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Official list of idioms: https://kotlinlang.org/docs/idioms.html

Preamble/Important notes¶

"Syntactic sugar" is a false concept. Each language feature affects how we use the language.
One can adopt different programming styles in Kotlin. Idiomatic kotlin is an unofficial combination of practices developed by experienced Kotlin users.
One of the key features of idiomatic kotlin is to use scopes instead of objects to provide incapsulation and limit visible mutable state.

What is idiomatic Kotlin (roughly)?¶

Less "ceremony". Pragmatic approach.
Hybrid object/functional paradigm (objects for data, functions for interactions).
Don't use mutable state wherever read-only is enough.
Don't use classes when functions are enough.
Don't use inheritance when composition is enough.
Function is a first-class citizen.
Scope is a first-class citizen.
Use extensions to separate inner workings from utilities.

Do not do¶

Scope pollution.
Scope function abuse.
Rely on initialization order.

String interpolation¶

Interpolated string allows constructing string fast

In [ ]:

println("This is a plain string")

In [ ]:

val arg = "Interpolated"
println("This is an $arg string")

In [ ]:

val number = 1

fun increment(num: Int) = num + 1
println("This is an $arg string number ${increment(number) + 1}")

In [ ]:

println("This is a string with \$a")

In [ ]:

println(
    """
    \\\
    This is a 
        multi
            line
                raw
                    string
""".trimIndent()
)

In [ ]:

println("""This is a raw string number ${number + 1} with \ and ${'$'} """)

Make `val`, not `var`!¶

val means read-only property

var means writable property

Unnecessary use of var is not recommended.

NOTE: In performance critical cases, explicit cycles are better, but mutable state should be encapsulated.

In [ ]:

/* Not recommended */
var counter = 0
for (i in 0..20) {
    counter += i
}
counter

In [ ]:

/* recommended */
val sum = (0..20).sum()
sum

In [ ]:

val intArray: IntArray = IntArray(21) { it -> it + 1 }
intArray.sum()

In [ ]:

var _a = 2
val a: Int get() = _a

println(a)
_a = 3
println(a)

Top level functions¶

Don't create a class when a function is enough.

Use access modifiers for incapsulation.

In [ ]:

/**
 * Functions could be defined everywhere in kotlin code. This is a so-called top-level function.
 */
fun doSomething() {
    println("I did it")
}

doSomething()

In [ ]:

interface AnInterface {
    fun doSomethingInAnObject()
}

object AnObject : AnInterface {
    /**
     * This is a member-function
     */
    override fun doSomethingInAnObject() {
        println("I did it in an object")
    }
}

AnObject.doSomethingInAnObject()

In [ ]:

fun doSomethingSpecial() {
    val base = "base"
    val special = "special"

    /**
     * This one is inside another function
     */
    fun String.doSomethingInside() {
        println("I did $special inside another function on $this")
    }
    base.doSomethingInside()
}
//"ddd".doSomethingInside()

doSomethingSpecial()

In [ ]:

fun returnFunction(): (String) -> Unit {
    fun printStringWithPrefix(str: String) {
        println("Prefixed: $str")
    }
    return ::printStringWithPrefix
    //return { printStringWithPrefix(it) }
    //return { println("Prefixed: $it") }
}

returnFunction()("Haskel, Boo!")

Unit¶

In [ ]:

fun returnUnit(): Unit {
    // no return statement `Unit` is returned implicitly
    val b = Unit
}

In [ ]:

fun doNothing() {

}

val a = doNothing()
a::class

Default parameters¶

In [ ]:

fun doSomethingWithDefault(a: Int = 0, b: String = "") {
    println("A string with a == $a and b == \"$b\"")
}

doSomethingWithDefault()
doSomethingWithDefault(4)
doSomethingWithDefault(2, "aaa")
doSomethingWithDefault(b = "fff", a = 8)
doSomethingWithDefault(6, b = "fff")
doSomethingWithDefault(a = 2, "fff")// don't do that

In [ ]:

fun integrate(
    from: Double = 0.0,
    to: Double = 1.0,
    step: Double = 0.01,
    function: (Double) -> Double,
): Double {
    require(to > from)
    require(step > 0)
    var sum = 0.0
    var pos = from
    while (pos < to) {
        sum += function(pos)
        pos += step
    }
    return sum * step
}

integrate { x ->
    x * x + 2 * x + 1
}
integrate(0.0, PI) { sin(it) }
// integrate(0.0, PI)  //error
integrate(0.0, step = 0.02, to = PI) { sin(it) }
//integrate(0.0, step = 0.02, PI) { sin(it) }

integrate(0.0, step = 0.02, function = { sin(it) }, to = PI)

In [ ]:

fun functionWithParameters() {
    println("without default")
}

//@JvmOverloads
@JvmName("functionWithDefaultParameters")
fun functionWithParameters(a: Int = 2) { // don't do that
    println("with default")
}

functionWithParameters()

Function body shortcut¶

In [ ]:

fun theSameAsBefore(a: Int = 0, b: String = ""): String = "A string with a == $a and b == $b"

class Context(val a: String, val b: String)

fun somethingWith(context: Context) = with(context) {
    println(a + b)
}

Trailing lambda¶

Scope is a separate important concept in Kotlin

In [ ]:

data class BuildResult internal constructor(val str: String, val int: Int, val optional: Double?) {
    init {
        require(optional != null) { "null!" }
    }
}

class Builder(
    var str: String = "",
    var int: Int = 0
) {
    var optional: Double? = null

    fun build() = BuildResult(str, int, optional)
}

fun BuildResult(defaultInt: Int, block: Builder.() -> Unit): BuildResult =
    Builder(int = defaultInt).apply(block).build()

val res = BuildResult(2) {
    str = "ff"
    optional = 1.0
}

res

Inheritance¶

In [ ]:

//Interfaces and objects

fun interface AnInterface {
    val a: Int
        get() = 4

    // set(value){
    //     println(value)
    // }
    fun doSomething() //= println("From interface")

    fun doSomething2() = Unit
}

abstract class AnAbstractClass(override val a: Int) : AnInterface {
    override final fun doSomething() = println("From a class")
    abstract fun doSomethingElse()
}

class AClass(override val a: Int) : AnInterface {
    override fun doSomething() = println("From a class")
}

class BClass(a: Int) : AnAbstractClass(a) {
    override fun doSomethingElse() = println("something else")
}

/**
 * A singleton (object) is a type (class) which have only one instance
 */
object AnObject : AnInterface {
    override fun doSomething(): Unit = TODO("Not yet implemented")
}

/**
 * Creating an instance
 */
val instance = AClass(3)

/**
 * Using singleton reference without constructor invocation
 */
val obj: AnInterface = AnObject

/**
 * Anonymous object
 */
val anonymous = object : AnInterface {
    override fun doSomething(): Unit = TODO("Not yet implemented")
    override fun doSomething2(): Unit = TODO("Not yet implemented")
}

val anonymous2 = AnInterface {
    println("Do something")
}

/**
 * The one that should not be named
 */
val voldemort = object {
    fun doSomething(): Unit = TODO()
}

// voldemort.doSomething()

In [ ]:

fun interface Float64Function : (Double) -> Double {
    override operator fun invoke(arg: Double): Double

    operator fun invoke(doubles: DoubleArray): DoubleArray = DoubleArray(doubles.size) { invoke(doubles[it]) }
}

val sin = object : Float64Function {
    override fun invoke(p1: Double): Double = sin(p1)
}

val cos = Float64Function { kotlin.math.cos(it) }

val tg = Float64Function(::tan)

sin(PI / 2)

sin(doubleArrayOf(0.0, PI / 2))

Declaration site variance¶

In consumes, out produces.

In [ ]:

interface Producer<out T> {
    fun produce(): T
}

interface Consumer<in T> {
    fun consume(value: T)
}

interface Doer<T> : Producer<T>, Consumer<T> {

}

Return on when¶

https://kotlinlang.org/docs/idioms.html#return-on-when-statement

In [ ]:

fun transform(color: String): Int = when (color) {
    "Red" -> 0
    "Green" -> 1
    "Blue" -> 2
    else -> throw IllegalArgumentException("Invalid color param value")
}

In [ ]:

fun transformIf(flag: Boolean): Int = if (flag) 1 else 0

Runtime type dispatch¶

In Java it is called DOP (Data Oriented Programming).

Also, sometimes (wrongly) called pattern matching.

In [ ]:

/**
 * Matching by type
 */
fun checkType(arg: Any): Unit = when (arg) {
    is String -> println("I am a String. Length is ${arg.length}")
    is Int -> println("I am an Int.")
    is Double -> println("I am a Double")
    //2==2 -> println("Wat?")
    else -> println("I don't know who am I?")
}

fun checkType2(arg: Any): Unit = when {
    arg is String -> println("I am a String")
    arg is Int -> println("I am an Int")
    arg is Double -> println("I am a Double")
    2 == 2 -> println("Wat?")
    else -> println("I don't know who am I?")
}

checkType(true)

Try-catch expression¶

https://kotlinlang.org/docs/idioms.html#try-catch-expression

In [ ]:

fun tryCatch() {
    val result: Int = try {
        22
    } catch (e: ArithmeticException) {
        throw IllegalStateException(e)
    } finally {
        println("finally")
    }

    // Working with result
}

In [ ]:

val res: Result<Int> = runCatching<Int> {
    //error("Error happened")
    4
}

println(res.exceptionOrNull())
(res.getOrNull() ?: 0) + 1

In [ ]:

val dataFlow = sequence {
    var i = 0
    while (true) {
        yield(i++)
    }
}

dataFlow.map {
    runCatching {
        if (it % 2 == 0) error("Even: $it") else it
    }
}.take(10).toList().joinToString(separator = "\n")

Nothing¶

In [ ]:

/**
 * The declaration if valid because [TODO] returns Nothing
 */
fun getSomething(): Int? = TODO()

fun findSomething(): Int {
    // early return is valid because `return` operator result is Nothing
    val found: Int = getSomething() ?: return 2
    return found + 2
}

//FIXME don't do that
fun dontDoThat() {
    if (true) return
    println(false)
}

fun checkCondition(): Int {
    fun conditionSatisfied() = false

    return if (conditionSatisfied()) {
        1
    } else {
        //error is Nothing
        error("Condition is not satisfied")
    }
}

In [ ]:

fun loop(block: () -> Unit): Nothing {
    while (true) {
        block()
    }
}

Data class¶

In [ ]:

class SimpleClass(val a: Int, val b: Double, c: Double) {
    override fun toString() = "SimpleClass(a=$a, b=$b)"
}


data class DataClass(val a: Int, var b: Double /*, d: Double */) {
    init {
        require(b >= 0) { "B should be positive" }
    }

    val c get() = b + 1
}

val simple = SimpleClass(1, 2.0, 3.0)

println(simple)

val data = DataClass(2, 2.0)
val copy = data.copy(b = 22.2)
println(copy)

Nullable truth¶

In [ ]:

/**
 * Nullable truth.
 */
fun nullableTruth() {
    class A(val b: Boolean?)

    val a: A? = A(null) // Compilator warning here
    //use
    a?.b == true
    //instead of
    a?.b ?: false

    // The old way
    val res = if (a == null) {
        false
    } else {
        if (a.b == null) {
            false
        } else {
            a.b
        }
    }
}

In [ ]:

import java.util.Optional
import kotlin.reflect.typeOf

println(typeOf<Optional<Boolean>>() == typeOf<Optional<Optional<Boolean>>>())

println(typeOf<Boolean?>() == typeOf<Boolean??>())

In [ ]:

mapOf<String, String>().get("f")!!

Safe call¶

In [ ]:

import java.io.File

/**
 * Safe call and elvis operator
 */
fun idiom12() {
    val files = File("Test").listFiles()
    println(files?.size ?: "empty")
}
idiom12()

In [ ]:

fun printNotNull(any: Any) = println(any)

// printNotNull(null) does not work

val value: Int? = 2
//val value: Int? by lazy { 2 }

//printNotNull(value) // Error
if (value != null) {
    //not guaranteed to work with mutable variable
    printNotNull(value)
}

var value1: Int? = 2

// Safe call here
value1?.let {
    printNotNull(it) // execute this block if not null
    //printNotNull(value1) // value is not null here
}

Nullable assignment¶

In [ ]:

/**
 * Dart-like (?=) nullable assignment
 */
fun idiom11() {
    var x: String? = null
    x = x ?: "Hello"
}

Extension functions¶

In [ ]:

/**
 * Extension function on a String. It is not monkey-patching.
 */
fun String.countOs(): Int = count { it == 'ы' } // implicit this points to String

fun Int.printMe() = println(this) // explicit this

fun Any?.doSomething(): Int = TODO("Don't do that")

fun ((Int) -> Int).evalAndIncrement(): (Int) -> Int = { this.invoke(it) + 1 }

// val intFunction: (Int)->Int = {it -1}

// val modifiedFunction = intFunction.evalAndIncrement()

// println(modifiedFunction(0))

"вылысыпыдыстычка".countOs().printMe()

In [ ]:

infix fun <T : Comparable<T>> ClosedRange<T>.intersect(other: ClosedRange<T>): ClosedRange<T>? {
    val start = maxOf(this.start, other.start)
    val end = minOf(this.endInclusive, other.endInclusive)
    return if (end >= start) start..end else null
}

(0..8).intersect(6..12)
0..8 intersect 6..12

In [ ]:

0.0..8.0 intersect 2.9..6.1

In [ ]:

fun List<String>.concat() = joinToString(separator = "")
listOf("a", "b", "c").concat()
listOf(1, 2, 3).concat()

In [ ]:

val functionWithReciever: List<String>.(arg: Int) -> Unit = { arg ->
    println(get(arg))
}

listOf("1", "2", "3").functionWithReciever(1)

Extension properties¶

Remember property usage guidelines.

In [ ]:

/**
 * Extension property (must be virtual)
 */
val List<Number>.odd get() = filter { it.toInt() % 2 == 1 }

List(10) { it }.odd

In [ ]:

var MutableMap<String, String>.a: String?
    get() = this.get("a")
    set(value) {
        if (value == null) {
            this.remove("a")
        } else {
            this.set("a", value)
        }
    }

val map = mutableMapOf(
    "a" to "a",
    "b" to "b"
)
val map2 = mutableMapOf(
    Pair("a", "a"),
    Pair("b", "b")
)

map.a = "6"

map.a

In [ ]:

/**
 * Extension variable (also must be virtual)
 */
var Array<Int>.second: Int
    get() = this[1]
    set(value) {
        this[1] = value
    }


val array = Array(5) { it }
array.second = 9
array

Scope functions (run, with, let)¶

In [ ]:

object AClass {
    val a = "a"
    val b = "b"
    val c = "c"
}

fun getAClass(): AClass? = null

/**
 * [with]/[run]/[let] are used to create a scope with given argument or receiver
 */


// Simple print of AClass
println("a = ${AClass.a}, b = ${AClass.b}, c = ${AClass.c}")

// Using `with`
val res = with(AClass) {
    // AClass type is the receiver in this scope
    println("a = ${this.a}, b = $b, c = $c")
    /*return@with*/ "some value"
}

res

In [ ]:

object AContext {
    fun AClass.abc() = a + b + c // warning additional concatenation
}

fun printAbc(aClass: AClass) = with(AContext) {
    aClass.abc()
}

In [ ]:

//using `run`
getAClass()?.takeIf { it.a.isNotEmpty() }?.run {
    // the same as above
    println("a = $a, b= $b, c = $c")
    2
}

val runResult: Int = run {

}

//Using `let` to compose result. Not recommended using without a need
val letResult = getAClass()?.let { arg ->
    arg.c + arg.a
}

In [ ]:

//Don't do that
fun scopeAbuse(str: String?) =
    str.takeIf { it?.isNotEmpty() == true }?.let { it.substring(0..4) }?.let { it.matches(".*".toRegex()) }

fun withoutAbuse(str: String?): Boolean? = if (str?.isNotEmpty() == true) {
    val substring = str.substring(0..4)
    substring.matches(".*".toRegex())
} else {
    null
}

Scope functions (also, apply)¶

In [ ]:

/**
 * Using apply/also to add a finalizing action
 */
var i = 2

/**
 * [also] block does not affect the result
 */
fun getAndIncrement() = i.also { i += 1 } //don't do that

fun incrementAndPring(arg: Int) = (arg + 1).also{
    println(it)
}

println(getAndIncrement())
println(i)

In [ ]:

class Rectangle {
    var length: Number = 0
    var breadth: Number = 0
    var color: Int = 0xffffff
}

/**
 * Configure properties of an object (apply)
 * https://kotlinlang.org/docs/idioms.html#configure-properties-of-an-object-apply
 */
val myRectangle = Rectangle().apply {
    length = 4
    breadth = 5
    color = 0xFAFAFA
}

fun Rectangle(block: Rectangle.() -> Unit): Rectangle = Rectangle().apply(block)

val myRectangle2 = Rectangle {
    length = 4
    breadth = 5
    color = 0xFAFAFA
}

fun Rectangle(length: Number) = Rectangle().apply { this.length = length }

Rectangle(8)

In [ ]:

Lists¶

Do not cast List to MutableList!

In [ ]:

/**
 * Lists and mutable lists
 */


/**
 * This method creates a read-only list of strings. One should note that the type of the list is not specified
 */
val list: List<String> = listOf("a", "b", "c")

println(list[0])
println(list.get(1))
//println(list.get(4))
println(list.getOrNull(4) ?: "nothing")
list::class

In [ ]:

/**
 * This one creates a mutable list
 */
val mutableList: MutableList<String> = mutableListOf("a", "b", "c")
mutableList[2] = "d"
mutableList.add("e")
mutableList += "f"
mutableList

In [ ]:

//don't do that ever
fun doBadThingWithList(list: List<String>): List<String> = (list as MutableList<String>).apply { add("something") }

doBadThingWithList(listOf("a", "b", "c"))

In [ ]:

listOf("a")::class

In [ ]:

val mutableList = mutableListOf(1, 2, 3)

fun consumeList(list: List<Int>) {
    println(list.joinToString())
}
consumeList(mutableList)
mutableList.add(4)
consumeList(mutableList)

In [ ]:

listOf(1, 2) + listOf(3, 4)

In [ ]:

var roList = listOf("a", "b")
roList += setOf("c")
roList

In [ ]:

val writableList = mutableListOf("a", "b") // Warning!
writableList += setOf("c")
writableList

In [ ]:

/**
 * This one creates a mutable ArrayList.
 */
val arrayList: ArrayList<String> = arrayListOf("a", "b", "c")

//ArrayList<Int>()

//Danger zone
//
//val newList: List<String> = list + "f" + mutableList
//
//println(newList)

In [ ]:

//Bonus

val lambdaList = List(3) { it.toString() }
println(lambdaList)

val builderList: List<Int> = buildList {
    add(2)
    add(8)
    remove(8)
}

builderList

In [ ]:

// val sequentialList = List(20){it}
// sequentialList.zipWithNext{ l, r -> r - l }.forEach{println(it)}

Shortcut collection builders¶

In [ ]:

/**
 * Use shortcut function to provide default values
 */
fun doSomething(additionalArguments: List<String> = emptyList()) {
    TODO()
    emptyArray<String>()
    emptySet<String>()
    emptyMap<String, String>()
}
emptyList<String>()::class

In [ ]:

Maps¶

In [ ]:

val map = mutableMapOf(
    "key" to "a",
    "key2" to "b",
)

//The map could be accessed via indexing operation
println(map::class)
println(map["key"])
map["key"] = "fff"
println(map["key"])
println(map["key3"])

In [ ]:

//val entry: MutableMap.MutableEntry<String, String> = map.iterator().next()

//map.entries.first().component2()

/**
 * The destructuring declaration for maps and other objects that support `operator fun componentN`
 */
for ((k: String, v) in map) {
//val (k, v) = entry
//        val k = entry.component1()
//        val v = entry.component2()
    println("$k -> $v")
}

In [ ]:

map.forEach { (k, v) -> println("$k -> $v") }

// java version
map.forEach { k, v -> println("$k -> $v") }

In [ ]:

val (a, b) = Pair(1, 2)

val coord = doubleArrayOf(0.0, 1.0, 2.0)

val (x, y, z) = coord

data class Coordinate(val x: Double, val y: Int)

val (x1, y1) = Coordinate(1.0, 2)

Mutable type access decorator¶

To be replaced with qualified getter types soon

In [ ]:

class AClassWithList {
    var b: Double = 2.0
        private set

    init {
        b = 5.0
    }

    private val _list: MutableList<Int> = ArrayList<Int>()
    val list: List<Int> get() = _list

    fun add(int: Int) {
        require(int > 0)
        _list.add(int)
    }
}


val obj = AClassWithList()

// obj.b = 10.0 //error
obj.add(42)
obj.list.add(43)

Wrap mutable logic / Kotlin builder pattern¶

In [ ]:

val list = buildList {
    repeat(10) {
        add(it)
    }
}
println(list)

In [ ]:

data class ImmutableObject(val a: String, val b: String, val c: Int)

class ImmutableObjectBuilder(a: String) {
    var a: String = a
    var b: String = ""
    var c = 0

    fun build() = ImmutableObject(a, b, c)
}

/*inline*/ fun ImmutableObject(a: String, block: ImmutableObjectBuilder.() -> Unit): ImmutableObject =
    ImmutableObjectBuilder(a).apply(block).build()

ImmutableObject("aValue") {
    c = 99
}

Contains operator and ranges¶

In [ ]:

val emailsList = emptyList<String>()

// When used directly infix in operator checks if the element is contained in a collection
//using `operator fun contains`

if ("john@example.com" in emailsList) {
    println("is in list")
}

if ("jane@example.com" !in emailsList) {
    println("not in list")
}

In [ ]:

import java.time.*

class DateRange(val start: Instant, val end: Instant)

operator fun DateRange.contains(value: Instant): Boolean = value > start && value < end


println(Instant.now() in DateRange(Instant.EPOCH, Instant.MAX))

In [ ]:

// Another (different) use of `in` is iteration over range or collection using
// using `operator fun iterator`

for (i in 1..100) {
    println(i)
}  // closed range: includes 100

(1..100).forEach { i ->
    println(i)
} //the same, but with boxing

for (i in 1 ..< 100) {
    println(i)
} // half-open range: does not include 100

for (x in 2..10 step 2) {
    println(x)
}

for (x in 10 downTo 1) {
    println(x)
}

infix fun ClosedRange<Double>.step(step: Double): Sequence<Double> {
    //TODO check arguments
    var current = start
    return sequence {
        do {
            yield(current)
            current += step
        } while (current <= endInclusive)
    }
}

for (x in 0.0..10.0 step 0.5) {
    println(x)
}

Map-reduce¶

In [ ]:

val list: List<Int> = listOf(1, 2, 3, 4, 5, 6)

val result = list
    //.stream().parallel()
    //.asSequence()
    .filter { it % 2 == 0 } //select even numbers
    .map { it * it } // get square of each element
    //.onEach { println(it) }
    //.sumOf { it } //use one of reduce operations
    .reduce { acc: Int, i: Int -> acc + i }
    //.fold(0.0) { acc: Double, i: Int -> acc + i }

result

In [ ]:

val sequence = sequence {
    var counter = 1
    while (true) {
        yield(counter++)
        yield(counter++)
        // println(counter)
    }
}

val result = sequence
    .take(6)
    .filter { it % 2 == 0 } //select even numbers
    .map { it * it } // get square of each element
    //.onEach { println(it) }
    //.sumOf { it } //use one of reduce operations
    .reduce { acc: Int, i: Int -> acc + i }

result

Scoped resource usage¶

In [ ]:

import java.nio.file.*


val stream = Files.newInputStream(Paths.get("file.txt"))
// The resource is automatically closed  when leaving the scope
stream.bufferedReader().use { reader ->
    println(reader.readText())
}

Factory as parameter and companion factories¶

In [ ]:

interface Factory<T : Any> {
    fun build(str: String): T
}

data class IntContainer(val arg: Int) {

    companion object : Factory<IntContainer> {
        override fun build(str: String) = IntContainer(str.toInt())

        fun buildSpecial(str: String) = IntContainer(str.toInt())
    }
}

data class DoubleContainer(val arg: Double) {

    companion object : Factory<DoubleContainer> {
        override fun build(str: String) = DoubleContainer(str.toDouble())
    }
}

fun <T : Any> buildContainer(str: String, factory: Factory<T>): T = factory.build(str)


buildContainer("22", IntContainer)

Initialization¶

In [ ]:

open class Bad {
    val value: Int = requestValue()

    open fun requestValue(): Int {
        doSomethingElse()
        return 2
    }

    private fun doSomethingElse() {
        println(value)
    }
}

Bad()

In [ ]:

class BadString {
    val value: String = requestValue()

    fun requestValue(): String {
        doSomethingElse()
        return "2"
    }

    private fun doSomethingElse() {
        println(value)
    }
}

BadString()

In [ ]:

//Factory functions are preferred to the initialization logic

data class Good internal constructor(val value: Int) {
    init {
        //Initialization block is there to check arguments
        require(value >= 0)
    }

    companion object
}

private fun requestValue(): Int = TODO()

// This is the factory-function
fun Good(): Good = Good(requestValue())

// additional constructor-like builders could be added to the companion

@OptIn(ExperimentalUnsignedTypes::class)
fun Good.Companion.build(value: UInt) = Good(value.toInt())

Good.build(32U)

Delegates¶

In [ ]:

class ClassWithALazyProperty {
    //Use lazy delegate for something that should be calculated ones on first call
    val lazyValue by lazy {
        //Do dome heavy logic here
        println("Initialized")
        22
    }

    val getterValue: Int
        get() {
            println("got")
            return 33
        }
}

val lazyClass = ClassWithALazyProperty()
println(lazyClass.lazyValue)
println(lazyClass.lazyValue)
println(lazyClass.getterValue)
println(lazyClass.getterValue)

In [ ]:

//Using other delegates
val map = mutableMapOf("a" to 1, "b" to 2)

var a: Int by map

println(a)
a = 3
println(map)

Inline functions¶

In [ ]:

/**
 * Definition of inline function
 */
inline fun List<Int>.forEachOdd(block: (Int) -> Unit) = forEach {
    if (it % 2 == 1) block(it)
}


/**
 * The demonstration of use of inline [forEach] function with non-local return
 */
fun foo(): Int {
    listOf(1, 2, 3, 4, 5).forEachOdd {
        if (it == 3) return it // non-local return directly to the caller of foo()
        print("$it, ")
    }
    println("this point is unreachable")
    return 0
}
foo()

In [ ]:

/**
 * Using inline function for type reification during the compile time
 */
inline fun <reified T> List<T>.prettyPrint() = forEach {
    when (T::class) {
        Double::class -> println("Double: ${it as Double}")
        Int::class -> println("Int: ${it as Int}")
        else -> it.toString()
    }
}

inline fun <reified T> prettyPrintOne(arg: T) = listOf(arg).prettyPrint()

listOf(1, 2, 3).prettyPrint()

/**
 * **WARNING** inline functions are an advanced feature and should be used only for
 * reification or non-local return
 * NOT for optimization.
 */

Collections and boxing¶

In [ ]:

/**
 * Values are boxed. Each call is indirect
 */
val list: List<Double> = List(20) { it.toDouble() }

/**
 * Still boxed
 */
val genericArray: Array<Double> = Array(20) { it.toDouble() }

/**
 * Non-boxed
 */
val specializedArray: DoubleArray = DoubleArray(20) { it.toDouble() }

In [ ]:

68 KiB Raw Blame History

Preamble/Important notes¶

What is idiomatic Kotlin (roughly)?¶

Do not do¶

String interpolation¶

Make val, not var!¶

Top level functions¶

Unit¶

Default parameters¶

Function body shortcut¶

Trailing lambda¶

Inheritance¶

Declaration site variance¶

Return on when¶

Runtime type dispatch¶

Try-catch expression¶

Nothing¶

Data class¶

Nullable truth¶

Safe call¶

Nullable assignment¶

Extension functions¶

Extension properties¶

Scope functions (run, with, let)¶

Scope functions (also, apply)¶

Lists¶

Shortcut collection builders¶

Maps¶

Mutable type access decorator¶

Wrap mutable logic / Kotlin builder pattern¶

Contains operator and ranges¶

Map-reduce¶

Scoped resource usage¶

Factory as parameter and companion factories¶

Initialization¶

Delegates¶

Inline functions¶

Collections and boxing¶

68 KiB

Raw Blame History

Make `val`, not `var`!¶