Think Switch Case in Java● Expression; not a statement● Exhaustive matching

Kotlin - for

val items = listOf("IN2000", "is", "cool")

for (item in items) { println(item)}

for ((index, value) in items.withIndex()) { println("$index : $value")}

Kotlin - for (cont.)

for (i in 6 downTo 0 step 2) { println(i)}

>> 6>> 4>> 2>> 0

for (i in 1..3) { println(i)}

>> 1>> 2>> 3

Kotlin - while

val items = listOf("IN2000", "is", "cool")var index = 0

while (index < items.size) { println("item at $index is ${items[index]}") index++}

Basert på

https://developer.android.com/training/basics/firstapp/creating-project

Null safety

Kotlin - Null safety

“ I call it my billion-dollar

mistake. It was the

invention of the null

reference in 1965 ”

- Tony Hoare -

Kotlin - Null safety (cont.)

● What is null?○ Pointer to nothing?

● Where does this fit into the type system?● One of the most common causes of crashes in Java

Kotlin - Nullable types

T?

T

Kotlin - Nullable types (cont.)

val x: String = null

val x: String? = null

Error: Null can not be a value of a non-null type String

Kotlin - Smart casting

val x: String? = null

if (x != null) { println(x.length)}

Kotlin - Smart casting (cont.)

val x: String? = null

if (x == null) { // Can't use members of b in here} else { println(x.length)}

Kotlin - Null safe call operator

x?.foo()

if (x != null) x.foo() else null

x?.foo()?.bar()

Kotlin - Elvis operator (null default operator)

val z = x ?: y

Kotlin - Non-null assertion

x!!

x ?: throw NullPointerException()

Basert på

https://developer.android.com/training/basics/firstapp/creating-project

Collections

Kotlin - Standard collections

Immutable MutableList<T> MutableList<T>

Set<T> MutableSet<T>

Map<K, V> MutableMap<K, V>

Array<T>

Kotlin - Standard collections (cont.)

Immutable MutablelistOf() mutableListOf()

setOf() mutableSetOf()

mapOf() mutableMapOf()

arrayOf()

Kotlin - Collection interfaces

https://kotlinlang.org/docs/reference/collections-overview.html

Basert på

https://developer.android.com/training/basics/firstapp/creating-project

Classes and Objects

Kotlin - Class

class Person { val age = 10}

val person = Person()println(person.age)

>> 10

Kotlin - Class: Constructors

● Primary constructor

● Secondary constructor

● Primary constructor with properties

● init block

Kotlin - Class: Secondary Constructor

class Person { val age: Int constructor(age: Int) { this.age = age }}

Kotlin - Class: Primary Constructor

class Person(age: Int) { val age: Int = age}

Kotlin - Class: Primary Constructor with property

● Fields in primary constructor● No need for getters and setters

class Person(val age: Int)

Kotlin - Class: Init

● Init will run when object is created● Can have multiple init blocks● Can be used for complicated initialization logic

class Person(val age: Int) { init { println("Creating person with age: $age") }}

public class Person {

private String name; private int age;

public Person(String name, int age) { this.name = name; this.age = age; }

public String getName() { return name; }

public void setName(String name) { this.name = name; }

public int getAge() { return age; }

public void setAge(int age) { this.age = age; }

@Override public String toString() { return "Person{" + "name='" + name + '\'' + ", age=" + age + '}'; }

@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); }}

Kotlin - POJO Class

Kotlin - Data Class

data class Person(var name: String, var age: Int)

Kotlin - Getters and Setters

● Why do we have them in Java?● Encapsulation● Auto-generated, but also customizable

var stringRepresentation: Stringget() = this.toString()set(value) {

setDataFromString(value) }

Kotlin - Getters and Setters interoperability

public class Person {

private String name;

public Person(String name) {this.name = name;

}

public String getName() {return name;

}

public void setName(String name) {

this.name = name;}

}

val person = Person("Alice")

person.name // Same as person.getName()

person.name = "Bob" // Same as person.setName("Bob")

Kotlin - Inheritance

● : both used for extends and implements

● Classes and fields are final by default

● Uses primary constructor instead of super() when extending

open class Base(p: Int)

class Derived(p: Int) : Base(p)

Kotlin - Inheritance (cont.)

● override is a keyword

open class Shape { open fun draw() { /*...*/ } fun fill() { /*...*/ }}

class Circle() : Shape() { override fun draw() { /*...*/ }}

Kotlin - Abstract class and Interface

● Same as Java :)

Kotlin - Sealed classes

● An abstract class with known inheritors

● Exhaustive matching with when

sealed class Exprdata class Const(val number: Double) : Expr()data class Sum(val e1: Expr, val e2: Expr) : Expr()object NotANumber : Expr()

Kotlin - Objects

● Used to create Singletons● Managing state● Lazily initialized

object DataProviderManager { fun registerDataProvider(provider: DataProvider) { // ... }

val allDataProviders: Collection<DataProvider> get() = // ...}

Kotlin - Companion Objects

Object declared inside a class

class MyClass { companion object { fun create(): MyClass = MyClass() }}

val instance = MyClass.create() Called like this

● Alternative to static

Kotlin - Nested Class

Classes can be nested into other classes

class Outer { private val bar: Int = 1 class Nested { fun foo() = 2 }}

val demo = Outer.Nested().foo()

>> 2

Kotlin - Inner Class

An inner class can access members of its outer class

class Outer { private val bar: Int = 1 inner class Inner { fun foo() = bar }}

val demo = Outer().Inner().foo()

>> 1

Kotlin - Anonymous Objects

● Made with the use of Object Expression● Used a lot in Android Studio

○ For instance: onClickListener()

window.addMouseListener(object : MouseAdapter() {

override fun mouseClicked(e: MouseEvent) { ... }

override fun mouseEntered(e: MouseEvent) { ... }})

Kotlin - Enums

● Class with known possible values● Type-safety

enum class Direction { NORTH, SOUTH, WEST, EAST}

Kotlin - Why we need enums

fun move(pos: Pair<Int, Int>, direction: String) = when (direction) { "NORTH" -> pos.first to pos.second+1 "SOUTH" -> pos.first to pos.second-1 "WEST" -> pos.first-1 to pos.second "EAST" -> pos.first+1 to pos.second }

‘when’ expression must be exhaustive, add necessary ‘else’ branch

Kotlin - Why we need enums (cont.)

fun move(pos: Pair<Int, Int>, direction: String) = when (direction) { "NORTH" -> pos.first to pos.second+1 "SOUTH" -> pos.first to pos.second-1 "WEST" -> pos.first-1 to pos.second "EAST" -> pos.first+1 to pos.second else -> throw IllegalArgumentException() }

Kotlin - Why we need enums (cont.)

fun move(pos: Pair<Int, Int>, direction: Direction) = when (direction) { Direction.NORTH -> pos.first to pos.second+1 Direction.SOUTH -> pos.first to pos.second-1 Direction.WEST -> pos.first-1 to pos.second Direction.EAST -> pos.first+1 to pos.second }

enum class Direction { NORTH, SOUTH, WEST, EAST}

Kotlin - Enums with properties

enum class Color(val rgb: Int) { RED(0xFF0000), GREEN(0x00FF00), BLUE(0x0000FF)}

Basert på

https://developer.android.com/training/basics/firstapp/creating-project

Operator overloading and destructuring

Kotlin - Operator overloading

● Defines what an operator cando for other types

● Looks after a function that is modified

● unaryMinus() → unary negation function

data class Point(val x: Int, val y: Int)

operator fun Point.unaryMinus() = Point(-x, -y)

val point = Point(10, 20)

fun main() { println(-point)}

>> Point(x=-10, y=-20)

Kotlin - Destructuring

● Convenient syntax● Often used in loops

val (name, age) = person

val name = person.component1()val age = person.component2()

val name = person.nameval age = person.age

Basert på

https://developer.android.com/training/basics/firstapp/creating-project

Functional Kotlin

Functional - What is functional programming?

● A paradigm

● Style

● Focus on functions

● Higher orders functions

● Immutable values

Functional - Functional languages

● ML

○ Haskell

○ Elm

● Lisp

○ Clojure

● Multi Paradigm

○ Kotlin

○ Scala

Kotlin - Functional programming

“ [...] functional programming can be viewed as a style of programming in which the basic method of computation is the application of functions to arguments.”

- Hutton, Graham (2005). Programming in Haskell.

Functional - In general

● Function as a value○ Higher order functions

● Pure functions○ No side effects

● Anonymous functions (lambda)

● Function composition

Programming without side-effects

● How do we change a value?

● Make a new one!

● What about more complex data structures?

var x = 5x += 1

val x = 5val y = x + 1

Without side-effectsWith side-effects

Functional Kotlin - In this course

● Functional in the LARGE○ Managed effects

○ Monads

● Functional in the SMALL

○ Higher order functions

○ Limited mutations

● We program functional in the SMALL

Kotlin - Higher Order Function

● Takes a function as a parameter

fun repeat(times: Int, action: (Int) -> Unit)

repeat(3) { println("Hello")}

>> Hello>> Hello>> Hello

repeat(0) { error("We should not get here!")}

>>

Kotlin - Lambda Expressions

val add: (Int, Int) -> Int = { x: Int, y: Int -> x + y }

val add = { x, y -> x + y } Without annotations

● Arguments and body are separated with an arrow● Annotations are optional● Can span multiple lines● Return value is the last expression in the body

Kotlin - Lambda type signature

● (Int) -> String

● Parameters in parentheses

● Return-type after arrow (→)

● Read as “from Int to String”

● val onClick: () -> Unit

● val add: (Int, Int) -> Int

Kotlin - Trailing lambdas

Can place lambda outside argument list

f(x, {...})

f(x) {...}

g {...}

Kotlin - it

Implicit single argument

{ x -> x}

{ it }

Kotlin - Working with Immutable Lists

fun squareList(items: List<Int>): List<Int> { val newItems = mutableListOf<Int>() for (item in items) { newItems += item * item } return newItems}

squareList(listOf(1, 2, 3))

>> [1, 4, 9]

Kotlin - Working with Immutable Lists (cont.)

listOf(1, 2, 3).map { it * it }

>> [1, 4, 9]

Kotlin - Map Filter Fold

listOf(1, 2, 3).map { it + 1 }

>> [2, 3, 4]

listOf(1, 2, 3).filter { it % 2 == 0 }

>> [2]

listOf(1, 2, 3).fold(1) { acc, e -> acc * e }

>> 6

Kotlin - Map Filter Fold (cont.)

val sumSquareEven = items .map { it * it } .filter { it % 2 == 0 } .fold(0) { a, v -> a + v }

Kotlin - Map Filter Fold (cont.)

val square = { x: Int -> x * x }val isEven = { x: Int -> x % 2 == 0 }val sumSquareEven = items .map(square) .filter(isEven) .fold(0, Int::plus) // or just sum()

Kotlin - Map Filter Fold (type signatures)

fun <T, R> Iterable<T>.map(transform: (T) -> R): List<R>

fun <T> Iterable<T>.filter(predicate: (T) -> Boolean): List<T>

fun <T, R> Iterable<T>.fold(initial: R, operation: (acc: R, T) -> R): R

Kotlin - Arrow

Functional programmering @ UiO

● IN2040 – Functional programming

● INF3110 – Programming Languages

Basert på

https://developer.android.com/training/basics/firstapp/creating-project

Scope functions

Kotlin - Scope functions

● Execute a block of code within a context

● Let● Run● With● Apply● Also

Kotlin - let

● The context object is available as an argument (it)● The return value is the lambda result● Useful in combination with ?.

fun <T, R> T.let(block: (T) -> R): R

val str: String? = "Hello"val length = str?.let { println("calculating length of $it") it.length}

Kotlin - run

● The context object is available as a receiver (this)● The return value is the lambda result

fun <T, R> T.run(block: T.() -> R): R

val str: String? = "Hello"val length = str?.run { println("calculating length of $this") length}

Kotlin - with

● The context object is passed as an argument ● Available as a receiver (this) inside the lambda● Return value is the lambda result

with (person) { println("Name: $name") println("Age: $age")}

fun <T, R> with(receiver: T, block: T.() -> R): R

Kotlin - apply

● The context object is available as a receiver (this)● The return value is the object itself

fun <T> T.apply(block: T.() -> Unit): T

val adam = Person("Clark").apply { age = 32 city = "Denver"}

Kotlin - also

● The context object is available as an argument (it)● The return value is the object itself

fun <T> T.also(block: (T) -> Unit): T

val numbers = mutableListOf("one", "two", "three")numbers .also { println("Elements before adding: $it") } .add("four")

>> Elements before adding: [one, two, three]

Kotlin - Scope functions overview

Kotlin - Flowchart

Source: https://medium.com/@elye.project/mastering-kotlin-standard-functions-run-with-let-also-and-apply-9cd334b0ef84

Basert på

https://developer.android.com/training/basics/firstapp/creating-project

Summary

Kotlin - Summary

● Forces you to make better decisions

○ Mutability

○ open, override, sealed classes

○ exhaustive matching

● Null safety

● Expression based

● Supports a functional style

Kotlin

Slides and examples are based on the official Kotlin documentation

https://kotlinlang.org/docs/reference/