Kotlin Cheat Sheet

4 min readDec 31, 2021

Hi Everyone, this time I come up with something new rather than Understanding concept. I prepared Cheat Sheet for Kotlin Programming Language for quick reference.

Kotlin is a cross-platform, statically typed, general-purpose programming language with type inference. Kotlin is a new programming language for the JVM. It produces Java bytecode, supports Android and generates JavaScript.

Basics

You do not need ; to break statements.
There are two keywords for variable declaration, var and val.
var : var is like a general variable and can be assigned multiple times and is known as the mutable variable in Kotlin
val : val is a constant variable and can not be assigned multiple times and can be Initialized only single time and is known as the immutable variable in Kotlin.
The keyword void common in Java or C# is called Unit in Kotlin.
Unlike Java or C#, you declare the type of a variable after the name, e.g. var firstName : String

Strings :

val name = "Peter"
val newVar = "Hello, " + name
val helloName = "Hello, $name"
val upercaseName = "Hello, ${name.toUpperCase()

2. Booleans :

val trueBoolean = true
val falseBoolean = false
val andCondition = trueBoolean && falseBoolean
val orCondition = trueBoolean || falseBoolean

3. Floats :

val intNumber = 10
val doubleNumber = 10.0
val longNumbe = 10L
val floatNum = 10.0F

4. Mutability :

var mutableString: String = "Hello"
val immutableString: String = Hello"

5. Companion objects Syntax :

class MyClass {
    companion object Factory {
        fun create(): MyClass = MyClass()
    }
}

6. Null Safety :

Nullable properties

val cannotBeNull: String = null // Invalid
val canBeNull: String? = null // Validval cannotBeNull: Int = null // Invalid
val canBeNull: Int? = null // Valid

Null Checking

val name: String? = "Hello"if (name != null && name.length > 0) {
    print("String length is ${name.length}")
} else {
    print("String is empty.")
}

!! operator

val opt = name!!.length

the not-null assertion operator (!!) converts any value to a non-null type and throws an exception if the value is null. You can write name!!, and this will return a non-null value of name (for example, a String in our example) or throw an NPE if name is null

Safe Casts

val opt: Int? = a as? Int

Regular casts may result in a ClassCastException if the object is not of the target type. Another option is to use safe casts that return null if the attempt was not successful.

7. Classes :

Classes in Kotlin are declared using the keyword class

class Student{ /*...*/ }

Primary Constructor

class Student(val name: String, val age: Int)
val student1 = Student("Peter", 11)

Secondary Constructor

class Student(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 Student(val name: String, val age: Int? = null)

Enum Class

enum class Color(val rgb: Int) {
    RED(0xFF0000),
    GREEN(0x00FF00),
    BLUE(0x0000FF)
}

Data Class

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

8. Functions :

Kotlin functions are declared using the fun keyword:

fun printStudentName() {
    print("Richa")
}

Parameters & Return Types

fun printStudentName(student: Student) {
    print(student.name)
}

Default Parameters

fun getStudentName(student: Student, intro: String = "Hello,") {
    return "$intro ${student.name}"
}

9. Collections

List

val numbers = listOf("one", "two", "three", "four")

Set

val numbers = setOf(1, 2, 3, 4)

Map

A Map stores key-value pairs (or entries); keys are unique, but different keys can be paired with equal values.

val numbersMap = mapOf("key1" to 1, "key2" to 2, "key3" to 3, "key4" to 1)

10. Control Flows

Conditions and loops

“if” Statement

if is an expression: it returns a value

var max = a
if (a < b) max = b

// With else
var max: Int
if (a > b) {
    max = a
} else {
    max = b
}

// As expression
val max = if (a > b) a else b

when

when defines a conditional expression with multiple branches.

when (x) {
    1 -> print("x == 1")
    2 -> print("x == 2")
    else -> {
        print("x is neither 1 nor 2")
    }
}

for

for (item in collection) print(item)

If you want to know more about null concept please go through my blog Kotlin: Null Safety Concept for basic understanding.

Happy Reading :)