cheatsheets/kotlin.md

---
title: Kotlin
updated: 2018-12-06
category: Java & JVM
prism_languages: [kotlin]
intro: |
    [Kotlin](http://kotlinlang.org/) is a statically typed programming language for modern multiplatform applications.
---

Variables
---------
{: .-three-column}

### Mutability

```kotlin
var mutableString: String = "Adam"
val immutableString: String = "Adam"
val inferredString = "Adam"
```

### Strings

```kotlin
val name = "Adam"
val greeting = "Hello, " + name
val greetingTemplate = "Hello, $name"
val interpolated = "Hello, ${name.toUpperCase()}"
```

### Numbers

```kotlin
val intNum = 10
val doubleNum = 10.0
val longNum = 10L
val floatNum = 10.0F
```

### Booleans

```kotlin
val trueBoolean = true
val falseBoolean = false
val andCondition = trueBoolean && falseBoolean
val orCondition = trueBoolean || falseBoolean
```

### Static Fields

```kotlin
class Person {
    companion object {
        val NAME_KEY = "name_key"
    }
}

val key = Person.NAME_KEY
```

Null Safety
-----------
{: .-two-column}

### Nullable properties

```kotlin
val cannotBeNull: String = null // Invalid
val canBeNull: String? = null // Valid

val cannotBeNull: Int = null // Invalid
val canBeNull: Int? = null // Valid
```

### Checking for null

```kotlin
val name: String? = "Adam"

if (name != null && name.length > 0) {
    print("String length is ${name.length}")
} else {
    print("String is empty.")
}
```

### Safe Operator

```kotlin
val nullableStringLength: Int? = nullableString?.length
val nullableDepartmentHead: String? = person?.department?.head?.name
```

### Elvis Operator

```kotlin
val nonNullStringLength: Int = nullableString?.length ?: 0
val nonNullDepartmentHead: String = person?.department?.head?.name ?: ""
val nonNullDepartmentHead: String = person?.department?.head?.name.orEmpty()
```

### Safe Casts
```kotlin
// Will not throw ClassCastException
val nullableCar: Car? = (input as? Car)
```

Collections
-----------
{: .-two-column}

### Creation

```kotlin
val numArray = arrayOf(1, 2, 3)
val numList = listOf(1, 2, 3)
val mutableNumList = mutableListOf(1, 2, 3)
```

### Accessing

```kotlin
val firstItem = numList[0]
val firstItem = numList.first()
val firstItem = numList.firstOrNull()
```

### Maps

```kotlin
val faceCards = mutableMapOf("Jack" to 11, "Queen" to 12, "King" to 13)
val jackValue = faceCards["Jack"] // 11
faceCards["Ace"] = 1
```

### Mutability

```kotlin
val immutableList = listOf(1, 2, 3)
val mutableList = immutableList.toMutableList()

val immutableMap = mapOf("Jack" to 11, "Queen" to 12, "King" to 13)
val mutableMap = immutableMap.toMutableMap()
```

### Iterating

```kotlin
for (item in myList) {
    print(item)
}

myList.forEach {
    print(it)
}

myList.forEachIndexed { index, item -> 
    print("Item at $index is: $item")
}
```

### Filtering & Searching

```kotlin
val evenNumbers = numList.filter { it % 2 == 0 }
val containsEven = numList.any { it % 2 == 0 }
val containsNoEvens = numList.none { it % 2 == 0 }
val containsNoEvens = numList.all { it % 2 == 1 }
val firstEvenNumber: Int = numList.first { it % 2 == 0 }
val firstEvenOrNull: Int? = numList.firstOrNull { it % 2 == 0 }
val fullMenu = objList.map { "${it.name} - $${it.detail}" }
```

Note: `it` is the [implicit name for a single parameter](https://kotlinlang.org/docs/reference/lambdas.html#it-implicit-name-of-a-single-parameter).

Functions
---------
{: .-two-column}

### Parameters & Return Types

```kotlin
fun printName() {
    print("Adam")
}

fun printName(person: Person) {
    print(person.name)
}

fun getGreeting(person: Person): String {
    return "Hello, ${person.name}"
}

fun getGreeting(person: Person): String = "Hello, ${person.name}"
fun getGreeting(person: Person) = "Hello, ${person.name}"
```

### Higher Order Functions

```kotlin
fun callbackIfTrue(condition: Boolean, callback: () -> Unit) {
    if (condition) {
        callback()
    }
}

callbackIfTrue(someBoolean) {
    print("Condition was true")
}
```

### Extension Functions

```kotlin
fun Int.timesTwo(): Int {
    return this * 2
}

val four = 2.timesTwo()
```

### Default Parameters

```kotlin
fun getGreeting(person: Person, intro: String = "Hello,"): String {
    return "$intro ${person.name}"
}

// Returns "Hello, Adam"
val hello = getGreeting(Person("Adam"))

// Returns "Welcome, Adam"
val welcome = getGreeting(Person("Adam"), "Welcome,")
```

### Named Parameters

```kotlin
class Person(val name: String = "", age: Int = 0)

// All valid
val person = Person()
val person = Person("Adam", 100)
val person = Person(name = "Adam", age = 100)
val person = Person(age = 100)
val person = Person(age = 100, name = "Adam")
```

### Static Functions

```kotlin
class Fragment(val args: Bundle) {
    companion object {
        fun newInstance(args: Bundle): Fragment {
            return Fragment(args)
        }
    }
}

val fragment = Fragment.newInstance(args)
```

* [Companion Objects](https://kotlinlang.org/docs/reference/object-declarations.html#companion-objects)

Classes
-------
{: .-two-column}

### Primary Constructor

```kotlin
class Person(val name: String, val age: Int)
val adam = Person("Adam", 100)
```

### Secondary Constructors

```kotlin
class Person(val name: String) {
    private var age: Int? = null

    constructor(name: String, age: Int) : this(name) {
        this.age = age
    }
}

// Above can be replaced with default params
class Person(val name: String, val age: Int? = null)
```

### Inheritance & Implementation

```kotlin
open class Vehicle
class Car : Vehicle()

interface Runner {
    fun run()
}

class Machine : Runner {
    override fun run() {
        // ...
    }
}
```

Control Flow
------------
{: .-two-column}

### If Statements

```kotlin
if (someBoolean) {
    doThing()
} else {
    doOtherThing()
}
```

### For Loops

```kotlin
for (i in 0..10) { } // 1 - 10
for (i in 0 until 10) // 1 - 9
(0..10).forEach { }
for (i in 0 until 10 step 2) // 0, 2, 4, 6, 8
```

### When Statements

```kotlin
when (direction) {
    NORTH -> {
        print("North")
    }
    SOUTH -> print("South")
    EAST, WEST -> print("East or West")
    "N/A" -> print("Unavailable")
    else -> print("Invalid Direction")
}
```

### While Loops

```kotlin
while (x > 0) {
    x--
}

do {
    x--
} while (x > 0)
```

Destructuring Declarations
--------------------------
{: .-two-column}

### Objects & Lists

```kotlin
val person = Person("Adam", 100)
val (name, age) = person

val pair = Pair(1, 2)
val (first, second) = pair

val coordinates = arrayOf(1, 2, 3)
val (x, y, z) = coordinates
```

### ComponentN Functions

```kotlin
class Person(val name: String, val age: Int) {
	operator fun component1(): String {
		return name
	}

	operator fun component2(): Int {
		return age
	}
}
```

References
----------
{: .-one-column}

* [Defining Variables](https://kotlinlang.org/docs/reference/basic-syntax.html#defining-variables) _(kotlinlang.org)_
* [Strings Documentation](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-string/index.html) _(kotlinlang.org)_
* [String Templates](https://kotlinlang.org/docs/reference/basic-types.html#string-templates) _(kotlinlang.org)_
* [Basic Types](https://kotlinlang.org/docs/reference/basic-types.html) _(kotlinlang.org)_
* [Companion Objects](https://kotlinlang.org/docs/reference/object-declarations.html#companion-objects) _(kotlinlang.org)_
* [Null Safety](https://kotlinlang.org/docs/reference/null-safety.html) _(kotlinlang.org)_
* [Collections Overview](https://kotlinlang.org/docs/reference/collections.html) _(kotlinlang.org)_
* [Collections Documentation](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/index.html) _(kotlinlang.org)_
* [Functions Documentation](https://kotlinlang.org/docs/reference/functions.html) _(kotlinlang.org)_
* [Classes Documentation](https://kotlinlang.org/docs/reference/classes.html) _(kotlinlang.org)_
* [Destructuring Declarations](https://kotlinlang.org/docs/reference/multi-declarations.html) _(kotlinlang.org)_