Added Kotlin cheatsheet. (#724)
This commit is contained in:
Adam McNeilly
chad d
parent
dccb59103e
commit
04ea319247
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@ -14,6 +14,7 @@ names:
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- JavaScript
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- JavaScript libraries
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- Jekyll
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- Languages
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- Ledger
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- Markup
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- macOS
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@ -0,0 +1,400 @@
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---
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title: Kotlin
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layout: 2017/sheet
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category: Languages
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intro: |
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[Kotlin](http://kotlinlang.org/) is a statically typed programming language for modern multiplatform applications.
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---
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Variables
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---------
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{: .-three-column}
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### Mutability
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```kotlin
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var mutableString: String = "Adam"
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val immutableString: String = "Adam"
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val inferredString = "Adam"
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```
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### Strings
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```kotlin
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val name = "Adam"
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val greeting = "Hello, " + name
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val greetingTemplate = "Hello, $name"
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```
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### Numbers
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```kotlin
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val intNum = 10
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val doubleNum = 10.0
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val longNum = 10L
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val floatNum = 10.0F
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```
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### Booleans
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```kotlin
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val trueBoolean = true
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val falseBoolean = false
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val andCondition = trueBoolean && falseBoolean
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val orCondition = trueBoolean || falseBoolean
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```
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### Static Fields
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```kotlin
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class Person {
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companion object {
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val NAME_KEY = "name_key"
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}
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}
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val key = Person.NAME_KEY
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```
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Null Safety
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-----------
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{: .-two-column}
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### Nullable properties
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```kotlin
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val cannotBeNull: String = null // Invalid
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val canBeNull: String? = null // Valid
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val cannotBeNull: Int = null // Invalid
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val canBeNull: Int? = null // Valid
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```
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### Checking for null
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```kotlin
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val name: String? = "Adam"
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if (name != null && name.length > 0) {
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print("String length is ${name.length}")
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} else {
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print("String is empty.")
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}
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```
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### Safe Operator
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```kotlin
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val nullableStringLength: Int? = nullableString?.length
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val nullableDepartmentHead: String? = person?.department?.head?.name
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```
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### Elvis Operator
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```kotlin
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val nonNullStringLength: Int = nullableString?.length ?: 0
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val nonNullDepartmentHead: String = person?.department?.head?.name ?: ""
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val nonNullDepartmentHead: String = person?.department?.head?.name.orEmpty()
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```
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### Safe Casts
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```kotlin
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// Will not throw ClassCastException
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val nullableCar: Car? = (input as? Car)
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```
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Collections
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-----------
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{: .-two-column}
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### Creation
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```kotlin
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val numArray = arrayOf(1, 2, 3)
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val numList = listOf(1, 2, 3)
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val mutableNumList = mutableListOf(1, 2, 3)
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```
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### Accessing
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```kotlin
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val firstItem = numList[0]
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val firstItem = numList.first()
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val firstItem = numList.firstOrNull()
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```
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### Maps
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```kotlin
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val faceCards = mutableMapOf("Jack" to 11, "Queen" to 12, "King" to 13)
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val jackValue = faceCards["Jack"] // 11
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faceCards["Ace"] = 1
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```
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### Mutability
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```kotlin
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val immutableList = listOf(1, 2, 3)
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val mutableList = immutableList.toMutableList()
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val immutableMap = mapOf("Jack" to 11, "Queen" to 12, "King" to 13)
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val mutableMap = immutableMap.toMutableMap()
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```
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### Iterating
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```kotlin
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for (item in myList) {
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print(item)
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}
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myList.forEach {
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print(it)
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}
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myList.forEachIndexed { index, item ->
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print("Item at $index is: $item")
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}
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```
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### Filtering & Searching
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```kotlin
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val evenNumbers = numList.filter { it % 2 == 0 }
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val containsEven = numList.any { it % 2 == 0 }
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val containsNoEvens = numList.none { it % 2 == 0 }
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val containsNoEvens = numList.all { it % 2 == 1 }
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val firstEvenNumber: Int = numList.first { it % 2 == 0 }
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val firstEvenOrNull: Int? = numList.firstOrNull { it % 2 == 0 }
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```
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Note: `it` is the [implicit name for a single parameter](https://kotlinlang.org/docs/reference/lambdas.html#it-implicit-name-of-a-single-parameter).
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Functions
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---------
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{: .-two-column}
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### Parameters & Return Types
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```kotlin
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fun printName() {
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print("Adam")
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}
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fun printName(person: Person) {
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print(person.name)
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}
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fun getGreeting(person: Person): String {
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return "Hello, ${person.name}"
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}
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fun getGreeting(person: Person): String = "Hello, ${person.name}"
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fun getGreeting(person: Person) = "Hello, ${person.name}"
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```
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### Higher Order Functions
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```kotlin
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fun callbackIfTrue(condition: Boolean, callback: () -> Unit) {
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if (condition) {
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callback()
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}
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}
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callbackIfTrue(someBoolean) {
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print("Condition was true")
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}
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```
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### Extension Functions
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```kotlin
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fun Int.timesTwo(): Int {
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return this * 2
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}
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val four = 2.timesTwo()
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```
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### Default Parameters
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```kotlin
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fun getGreeting(person: Person, intro: String = "Hello,") {
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return "$intro ${person.name}"
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}
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// Returns "Hello, Adam"
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val hello = getGreeting(Person("Adam"))
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// Returns "Welcome, Adam"
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val welcome = getGreeting(Person("Adam"), "Welcome,")
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```
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### Named Parameters
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```kotlin
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class Person(val name: String = "", age: Int = 0)
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// All valid
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val person = Person()
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val person = Person("Adam", 100)
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val person = Person(name = "Adam", age = 100)
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val person = Person(age = 100)
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val person = Person(age = 100, name = "Adam")
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```
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### Static Functions
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```kotlin
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class Fragment(val args: Bundle) {
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companion object {
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fun newInstance(args: Bundle): Fragment {
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return Fragment(args)
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}
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}
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}
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val fragment = Fragment.newInstance(args)
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```
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* [Companion Objects](https://kotlinlang.org/docs/reference/object-declarations.html#companion-objects)
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Classes
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-------
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{: .-two-column}
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### Primary Constructor
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```kotlin
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class Person(val name: String, val age: Int)
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val adam = Person("Adam", 100)
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```
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### Secondary Constructors
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```kotlin
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class Person(val name: String) {
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private var age: Int? = null
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constructor(name: String, age: Int) : this(name) {
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this.age = age
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}
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}
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// Above can be replaced with default params
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class Person(val name: String, val age: Int? = null)
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```
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### Inheritance & Implementation
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```kotlin
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open class Vehicle
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class Car : Vehicle()
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interface Runner {
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fun run()
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}
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class Machine : Runner {
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override fun run() {
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// ...
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}
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}
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```
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Control Flow
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------------
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{: .-two-column}
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### If Statements
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```kotlin
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if (someBoolean) {
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doThing()
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} else {
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doOtherThing()
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}
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```
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### For Loops
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```kotlin
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for (i in 0..10) { } // 1 - 10
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for (i in 0 until 10) // 1 - 9
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(0..10).forEach { }
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for (i in 0 until 10 step 2) // 0, 2, 4, 6, 8
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```
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### When Statements
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```kotlin
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when (direction) {
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NORTH -> {
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print("North")
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}
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SOUTH -> print("South")
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EAST, WEST -> print("East or West")
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"N/A" -> print("Unavailable")
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else -> print("Invalid Direction")
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}
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```
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### While Loops
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```kotlin
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while (x > 0) {
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x--
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}
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do {
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x--
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} while (x > 0)
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```
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Destructuring Declarations
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--------------------------
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{: .-two-column}
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### Objects & Lists
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```kotlin
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val person = Person("Adam", 100)
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val (name, age) = person
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val pair = Pair(1, 2)
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val (first, second) = pair
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val coordinates = arrayOf(1, 2, 3)
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val (x, y, z) = coordinates
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```
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### ComponentN Functions
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```kotlin
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class Person(val name: String, val age: Int) {
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operator fun component1(): String {
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return name
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}
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operator fun component2(): Int {
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return age
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}
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}
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```
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References
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----------
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{: .-one-column}
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* [Defining Variables](https://kotlinlang.org/docs/reference/basic-syntax.html#defining-variables) _(kotlinlang.org)_
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* [Strings Documentation](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-string/index.html) _(kotlinlang.org)_
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* [String Templates](https://kotlinlang.org/docs/reference/basic-types.html#string-templates) _(kotlinlang.org)_
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* [Basic Types](https://kotlinlang.org/docs/reference/basic-types.html) _(kotlinlang.org)_
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* [Companion Objects](https://kotlinlang.org/docs/reference/object-declarations.html#companion-objects) _(kotlinlang.org)_
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* [Null Safety](https://kotlinlang.org/docs/reference/null-safety.html) _(kotlinlang.org)_
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* [Collections Overview](https://kotlinlang.org/docs/reference/collections.html) _(kotlinlang.org)_
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* [Collections Documentation](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/index.html) _(kotlinlang.org)_
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* [Functions Documentation](https://kotlinlang.org/docs/reference/functions.html) _(kotlinlang.org)_
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* [Classes Documentation](https://kotlinlang.org/docs/reference/classes.html) _(kotlinlang.org)_
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* [Destructuring Declarations](https://kotlinlang.org/docs/reference/multi-declarations.html) _(kotlinlang.org)_
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