Kotlin

Language Overview

Class

Similar to Java class

class Person(val name: String, val age: Int) // concise constructor

// Full primary constructor
class Person(name: String) {
    init {
        this.name = name
    }
}

// Changing visibility
class InternalComponent
internal constructor(name: String) {
    ...
}

// Secondary constructor
class Rectangle(val height: Int, val width: Int) {
    constructor(side: Int) : this(side, side) { ... }
}

Class modifiers

Class Delegation

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() // calls controller.logger.logAll()
}

Data class

Similar to class, but also includes definition for copy, equals, hashCode and toString methods

data class Person(val name: String, val address: String)

contact.copy(address = "new address")

Enum class

enum class Color {
    BLUE, ORANGE, RED
}

enum class Color(
    val r: Int,
    val g: Int,
    val b: Int
) {
    BLUE(0, 0, 255), ORANGE(255, 165, 0), RED(255, 0, 0);
    
    fun rgb() = (r * 256 + g) * 256 + b
}

Sealed class

Restricts the class hierarchy: all subclasses must be located in the same file

sealed class Expr
class Num(val value: Int) : Expr()
class Sum(val left: Expr, val right: Expr) : Expr()

fun eval(e: Expr): Int = when (e) {
    is Num -> e.value
    is Sum -> eval(e.left) + eval(e.right)
}
// if Expr was not sealed, when would need an else branch

Conditionals

If

If is an expression in Kotlin - it returns a value you can assign to a variable

val max = if (a > b) a else b

Ternary operator

There is no ternary operator in Kotlin

When

Similar to Java's switch statement, useful with many conditions

// Checking conditions
val (description, colour) = when {
    degrees < 10 -> "cold" to BLUE
    degrees < 25 -> "mild" to ORANGE
    else -> "hot" to RED
}

// Checking values
enum class Color {
    BLUE, ORANGE, RED
}

fun getDescription(color: Color): String =
    when (color) {
        Color.BLUE -> "cold"
        Color.ORANGE -> "mild"
        Color.RED -> "hot"
    }
}

// Multiple values
fun respondToInput(input: String) = when (input) {
    "y", "yes" -> "I'm glad you agree"
    "n", "no" -> "Sorry to hear that"
    else -> "I don't understand you"
}

// Match on any object (using equals)
fun mix(c1: Color, c2: Color) =
    when (setOf(c1, c2)) {
        setOf(RED, YELLOW) -> ORANGE
        setOf(YELLOW, BLUE) -> GREEN
        setOf(BLUE, VIOLET) -> INDIGO
        else -> throw Exception("Dirty color")
    }

// Checking types
when (pet) {
    is Cat -> pet.meow() // equivalent to instanceof
    is Dog -> pet.woof() // smart cast to the right type
}

when (val pet = getMyFavouritePet()) {
    is Cat -> pet.meow()
    is Dog -> pet.woof()
}

Constants

Compile-time constants

The value will be inlined everywhere in the resulting bytecode. It's possible only for primitive types and String.

const val answer = 42

@JvmField

Exposes a Kotlin property as a field in Java, so no getter and setter are generated.

@JvmField
val prop = MyClass()

// same as following in Java
public static final MyClass prop = new MyClass();

Exceptions

There is no difference between checked and unchecked exceptions

val percentage = 
    if (number in 0..100)
        number
    else
        throw IllegalArgumentException("Error!")

Try is an expression

val number = try {
    Integer.parseInt(string)
} catch {e: NumberFormatException) {
    return
}

Functions

Default arguments

fun displaySeparator(character: Char = '*', size: Int = 10) {
    repeat(size) {
        print(character)
    }
}

displaySeparator(size = 5) // equivalent to displaySeparator('*', 5)
displaySeparator(character = '?') // equivalent to displaySeparator('?', 10)
displaySeparator(size = 3, character = '5') // valid, prints 555

Extension functions

They are defined outside of the class, but they can be called as a regular member of the class. They have to be imported in all the locations where the function is used. As they are static Java methods under the hood, no override is possible for extension functions.

fun String.lastChar() = this.get(this.length - 1)
// fun String.lastChar() = get(length - 1) is also valid

val c: Char = "abc".lastChar() // returns 'c'

Infix functions

infix fun Int.until(to: Int): IntRange

1.until(10)
1 until 10

infix fun <A, B> A.to(that: B) = Pair(this, that)

"ANSWER".to(42)
"hot" to RED

Inline functions

The compiler will substitute the body of an inline function instead of calling it, which makes the performance overhead for inline functions to zero.

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

@kotlin.internal.InlineOnly
public inline fun <R> run(block: () -> R): R = block()

// the InlineOnly annotation specifies that the function should
// not be called directly but always inlined

Named arguments

println(listOf('a', 'b', 'c').joinToString(
    separator="",
    prefix="(",
    postfix=")"
)) // will print (abc)

Hello, world!

package intro

fun main() {
    val name = "Kotlin"
    println("Hello, $name!") // will print Hello, Kotlin!
}

Lambdas

They replace anonymous classes in Java, and allow functional programming

// Basic syntax
{ x: Int, y: Int -> x + y }

// The following statements are all equivalent
list.any({ i: Int -> i > 0})
list.any() { i: Int -> i > 0 } // valid because lambda is the last argument
list.any { i: Int -> i > 0 } // empty parentheses can be omitted
list.any { i -> i > 0 } // type can be omitted if it can be inferred
list.any { it > 0 } // if lambda has a single argument

// Descructuring declarations
map.mapValues { (key, value) -> "$key -> $value!" }
map.mapValues { (_, value) -> "$value!" } // if one variable is not needed

{ println("hey!") }() // possible, but strange
run { println("hey!") } // usual way to run lambda

fun duplicateNonZero(list: List<Int>): List<Int> {
    return list.flatMap {
        if (it == 0) return listOf()
        listOf(it, it)
    }
}

println(duplicateNonZero(listOf(3, 0, 5)) // prints [] because the return inside
                                          // the lambda makes the fun return
 
// Correct versions                                         
list.flatMap {
    if (it == 0) return@flatMap listOf<Int>()
    listOf(it, it)
}

list.flatMap l@{
    if (it == 0) return@l listOf<Int>()
    listOf(it, it)
}

Lambda with receiver

Similar to an extensions function, it has an implicit this set to the first argument of the with extension function

val sb = StringBuilder()
with (sb) {
    appendln("Alphabeth: ")
    for (c in 'a'..'z') {
        append(c)
    }
    toString()
}

inline fun buildString(
    builderAction: StringBuilder.() -> Unit
) {
    val stringBuilder = StringBuilder()
    stringBuilder.builderAction()
    return stringBuilder.toString()
}

val s = buildString {
    this.append("...")
}

Lateinit

lateinit allows non-nullable properties to be initialized outside the constructor method. It cannot be a val, because that would violate immutability, and cannot be nullable or be a primitive type, as it's initialized to null under the hood. We can check if the property has been inialized by using the isinitialized property.

class KotlinActivity: Activity() {
    lateinit var myData: MyData = null
    
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        
        myData = intent.getParcelableExtra("MY_DATA")
    }
}

Loops

For

val list = listOf("a", "b", "c")
for (s: String in list) {
    print(s)
}

val map = mapOf(1 to "one", 2 to "two", 3 to "three")
for ((key, value) in map) {
    println("$key = $value")
}

val list = listOf("a", "b", "c")
for ((index, element) in list.withIndex()) {
    println("$index: $element")
}

// includes the upper bound - prints 123456789
for (i in 1..9) {
    print(i)
}

// excludes the upper bound - prints 12345678
for (i in 1 until 9) {
    print(i)
}

// iterating with a step - prints 97531
for (i in 9 downTo 1 step 2) {
    print(i)
}

// iterating over a string - prints bcd
for (ch in "abc") {
    print(ch + 1)
}

While

while (condition) {
 /*...*/
}
 
do {
 /*...*/
} while (condition)

Main with arguments

package intro

fun main(args: Array<String>) {
    val name = if (args.size > 0) args[0] else "Kotlin"
    println("Hello, $name!")
}

Multiline strings

val q = """To code,
    or not to code?.."""
println(q) // will print To code,
           //                or not to code?..
           
val r = """To code,
    or not to code?..""".trimMargin()
println(q) // will print To code,
           //            or not to code?..

Nullability

The modern approach is to make a NPE a compile-time error rather than a run-time error

val s1: String = "always not null"
val s2: String? = null

s1.length // OK
s2.length // Compiler error, s2 might be null
val length: Int? = s2?.length // equal to if (s2 != null) s2.length else null
val length: Int = s2?.length ?: 0 // ?: is called the Elvis operator

val s: String?
if (s == null) return
s.length // smart cast, it it passed the check above we know it's not null
s!! // if (s != null) s else null

if (any is String) {
    any.toUpperCase() // smart cast, any is casted to String in this block
}

(any as? String)?.toUpperCase() // safe cast, equivalent to
                                // if (a is String) a else null

Object

It's the Kotlin way to express a Singleton

object KSingleton {
    fun foo() {}
}

KSingleton.foo() // from Kotlin
KSingleton.INSTANCE.foo() // from Java

Companion object

Special object inside a class, useful to implement static methods in Kotlin. They can also implement a interface.

class A {
    companion object {
        fun foo() = 1
    }
}

fun main(args: Array<String>) {
    A.foo()
}

class C {
    companion object {
        @JvmStatic fun foo() {}
        fun bar() {}
    }
}

// from Java
C.foo(); // works because of the @JvmStatic annotation
C.Companion.foo(); // accessing the companion object from Java
C.Companion.bar(); // accessing the companion object from Java

Object expression

Replaces anonymous classes with multiple methods (if the anonymous class has a single method, it can be replaced by a lambda expresssion). This is not a singleton.

window.addMouseListener(
    object : MouseAdapter() {
        override fun mouseClicked(e: MouseEvent) {
            // ...
        }
        
        override fun mouseEntered(e: MouseEvent) {
            // ...
        }
    }
)

Properties

class Contact (
    val name: String, // read-only, property + getter
    var address: String // mutable, property + getter + setter
)

println(contact.address)
contact.address = "..."

class Rectangle(val height: Int, val width: Int) {
    val isSquare: Boolean
        get() {
            return height == width
        }
}

val rectangle = Rectangle(2, 3)
println(rectangle.isSquare) // false

class StateLogger {
    var state = false
        set(value) {
            println("state has changed: $field -> $value")
            field = value
        }
}

class LengthCounter {
    var counter: Int = 0
        private set
        
    fun addWord(word: String) {
        counter += word.length
    }
}

// Extension properties
val String.lastIndex: Int
    get() = this.length - 1
    
val String.indices: IntRange
    get() = 0..lastIndex
    
"abc".lastIndex // 2
"abc".indices // 0..2

val lazyValue: String by lazy {
    println("computed!")
    "Hello"
}

Ranges

fun isLetter(c: Char) = c in 'a'..'z' || c in 'A'..'Z'
isLetter('q') // true
isLetter('*') // false

fun isNotDigist(c: Char) = c !in '0'..'9'
isNotDigit('x') // true

fun recognize(c: Char) = when (c) {
    in '0'..'9' -> "It's a digit!"
    in 'a'..'z', in 'A'..'Z' -> "It's a letter!"
    else -> "I don't know..."
}
recognize('$') // I don't know...

Sequences

Allow lazy operations on collections without creating intermediate collections. They will apply "vertical" evaluation (the first element goes through the whole chain, then the second, etc.) rather than the "horizontal" evaluation collections will perform. Nothing happens until there is a terminal operation.

listOf(1, 2, 3, 4)    // [1, 2, 3, 4]
    .map { it * it }  // [1, 4, 9, 16]
    .find { it > 3 }  // 4
    
listOf(1, 2, 3, 4)
    .asSequence()    // asSequence will apply map and find
    .map { it * it } // to 1 and 2, stopping after 2 because
    .find { it > 3 } // a result is found

val numbers = generateSequence(3) { n ->
    (n + 1).takeIf { n < 7 }
}

println(numbers.first())  // does not run the lambda
println(numbers.toList()) // runs the lambda 4 times

val numbers = sequence {
    var x = 0
    while (true) {
        yield(x++)
    }
}
numbers.take(5).toList() // [0, 1, 2, 3, 4]

String templates

package intro

fun main(args: Array<String>) {
    val name = if (args.size > 0) args[0] else "Kotlin"
    println("Hello, $name!")
}

fun main(args: Array<String>) {
    println("Hello, ${args.getOrNull(0)!") // prints Hello, null! if args is empty
}

Variables

val variable1 // read-only, corresponds to final value in Java
var variable2 // mutable

Resources

Articles

• Learn Kotlin in Y minutes

Awesome

• Awesome Kotlin

Courses

• Kotlin Course - Tutorial for Beginners - freeCodeCamp

• Kotlin for Java developers - Coursera