Kotlin Cheat Sheet

Below is a concise Kotlin cheat sheet covering the basics, including syntax for variable declaration, control flows, functions, classes, and more. Kotlin is a statically typed programming language that runs on the Java Virtual Machine and can be used for developing Android apps, server-side applications, and much more.

Variables
val immutableVariable: String = "I cannot be reassigned"
var mutableVariable: Int = 42
// Type inference
val inferredType = "The compiler knows this is a String"

Basic Types
val myString: String = "Hello, Kotlin!"
val myInt: Int = 123
val myDouble: Double = 123.45
val myBoolean: Boolean = true
val myLong: Long = 123L
val myFloat: Float = 123.45F
val myChar: Char = 'A'

Arrays and Collections
val myArray = arrayOf(1, 2, 3)
val myList = listOf("a", "b", "c") // Immutable list
val myMutableList = mutableListOf("a", "b", "c") // Mutable list
val myMap = mapOf("key1" to "value1", "key2" to "value2")
val myMutableMap = mutableMapOf("key1" to "value1")

Control Flow
If Expression
val max = if (a > b) a else b


When Expression (Switch Case)
when (x) {
    1 -> print("x == 1")
    2 -> print("x == 2")
    else -> { // Note the block
        print("x is neither 1 nor 2")
    }
}

For Loop
for (item in collection) print(item)
for (i in 1..10) print(i) // Range inclusive

While Loop
while (x > 0) {
    x--
}

do {
    val y = retrieveData()
} while (y != null) // Executes at least once

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

// Single-expression function
fun multiply(a: Int, b: Int) = a * b

// Default parameters
fun greet(name: String, msg: String = "Hello") = println("$msg $name")

// Named arguments
greet(name = "World", msg = "Hi")

Classes and Objects
class Person(val firstName: String, val lastName: String) {
    var age: Int = 0

    fun greet() {
        println("Hello, my name is $firstName $lastName.")
    }
}

val person = Person("John", "Doe")
person.age = 30
person.greet()

// Data class (automatically provides equals(), hashCode(), toString(), copy())
data class User(val name: String, val age: Int)

Null Safety
var nullableVar: String? = "This can be null"
nullableVar = null

// Safe call operator
println(nullableVar?.length)

// Elvis operator
val len = nullableVar?.length ?: -1

// Non-null assertion operator (!!)
val nonNullVar = nullableVar!!.length

Extension Functions
fun String.addExclamation(): String = this + "!"

val excited = "Hello".addExclamation() // "Hello!"

Lambdas and Higher-Order Functions
val sumLambda: (Int, Int) -> Int = { x, y -> x + y }
val result = sumLambda(2, 3)  // 5

// Higher-order function
fun operateOnNumbers(a: Int, b: Int, operation: (Int, Int) -> Int): Int {
    return operation(a, b)
}

val sumResult = operateOnNumbers(2, 3, sumLambda)

Coroutines for Asynchronous Programming
// Basic coroutine launch
GlobalScope.launch {
    delay(1000L) // Non-blocking delay for 1 second (only useful in coroutines)
    println("World!")
}

println("Hello,")