Kotlin Basics

This highlight just covers scenarios not entirely standardised to most programming languages eg it will not cover while loops etc.

Kotlin REPL

Brew install kotlin and then you'll have access to the REPL.

Use kotlinc with no arguments to enter the REPL.

To compile and run code from the command line:

kotlinc hello.kt -include-runtime -d hello.jar
java -jar hello.jar
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Hello, Kotlin

fun main(args: Array<String>) {
  println("Hello, Kotlin!")
}
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Shift Operators

1 shl 3 // shift left
32 shr 2 // shift right
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Math Functions

import kotlin.math.*

fun main(args: Array<String>) {
  sin(45 * PI / 180) // 0.7071067811865475
}

sqrt(4.0) // 2.0

max(1,2) // 2
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Naming

val number: Int = 10 // constants

const val reallyConstant: Int = 42 // top level keyword to mark it as compile-time constant

var variableNumber: Int = 42
variableNumber = 12 // allowed
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Type Conversion

var integer: Int = 100
var decimal: Double = 12.5
integer = decimal // THROWS error
integer = decimal.toInt() // works nicer
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To do implicit conversion:

val hourlyRate: Double = 19.5
val hoursWorked: Int = 10
val totalCost: Double = hourlyRate * hoursWorked
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Chars Vs Strings

val characterA: Char = 'a' // must be single quotes
val stringDog: String = "Dog" // must be double quotes
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String interpolation happens with $:

var name = "Dick"
var message = "Hello my name is $name!" // "Hello my name is Dick!"
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Multiline strings:

let bigString = """
  |You can have a string
  |that contains multiple
  |lines
  |by
  |doing this.
  """.trimMargin()
println(bigString)
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The above will print:

You can have a string
that contains multiple
lines
by
doing this.
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Pair + Triples

val coordinates: Pair<Int, Int> = Pair(2, 3)
val coordinates3D = Triple(2, 3, 1)
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Number types

val a: Short = 12
val b: Byte = 120
val c: Int = -100000

val answer = a + b + c // Answer will be an Int
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Any, Unit, and Nothing

val anyNum: Any = 1
val anyString: Any = "Yo"
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Unit is a special type which only ever represents one value: the Unit object. It is similar to the void type in Java, except it makes working with generics easier:

fun add() {
  val result = 2 + 2
  println(result)
}

// same as
fun add(): Unit {
  val result = 2 + 2
  println(result)
}
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Nothing is a type that is helpful for declaring that a function not only doesn't return anything, but also never completes.

This can occur if a function either causes the program to stop completely by throwing an Exception or if it simply goes on forever without ever finishing.

Control Logic

Worth noting if/else can be single line ie val min = if (a < b) a else b.

Range

val closedRange = 0..5 // (0,1,2,3,4,5)
val halfOpenRange = 0 until 5 // (0,1,2,3,4)
val decreasingRange = 5 downTo 0 // (5,4,3,2,1,0)
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Repeat

sum = 1
var lastSum = 0
repeat(10) {
  val temp = sum
  sum += lastSum
  lastSum = temp
}
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Stepping

sum = 0
for (i in 1..count step 2) {
  sum += i
}
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You could even downTo step:

sum = 0
for (i in count downTo 1 step 2) {
  sum += i
}
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Labelled Statements

sum = 0
for (row in 0 until 8) {
  if (row % 2 == 0) {
    continue
  }
  for (column in 0 until 8) {
    sum += row * column
  }
}
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When Expressions

val number = 10
when (number) {
  0 -> println("Zero")
  else -> println("Non-zero")
}

// multi-valued whens
val string = "Dog"
when (string) {
  "Cat", "Dog" -> println("Animal is a house pet.")
  else -> println("Animal is not a house pet.")
}

// when in range
timeOfDay = when (hourOfDay) {
  in 0..5 -> "Early morning"
  in 6..11 -> "Morning"
  in 12..16 -> "Afternoon"
  in 17..19 -> "Evening"
  in 20..23 -> "Late evening"
  else -> "INVALID HOUR!"
}

// conditions in when statements
when {
  x == 0 && y == 0 && z == 0 -> println("Origin")
  y == 0 && z == 0 -> println("On the x-axis at x = $x")
  x == 0 && z == 0 -> println("On the y-axis at y = $y")
  x == 0 && y == 0 -> println("On the z-axis at z = $z")
  else -> println("Somewhere in space at x = $x, y = $y, z = $z")
}
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Functions

Note that parameters are constants by default.

fun printMyName() {
  println("My name is Dick Lucas.")
}

// with params
fun printMultipleOfFive(value: Int) {
  println("$value * 5 = ${value * 5}")
}
printMultipleOfFive(10)

// string interpolated
fun printMultipleOf(multiplier: Int, andValue: Int) {
  println("$multiplier * $andValue = ${multiplier * andValue}")
}
printMultipleOf(4, 2)
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If a function consists solely of a single expression, you can assign the expression to the function using = while at the same time not using braces, a return type, or a return statement:

fun multiplyInferred(number: Int, multiplier: Int) = number * multiplier
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Overloading

fun getValue(value: Int): Int {
  return value + 1
}
fun getValue(value: String): String {
  return "The value is $value"
}
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Functions As Variables

fun add(a: Int, b: Int): Int {
  return a + b
}

var function = ::add
function(4,2) // works
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