Added Kotlin cheatsheet. (#724)

_data/categories.yml

@@ -14,6 +14,7 @@ names:
   - JavaScript
   - JavaScript libraries
   - Jekyll
+  - Languages
   - Ledger
   - Markup
   - macOS

kotlin.md

@@ -0,0 +1,400 @@
+---
+title: Kotlin
+layout: 2017/sheet
+category: Languages
+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"
+```
+
+### 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 }
+```
+
+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, ${person.name}"
+```
+
+### Higher Order Functions
+
+```kotlin
+fun callbackIfTrue(condition: Boolean, callback: () -> Unit) {
+    if (condition) {
+        callback()
+    }
+}
+
+callbackIfTrue(someBoolean) {
+    print("Condition was true")
+}
+```
+
+### Extension Functions
+
+```kotlin
+fun Int.timesTwo(): Int {
+    return this * 2
+}
+
+val four = 2.timesTwo()
+```
+
+### Default Parameters
+
+```kotlin
+fun getGreeting(person: Person, intro: String = "Hello,") {
+    return "$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)_