Kotlin Cheatsheet

A Kotlin cheat sheet is a quick reference guide that provides a concise summary of the most important Kotlin syntax and features. Most of the content in this cheat sheet has been collected from the official Kotlin documentation while avoiding detailed explanations. This cheat sheet is designed for quick review purposes and not for providing detailed information, so I suggest that you also read the official Kotlin documentation.

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Kotlin logotype

Table of Contents

Introduction

Kotlin is a modern, open-source programming language that is used for building multi-platform applications. It is concise, expressive, and powerful, with features such as null safety, extension functions, lambdas, and many others. This cheat sheet will cover some of the essential Kotlin concepts.

First Kotlin program

This is an example of printing “Hello world” in Kotlin:

fun main() {
    println("Hello world")
}

Get input from user

To get input from the user in Kotlin, you can use the readLine() function. This is an example:

fun main() {
    print("Enter your name: ")
    val name = readLine()
    println("Hello, $name!")
}

Comments

// This is an end-of-line comment

/* This is a block comment
   on multiple lines. */

Block comments in Kotlin can be nested.

/* The comment starts here
/* contains a nested comment *⁠/
and ends here. */

KDoc

KDoc is the documentation format for Kotlin, similar to Javadoc for Java. KDoc is used to generate documentation for Kotlin classes, functions, and properties. KDoc comments start with the /** and end with */. This is an example:

/**
 * Calculates the sum of two integers.
 *
 * @param a The first integer to add.
 * @param b The second integer to add.
 * @return The sum of the two integers.
 */
fun sum(a: Int, b: Int): Int {
    return a + b
}

Variables

In Kotlin, variables can be declared using either the var or val keyword.

var variables are mutable, meaning their value can be changed after they are initialized.

var x = 5
x = 10

val variables, on the other hand, are immutable, meaning their value cannot be changed after they are initialized.

val y = 5
y = 10 // This will result in a compilation error

Data types

Here’s a brief overview of the most commonly used data types:

    val booleanVar: Boolean = true
    val byteVar: Byte = 127
    val shortVar: Short = 32767
    val intVar: Int = 2147483647
    val longVar: Long = 9223372036854775807L
    val floatVar: Float = 3.14f
    val doubleVar: Double = 3.14159265358979323846
    val charVar: Char = 'A'
    val stringVar: String = "Hello, world!"

Type Inference

Kotlin supports type inference, which means the compiler can infer the type of a variable from its initial value.

val x = 5 // The type of x is inferred to be Int
val y = "hello" // The type of y is inferred to be String

Variables in Kotlin can also be declared without an initial value, but in that case, the type must be explicitly specified:

var z: Double // Valid, z has no initial value
// println(z) // Invalid, z is not initialized and has no value yet
z = 3.14 // Valid, z is initialized with a value

Type Conversion

Kotlin provides several methods for converting between data types. Here’s an example in Kotlin that demonstrates various methods of type conversion.

    val str: String = "123"
    val num: Int = str.toInt() // Convert String to Int

    val dbl: Double = 123.45
    val int: Int = dbl.toInt() // Convert Double to Int

    val lng: Long = 9876543210
    val flt: Float = lng.toFloat() // Convert Long to Float

    val bol: Boolean = true
    val strBol: String = bol.toString() // Convert Boolean to String

    val char: Char = 'A'
    val intChar: Int = char.toInt() // Convert Char to Int // Conversion of Char to Number is deprecated. Use Char.code property instead.

    val byte: Byte = 127
    val short: Short = byte.toShort() // Convert Byte to Short

String templates 

val name= "Ali"
val result= "My name is $name"

Character escape 

\n insert new line
\t inserts a tab
\r inserts carriage return

Operators

Arithmetic operators

  val a = 10
  val b = 5
  println(a + b) // 15
  println(a - b) // 5
  println(a * b) // 50
  println(a / b) // 2
  println(a % b) // 0

Comparison operators

  val c = 10
  val d = 5
  println(c > d) // true
  println(c >= d) // true
  println(c < d) // false
  println(c <= d) // false
  println(c == d) // false
  println(c != d) // true

Assignment operators

  var h = 10
  h += 5
  println(h) // 15
  h -= 5
  println(h) // 10
  h *= 2
  println(h) // 20
  h /= 4
  println(h) // 5
  h %= 3
  println(h) // 1

Logical operators

  val i = true
  val j = false
  println(i && j) // false
  println(i || j) // true
  println(!i) // false

Bitwise operators

  val k = 0b1010
  val l = 0b1100
  println(k and l) // Prints "8" (0b1000)
  println(k or l) // Prints "14" (0b1110)
  println(k xor l) // Prints "6" (0b0110)

Range operator

  val o = 1..10
  println(o.contains(5)) // Prints "true"
  println(o.contains(11)) // Prints "false"

Control flow

If-else 

if (condition) {
    // Code to execute if condition is true
} else {
    // Code to execute if condition is false
}

When 

when (value) {
    condition1 -> // Code to execute if value matches condition1
    condition2 -> // Code to execute if value matches condition2
    else -> // Code to execute if value does not match any condition
}

Conditional Expression

Given if and when are expressions, we can directly assign them to variables.

val seasonFirstMonth = when(season) {
    "summer" -> 6,
    "winter" -> 12,
    "automn" -> 9,
    "spring" -> 3,
    else -> error("There is only 4 seasons")
}

Thus, it allows a similar ternary operator using a regular if.

val max = if (a > b) a else b

For loop 

for (item in collection) {
    // Code to execute for each item in collection
}

Ranges

There is a set of tools for defining ranges in Kotlin.


for(i in 0..3) {             
    print(i)
}

for(i in 0 until 3) {       
    print(i)
}

for(i in 2..8 step 2) {     
    print(i)
}

for (i in 3 downTo 0) {     
    print(i)
}

Char ranges are also supported.

for (c in 'a'..'d') {   
    print(c)
}

Ranges are also useful in if statements.

if (x in 1..5) {           
    print("x is in range from 1 to 5")
}

While 

while (condition) {
    // Code to execute as long as condition is true
}

Do While 

do {
    // Code to execute at least once
} while (condition)

Break and Continue 

for (i in 1..10) {
    if (i == 5) {
        break // Exit loop when i is equal to 5
    }
    if (i % 2 == 0) {
        continue // Skip even numbers and continue to the next iteration
    }
    // Code to execute for each odd number between 1 and 10
}

Exceptions

To throw an exception object, use the throw expression:

throw Exception("Exception...")

To catch an exception, use the try
catch expression:

try {
    // some code
} catch (e: SomeException) {
    // handler
} finally {
    // optional finally block
}

The Nothing type: This type has no values and is used to mark code locations that can never be reached. In your own code, you can use Nothing to mark a function that never returns.

fun fail(message: String): Nothing {
    throw IllegalArgumentException(message)
}

Functions

Function Declaration 

fun sayHello() {
    println("Hello!")
}

fun greet(name: String) {
    println("Hello, $name!")
}

Default arguments and named arguments 

fun greet(name: String = "World", greeting: String = "Hello") {
    println("$greeting, $name!")
}

fun main() {
    // calling function with default arguments
    greet() // output: Hello, World!

    // calling function with named arguments
    greet(greeting = "Hi", name = "Ali") // output: Hi, Ali!

    // calling function with some named arguments
    greet(name = "Reza") // output: Hello, Reza!
}

Function Return Types 

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

fun multiply(a: Int, b: Int) = a * b

Unit-returning functions If a function does not return a value, its return type is Unit.

fun printHello(): Unit {
  print("Hello")
}

Or

fun printHello() {
   print("Hello")
}

Local functions

Kotlin supports local functions, which are functions inside other functions. printMessage() is a local function defined within the main() function.

fun main() {
    fun printMessage(message: String) {
        println("Message: $message")
    }

    printMessage("Hello, world!")
}

Member functions

A member function is a function that is defined inside a class or object.

class Sample {
    fun foo() {//...}
}

Generic functions

Functions can have generic parameters, which are specified using angle brackets before the function name

fun <T> function(item: T){ 
 //Code
}

Lambda Expressions 

val sum = { a: Int, b: Int -> a + b }

val square: (Int) -> Int = { it * it }

Extension Functions and Properties

Extension Functions and Properties in Kotlin allow adding new functionality or properties to existing classes without modifying their source code.

// Extension function
fun String.reverse(): String {
    return this.reversed()
}

// Extension property
val String.firstChar: Char
    get() = this[0]

fun main() {
    val str = "Ali"
    println(str.reverse())  // Prints "ilA"
    println(str.firstChar)  // Prints "A"
}

Higher-Order Functions

A higher-order function is a function that takes another function as parameter and/or returns a function.

fun calculate(x: Int, y: Int, operation: (Int, Int) -> Int): Int { 
    return operation(x, y)                                         
}

fun sum(x: Int, y: Int) = x + y                                 

fun main() {
    val sumResult = calculate(1, 7, ::sum)                         
    val mulResult = calculate(1, 7) { a, b -> a * b }             
}

fun operation(): (Int) -> Int {                                     
    return ::square
}

fun square(x: Int) = x * x                                          

fun main() {
    val func = operation()                                          
    println(func(7))                                                
}

Operator overloading

Operator overloading in Kotlin allows you to define and use custom operators for your own classes and types.

data class Point(val x: Int, val y: Int) {
    operator fun plus(other: Point): Point {
        return Point(x + other.x, y + other.y)
    }
}

fun main() {
    val p1 = Point(1, 2)
    val p2 = Point(3, 4)
    val p3 = p1 + p2 // using the overloaded '+' operator
    println(p3) // Output: Point(x=4, y=6)
}

Variable number of arguments (varargs)

Varargs is a feature that allows you to pass a variable number of arguments of the same type to a function

fun printNumbers(vararg numbers: Int) {
    for (number in numbers) {
        println(number)
    }
}

fun main() {
    printNumbers(1, 2, 3) // prints 1, 2, 3
    printNumbers(4, 5, 6, 7, 8) // prints 4, 5, 6, 7, 8
}

Infix notation

Infix in Kotlin allows you to define functions that can be called using infix notation (i.e., without using parentheses and the dot notation).

infix fun Int.times(str: String) = str.repeat(this)

fun main() {
    val str = 5 times "Hello "
    println(str) // Output: "Hello Hello Hello Hello Hello "
}

Scope Functions

let can be used for scoping and null-checks. When called on an object, let executes the given block of code and returns the result of its last expression. The object is accessible inside the block by the reference it (by default) or a custom name.

val message: String? = "Hello"
message?.let {
    print(it.toUpperCase()) // Output: "HELLO"
}

Like let, run is another scoping function from the standard library. Basically, it does the same: executes a code block and returns its result. The difference is that inside run the object is accessed by this. This is useful when you want to call the object’s methods rather than pass it as an argument.

val message: String? = "Hello"
message?.run {
    print(this.toUpperCase()) // Output: "HELLO"
}

with is a non-extension function that can access members of its argument concisely: you can omit the instance name when referring to its members.

val person = Person("Ali", 24)
val message = with(person) {
    "My name is $name and I'm $age years old."
}

apply executes a block of code on an object and returns the object itself. Inside the block, the object is referenced by this. This function is handy for initializing objects.

val person = Person("Ali", 24)
person.apply {
    name = "Ali"
    age = 24
}

also works like apply: it executes a given block and returns the object called. Inside the block, the object is referenced by it, so it’s easier to pass it as an argument. This function is handy for embedding additional actions, such as logging in call chains.

val message: String? = "Hello"
message?.also {
    print(it.toUpperCase()) // Output: "HELLO"
}

Collections

Array

An array is a fixed-size collection of elements of the same data type.


// Declare an array of integers
val numbers = arrayOf(1, 2, 3, 4, 5)

// Declare an array of strings
val names = arrayOf("Alice", "Bob", "Charlie", "Dave")

// Declare an array of a specific size
val array = arrayOfNulls<Int>(10)

// Declare an array of integers with a specified size and initial value
val array = Array<Int>(7) { i -> i*i }
val filledArray = IntArray(5) { index -> index * 2 } // Other type: BooleanArray, ShortArray, DoubleArray and etc.


// Access an element at a specific index
val firstNumber = numbers[0]

// Access the last element of an array
val lastNumber = numbers[numbers.size - 1]

// Modify an element at a specific index
numbers[0] = 10

// Fill an array with a specific value
Arrays.fill(numbers, 0)


// Iterate over an array using a for loop
for (number in numbers) {
    println(number)
}

// Iterate over an array using a for loop with an index
for (i in names.indices) {
    println("${i}: ${names[i]}")
}

// Iterate over an array using a forEach loop
names.forEach { name ->
    println(name)
}

val index = numbers.indexOf(3)

numbers.sort()

names.reverse()

List 

val list = listOf(1, 2, 3, 4, 5)
val list2=mutableListOf(1, 2, 3, 4, 5)

Basic methods:

    val numbers = mutableListOf(1, 2, 3)
    numbers.add(4) // Adds the specified element to the end of the list
    numbers.remove(3) // Removes the first occurrence of the specified element from the list
    numbers[1] // Returns the element at the specified index in the list

Map 

val map = mapOf(1 to "one", 2 to "two", 3 to "three")
val map2= mutableListOf(1 to "one", 2 to "two", 3 to "three")

Basic methods:

    val numbers =  mutableMapOf("one" to 1, "two" to 2, "three" to 3)
    numbers.put("four", 4) // Associates the specified value with the specified key in the map
    numbers.remove("two") // Removes the mapping for the specified key from the map if it is present
    numbers.containsKey("two") // Returns true if the map contains the specified key

Set 

val set = setOf(1, 2, 3, 4, 5)
val set2=mutableSetOf(1, 2, 3, 4, 5)

Basic methods:

    val numbers =  mutableSetOf(1, 2, 3)
    numbers.add(4) // Adds the specified element to the set if it is not already present
    numbers.remove(3) // Removes the specified element from the set if it is present
    numbers.contains(1) //  Returns true if the set contains the specified element

Please for more collections and detaile read doc.

Class and Object

Classes 

class Person(val name: String, var age: Int) // class

val person = Person("Ali", 24) // object

Property and methods 

class Person(val name: String) {
   var age = 0 // property

   fun sayHello() { // method
      println("Hello, my name is $name")
   }
}

val person = Person("Ali")
person.age = 24
person.sayHello()

Getters and setters

Getter and setter in Kotlin are accessors used to retrieve and modify the value of a variable, respectively. The initializer, getter, and setter are optional.

class Person {
    var name: String = ""
        get() = field.uppercase(Locale.getDefault())
        set(value) {
            field = "Name: $value"
        }

    var age = 24 // has default getter and setter

    val username="Ali" // has default getter

}

For more details about properties, getter/setter, backing fields, backing propertiesï»ż and etc read this link.

Visibility modifiers

Late-initialized properties and variables

A lateinit variable is used when you know that a variable will be initialized before it is used, but you don’t want to assign an initial value at the time of declaration.

lateinit var myLateInitVar: String

// The variable is not initialized yet, so trying to access it will throw an exception
// println(myLateInitVar) // This line would throw a "lateinit property has not been initialized" exception

// Sometime later, the variable is initialized
myLateInitVar = "Hello World"

// Now we can access the variable without an exception
println(myLateInitVar) // Prints "Hello World"

Inheritance

Inheritance is the process by which one class acquires the properties and methods of another class.

open class Animal(val name: String) {
   open fun makeSound() {
      println("Animal sound")
   }
}

class Dog(name: String): Animal(name) {
   override fun makeSound() {
      println("Woof!")
   }
}

Interface and Abstract Class

Both interface and abstract class provide a way to define contracts or blueprints for classes to follow. They are used for abstraction.

interface Vehicle {
    fun start()
    fun stop()
    val name: String
}

abstract class Animal {
    abstract fun makeSound()
    open fun move() {
        println("Moving...")
    }
}

Abstraction

Abstraction is a mechanism for hiding the implementation details of an object and exposing only the necessary details to the user. In Kotlin, abstraction is achieved through interfaces and abstract classes.

Polymorphism

Polymorphism is the ability of an object to take many forms.

class Dog : Animal() {
    override fun makeSound() {
        println("Woof!")
    }
}

class Cat : Animal() {
    override fun makeSound() {
        println("Meow!")
    }
}

In this example, both Dog and Cat classes inherit from the Animal abstract class and provide their own implementation of the makeSound() method, which demonstrates polymorphism.

Object Expression and Declaration

An object expression creates an instance of an anonymous class, while an object declaration creates a singleton instance of a class.

val person = object {
    val name = "Ali"
    fun sayHello() {
        println("Hello, my name is $name")
    }
}

object Singleton {
    fun doSomething() {
        println("Doing something...")
    }
}

Companion objects An object declaration inside a class can be marked with the companion keyword.

class MyClass {
    companion object { }
}

Data class

A data class is a special class that is designed to hold data.

data class Person(val name: String, var age: Int)

val person = Person("Ali", 24)

Nested and Inner class 

class Outer {
   private val outerProperty = "Outer property"

   class Nested {
      fun foo() {
         // can't access outerProperty
      }
   }

   inner class Inner {
      fun bar() {
         println(outerProperty) // can access outerProperty
      }
   }
}

Type aliases

Type aliases provide alternative names for existing types. If the type name is too long you can introduce a different shorter name and use the new one instead.

Before using a type alias

val numbers = listOf(1, 2, 3, 4, 5)
numbers.filter { number: Int -> number % 2 == 0 }.map { number: Int -> "Number is $number" }

After using a type alias

typealias NumberPredicate = (Int) -> Boolean
typealias NumberMapper = (Int) -> String

val numbers = listOf(1, 2, 3, 4, 5)
val even: NumberPredicate = { number -> number % 2 == 0 }
val mapper: NumberMapper = { number -> "Number is $number" }
numbers.filter(even).map(mapper)

Enum

An enum is a type that represents a fixed set of values.

enum class Color {
   RED, GREEN, BLUE
}

Sealed class and interface

A sealed class/interface restricts the inheritance of its subclasses/interfaces to only within the same file.

sealed class Shape

class Circle: Shape()

// Can't subclass Shape outside of this file

Generics

A generic concept in Kotlin allows for type-safe programming by creating reusable classes, functions, and interfaces that can work with any data type.

class Box<T>(t: T) {
    var value = t
}

Delegation Pattern

Kotlin supports easy implementation of the delegation pattern on the native level without any boilerplate code.

interface Base {
    fun print()
}

class BaseImpl(val x: Int) : Base {
    override fun print() { print(x) }
}

class Derived(b: Base) : Base by b

fun main() {
    val b = BaseImpl(10)
    Derived(b).print()
}

Delegated properties

Delegated properties in Kotlin allow the definition of custom getters and setters for a property that can be delegated to another object, such as the lazy initialization of a property.

A lazy variable is initialized only when it is first accessed.

val myLazyVar: String by lazy {
    // Perform some expensive operation to initialize the variable
    "Hello World"
}
// The variable is not initialized until it is first accessed
println(myLazyVar) // Prints "Hello World"

Other Topics

Destructuring declarations

Destructuring declarations in Kotlin allow you to break down objects into individual variables in a single line of code.

 val person=Person("Ali",24)
 val (name, age) = person

Reflection

Reflection is a set of language and library features that allows you to introspect the structure of your program at runtime.

This is a example:


  // Obtain a Class object for the String class
    val stringClass = String::class.java

    // Get the fields of the String class and print their names
    val fields = stringClass.declaredFields
    for (field in fields) {
        println(field.name)
    }

    // Get the methods of the String class and print their names
    val methods = stringClass.declaredMethods
    for (method in methods) {
        println(method.name)
    }

Annotations

Annotations in Kotlin are special labels that can be applied to declarations such as classes, functions, and properties to provide additional information or metadata about the declaration at compile-time and runtime. This is a example:

@Deprecated("Use newMethod() instead", ReplaceWith("newMethod()"))
fun oldMethod() {
    // ...
}

Packages and imports

Packages are used to group related classes, functions, and other declarations together. Imports are used to make declarations from other packages accessible within your current file.

package com.example.models

class Person(val name: String, val age: Int) {
    // class implementation
}

import com.example.models.Person

fun main() {
    val person = Person("Ali", 24)
    println("Name: ${person.name}, Age: ${person.age}")
}

Null safety

Kotlin has null safety, which helps prevent null pointer exceptions. Kotlin provides operators to work with nullable types, including safe call, elvis, and not-null assertion operators. These features help developers write more reliable code and avoid null pointer exceptions.


    var nullableStr: String? = null
    var nonNullStr: String = "Hello"

    // safe call operator
    println(nullableStr?.length) // prints null

    // elvis operator
    val len = nullableStr?.length ?: -1
    println(len) // prints -1

    // not-null assertion operator
    // throws null pointer exceptions because nullableStr is null
    println(nullableStr!!.length)

    // this would not compile because nonNullStr is not nullable
    // nonNullStr = null

Equality

In Kotlin there are two types of equality:

data class Person(val name:String,val age:Int)

val person1=Person("Ali",24)
val person2=Person("Reza",27)
var person3=Person("Ali",24)

print(person1==person2) // false
print(person1!=person2) // true
print(person1===person2) // false
print(person1==person3) // true
print(person1===person3) // flase
print(person1!==person3) // true

person3=person1

print(person1===person3) // true

Comparable

You can compare objects using the Comparable interface. This interface defines a compareTo method that compares the current object with another object and returns an integer value that indicates the order of the objects.

data class Person(val name: String, val age: Int) : Comparable<Person> {
    override fun compareTo(other: Person): Int {
        return this.age.compareTo(other.age)
    }
}

val person1 = Person("Ali", 24)
val person2 = Person("Reza", 30)

if (person1 < person2) {
    println("${person1.name} is younger than ${person2.name}")
} else {
    println("${person1.name} is older than ${person2.name}")
}

When you use the sorted() or sort() function to sort a list of objects, Kotlin uses the compareTo() method of the objects to determine their natural order. If the objects in the list do not implement the Comparable interface, you will get a compile-time error.

val people = listOf(
    Person("Ali", 24),
    Person("Reza", 40),
    Person("Shabnam", 23)
)
val sortedPeople = people.sorted()
println(sortedPeople) // [Person(name=Shabnam, age=23), Person(name=Ali, age=24), Person(name=Reza, age=40)]

Regex

Regex, short for regular expression, is a sequence of characters that forms a pattern used to match and manipulate text.

Regex Basics

Kotlin provides several functions for working with regular expressions, including: matches, find, findAll, replace, split and etc.

This is a simple example:

    val phoneNumber="9112233445"
    val phoneNumber2="9178"
    val phoneNumber3="93abc"
    val regex = Regex("^\\d{10}$")
    println(regex.matches(phoneNumber)) // true
    println(regex.matches(phoneNumber2)) // false
    println(regex.matches(phoneNumber3)) // false