Kotlin-basic

[ezrohir]

Dictionaries / Maps

var namesWithAges = HashMap<String, Int>()
var namesWithAges = HashMap<String, Int>(20) //define capacity

// Assigning values:
namesWithAges.put("John Doe", 34)
//OR
namesWithAges["John Doe"] = 34

// Creating an immutable map:
val namesWithAges = mapOf("John Doe" to 34, "Jane Doe" to 29)

[ezrohir]

Functions

Prototype of a Kotlin function:

fun functionName(parameterName: DataType): ReturnType {
  //function body
}
fun greetUser(name: String): String {
  return "Hello ${name}!"
}

Default Argument

fun greetUser(name: String = "World"): String {
  return "Hello ${name}!"
}
greetUser() //returns Hello World!

[ezrohir]

Lambda

val square = { a:Int ->
  a * a
}
println(square(4)) //16

var multiple: (Int) -> Int = { it * it }
println(multiple(4)) //16
multiple = {it * it * it} //don't need '->'
println(multiple(2)) //8

  // 1 
  val lambda1: (String) -> Unit = { name: String -> println("Hello, $name") }
  // 2 
  val lambda2: (String) -> Unit = { name -> println("Hello, $name") }
  // 3 
  val lambda3: (String) -> Unit = { println("Hello, $it") }
  // 4
  val lambda4: (String) -> Unit = { println("Hello, World!") }
  // 5
  val lambda5: (Int, Int) -> Int = {  x, y -> 
    print(x)
    print(y)
    x + y
  }
  // 6
  val lambda6 = {x: Int, y: Int -> x + y }

[ezrohir]

Nullable / Optional Types

// kotlin Nullable Type
var authToken: String? = "some long string"
authToken = null

// Swift Optional Type
var authToken: String? = "some long string"
authToken = nil

Elvis Operator ?:

var id: Int? = null
val userId = id ?: -1 //userId is -1

fun readyForParade (participant : Bandmember) : Boolean {
  val trombonePlayer = participant as?  Bandmember ?: return false
  return trombonePlayer. practiceTime > 6000
}

[ezrohir]

when

Kotlin-when
val x = 3
when (x) {
  1 -> println("x == 1")
  2 -> println("x == 2")
else -> { 
  print("x is neither 1 nor 2")
  }
}
Swift-switch
let x = 3
switch x {
  case 1:
    print("x == 1")
  case 2:
    print("x == 2")
  default:
    print("x is neither 1 nor 2")
}

[ezrohir]

Class Extensions

Kotlin - Extension
fun Int.square(): Int {
  return this * this
}

println(5.square()) //prints 25

Swift - Extension
extension Int {
  func square() -> Int {
    return self * self
  }
}
print(5.square()) //prints 25

[ezrohir]

Interface / Protocol

Interface / Protocol

// Kotlin — Interface
interface Animal {
  var canFly: Boolean 

  fun eat()

  fun makeSound()
}
// Swift - Protocol

protocol Animal {
  var canFly: Bool {get}

  func eat()

  func makeSound()
}

Implementation

// Kotlin
interface Animal {
  val canFly: Boolean 

  get() = true 
}

// Swift
protocol Animal {
  var canFly: Bool {get}
}

extension Animal {
  var canFly: Bool {
    return false
  }
}