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Kotlin

A quick reference cheatsheet for Kotlin that includes usage, examples, and more.

#Introduction to Kotlin

#main()

fun main() {
  println("Greetings, askhacker.com!")
  // Code goes here
}

The main() function is the starting point of every Kotlin program and must be included in the code before execution

println("Greetings, earthling!")
print("Take me to ")
print("your leader.")

/*
Print:
Greetings, earthling!
Take me to your leader.
*/

#Notes

// this is a single line comment

/*
this
note
for
many
*/

#Execution order

fun main() {
  println("I will be printed first.")
  println("I will be printed second.")
  println("I will be printed third.")
}

#Data Types and Variables

#Mutable variables

var age = 25
age = 26

#Immutable variables

val goldenRatio = 1.618

#Type inference

// The following variables are assigned a literal value inside double quotes
// so the inferred type is String

var color = "Purple"

#String concatenation

var streetAddress = "123 Main St."
var cityState = "Brooklyn, NY"

println(streetAddress + " " + cityState)
// Print: 123 Main St. Brooklyn, NY

#String Templates

var address = "123 Main St."
println("The address is $address")
// prints: The address is 123 Main St.

#Built-in Properties and Functions

var monument = "the Statue of Liberty"

println(monument. capitalize())
// print: The Statue of Liberty
println(monument. length)
// print: 21

#Character escape

print("\"Excellent!\" I cried. \"Elementary,\" said he.")

// Print: "Excellent!" I cried. "Elementary," said he.
  • \n insert new line
  • \t inserts a tab
  • \r inserts carriage return
  • \' inserts a single quote
  • \" inserts a double quote
  • \\ inserts a backslash
  • \$ inserts a dollar sign

#Arithmetic Operators

5 + 7  // 12
9 -2   // 7
8 *4   // 32
25 /5  // 5
31 % 2 // 1

+ addition, - subtraction, * multiplication, / division, and % modulus

#Order of operations

5 + 8 *2 /4 -3 // 6
3 + (4 + 4) /2 // 7
4 *2 + 1 *7    // 15
3 + 18 /2 *1   // 12
6 -3 % 2 + 2   // 7

#Enhanced assignment operator

var batteryPercentage = 80

// long syntax
batteryPercentage = batteryPercantage + 10

// short syntax with augmented assignment operator
batteryPercentage += 10

#Increment and decrement operators

var year = 2019
year++   // 2020
year--   // 2019

#Math library

Math.pow(2.0, 3.0) // 8.0
Math.min(6, 9)     // 6
Math.max(10, 12)   // 12
Math. round(13.7)  // 14

#Conditional Expression

#If expression

var morning = true

if (morning) {
  println("Rise and shine!")
}
// Print: Rise and shine!

#Else-expression

var rained = false

if (rained) {
  println("No need to water the plants today.")
} else {
  println("The plant needs to be watered!")
}
// print: The plant needs watering!

#Else-If expressions

var age = 65

if (age < 18 ) {
  println("You are considered a minor")
} else if (age < 60) {
  println("You are considered an adult")
} else {
  println("You are considered senior")
}

// print: you are considered senior

#Comparison Operators

var myAge = 19
var sisterAge = 11
var cousinAge = 11

myAge > sisterAge  // true
myAge < cousinAge  // false
myAge >= cousinAge // true
myAge <= sisterAge // false

#Logical Operators

var humid = true
var raining = true
var jacket = false

println(!humid)
// print: false
println(jacket && raining)
// print: true
println(humid || raining)
// print: true

#AND operator: &&

var humid = true
var raining = true
var shorts = false
var sunny = false

// true AND true
println(humid && raining) // true
// true AND false
println(humid && shorts)  // false
// false AND true
println(sunny && raining) // false
// false AND false
println(shorts && sunny)  // false

#Or operator:||

var late = true
var skipBreakfast = true
var underslept = false
var checkEmails = false

// true OR true
println(skipBreakfast || late) // true
// true OR false
println(late || checkEmails)   // true
// false OR true
println(underslept || late)    // true
// false OR false
println(checkEmails || underslept) // false

#NOT operator

var hungry = true
var full = false

println(!hungry) // false
println(!full)   // true

#Evaluation order

!true && (false || true) // false
/*
(false || true) is evaluated first to return true.
Then, evaluate !true && true and return the final result false
*/

!false && true || false // true
/*
!false is evaluated first to return true.
Then true && true is evaluated, returning true.
then, true || evaluates to false and eventually returns true
*/

#Nested conditions

var studied = true
var well Rested = true

if (wellRested) {
  println("Good luck today!")
  if (studied) {
    println("You should prepare for the exam!")
  } else {
    println("Spend a few hours studying before the exam!")
  }
}

// Print: Good luck today!
// print: You should be ready for the exam!

#When expression

var grade = "A"

when (grade) {
  "A" -> println("Great job!")
  "B" -> println("Great job!")
  "C" -> println("You passed!")
  else -> println("Close! Be sure to prepare more next time!")
}
// print: Great job!

#Range operator

var height = 46 // inches

if (height in 1..53) {
  println("Sorry, you must be at least 54 inches to ride the coaster")
}
// Prints: Sorry, you must be at least 54 inches to ride the roller coaster

#Equality Operators

var myAge = 22
var sisterAge = 21

myAge == sisterAge // false
myAge != sisterAge // true

#Null-Safety

#Nullable vs Not-Nullable

var a: String = "Kotlin" // a can never be null
a = null // compilation error
var b: String? = "Kotlin" // b can be null
b = null // ok

#Safe-Calls

val a = "Kotlin"
val b: String? = null
println(a.length) // can be called safely, because a is never null
println(b?.length) // b?.length returns the length of b, or null if b is null
println(a?.length) // Unnecessary safe call

#Chaining Safe-Calls

bob?.department?.head?.name // chain returns null if any property is null

#Elvis Operator

val l = b?.length ?: -1 // if b is null, return the default value -1
// equval to:
val l: Int = if (b != null) b.length else -1

#Not Null Assertion Operator

val l = b!!.length // throws a NullPointerException, if b is null

#Collections

#Immutable list

var programmingLanguages = listOf("C#", "Java", "Kotlin", "Ruby")

#Mutable List

var fruits = mutableListOf("Orange", "Apple", "Banana", "Mango")

#Access List

var cars = listOf("BMW", "Ferrari", "Volvo", "Tesla")

println(cars[2]) // Prints: Volvo

#Size Attribute

var worldContinents = listOf("Asia", "Africa", "North America", "South America", "Antarctica", "Europe", "Australia")

println(worldContinents.size) // Prints: 7

#List Manipulation

var seas = listOf("Black Sea", "Caribbean Sea", "North Sea")
println(seas. contains("North Sea")) // Prints: true

// The contains() function performs a read operation on any list and determines if the element exists
seas.add("Baltic Sea") // Error: cannot write to immutable list
// The add() function can only be called on mutable lists, so the code above throws an error

#Immutable Sets

var primaryColors = setOf("Red", "Blue", "Yellow")

#Mutable Sets

var womenInTech = mutableSetOf("Ada Lovelace", "Grace Hopper", "Radia Perlman", "Sister Mary Kenneth Keller")

#Access Collection Elements

var companies = setOf("Facebook", "Apple", "Netflix", "Google")

println(companies.elementAt(3))
// Prints: Google
println(companies.elementAt(4))
// Returns and Error
println(companies.elementAtOrNull(4))
// Prints: null

#Immutable Map

var averageTemp = mapOf("winter" to 35,  "spring" to 60,  "summer" to 85, "fall" to 55)

#Mutable Mapping

var europeanDomains = mutableMapOf("Germany" to "de", "Slovakia" to "sk", "Hungary" to "hu", "Norway" to "no")

#Retrieve map keys and values

var oscarWinners = mutableMapOf("Parasite" to "Bong Joon-ho", "Green Book" to "Jim Burke", "The Shape Of Water" to "Guillermo del Toro")

println(oscarWinners.keys)
// Prints: [Parasite, Green Book, The Shape Of Water]

println(oscarWinners.values)
// Prints: [Bong Joon-ho, Jim Burke, Guillermo del Toro]
println(oscarWinners["Parasite"])
// Prints: Bong Joon-ho

#Add and remove map entries

var worldCapitals = mutableMapOf("United States" to "Washington D.C.", "Germany" to "Berlin", "Mexico" to "Mexico City", "France" to "Paris")

worldCapitals.put("Brazil", "Brasilia")
println(worldCapitals)
// Prints: {United States=Washington D.C., Germany=Berlin, Mexico=Mexico City, France=Paris, Brazil=Brasilia}

worldCapitals.remove("Germany")
println(worldCapitals)
// Prints: {United States=Washington D.C., Mexico=Mexico City, France=Paris, Brazil=Brasilia}

#Function

#Function

fun greet() {
  println("Hey there!")
}

fun main() {
  //Function call
  greet() //Prints: Hey there!
}

#Function Parameters

fun birthday(name: String, age: Int) {
   println("Happy birthday $name! You turn $age today!")
}

fun main() {
  birthday("Oscar", 26) 
  //Prints: Happy birthday Oscar! You turn 25 today!
  birthday("Amarah", 30) 
  //Prints: Happy birthday Amarah! You turn 30 today!
}

#Default Parameters

fun favoriteLanguage(name, language = "Kotlin") {
  println("Hello, $name. Your favorite programming language is $language")  
}

fun main() {
  favoriteLanguage("Manon") 
  //Prints: Hello, Manon. Your favorite programming language is Kotlin
  
  favoriteLanguage("Lee", "Java") 
  //Prints: Hello, Lee. Your favorite programming language is Java
}

#Named Parameters

fun findMyAge(currentYear: Int, birthYear: Int) {
   var myAge = currentYear -birthYear
println("I am $myAge years old.")
}

fun main() {
  findMyAge(currentYear = 2020, birthYear = 1995)
  //Prints: I am 25 years old.
  findMyAge(birthYear = 1920, currentYear = 2020)
  //Prints: I am 100 years old.
}

#Return Statement

//Return type is declared outside the parentheses
fun getArea(length: Int, width: Int): Int {
  var area = length *width

  //return statement
  return area
}

fun main() {
  var myArea = getArea(10, 8)
println("The area is $myArea.")
//Prints: The area is 80.
}

#Single expression function

fun fullName(firstName: String, lastName: String) = "$firstName $lastName"

fun main() {
  println(fullName("Ariana", "Ortega"))
  //Prints: Ariana Ortega
  println(fullName("Kai", "Gittens"))
  //Prints: Kai Gittens
}

#Function Literals

fun main() {
  //Anonymous Function:
  var getProduct = fun(num1: Int, num2: Int): Int {
return num1 *num2
  }
  println(getProduct(8, 3))
  //Prints: 24
//Lambda Expression
  var getDifference = { num1: Int, num2: Int -> num1 -num2 }
  println(getDifference(10, 3))
  //Prints: 7
}

#Class

#Class Example

//class with properties containing default values
class Student {
  var name = "Lucia"
  var semester = "Fall"
  var gpa = 3.95
}

//shorthand syntax without class body
class Student

#Class Instance

// Class
class Student {
  var name = "Lucia"
  var semester = "Fall"
var gpa = 3.95
}

fun main() {
  var student = Student()   
  // Instance
  println(student.name)     
  // Prints: Lucia
  println(student.semester) 
  // Prints: Fall
  println(student.gpa)      
  // Prints: 3.95  
}

#Primary Constructor

class Student(val name: String, val gpa: Double, val semester: String, val estimatedGraduationYear: Int) 

fun main() {
  var student = Student("Lucia", 3.95, "Fall", 2022) 
  println(student.name)     
  //Prints: Lucia
  println(student.gpa)
  //Prints: 3.95
  println(student.semester) 
  //Prints: Fall
  println(student.estimatedGraduationYear) 
  //Prints: 2022
}

#Initialization Block

class Student(val name: String, val gpa: Double, val semester: String, val estimatedGraduationYear: Int) {
  init {
    println("$name has ${estimatedGraduationYear -2020} years left in college.")
  }
}

fun main() {
  var student = Student("Lucia", 3.95, "Fall", 2022)
//Prints: Lucia has 2 years left in college. 
}

#Member Function

class Student(val name: String, val gpa: Double, val semester: String, val estimatedGraduationYear: Int) {

  init {
println("$name has ${estimatedGraduationYear -2020} years left in college.")
  }

  //member function
  fun calculateLetterGrade(): String {
    return when {
      gpa >= 3.0 -> "A"
      gpa >= 2.7 -> "B"
      gpa >= 1.7 -> "C"
gpa >= 1.0 -> "D"
      else -> "E"
    }
  }
}

//When the instance is created and the function is called, the when expression will be executed and return the letter grade
fun main() {
  var student = Student("Lucia", 3.95, "Fall", 2022)
//Prints: Lucia has 2 years left in college.
  println("${student.name}'s letter grade is ${student.calculateLetterGrade()}.")
  //Prints: Lucia's letter grade is A.
}

#See Also