Kotlin Cheat Sheet

fun main() {
    println("Hello, World!")
}

// Single line comment
/* Multi-line
   comment */
/** KDoc comment */

print("Name: ")
val name = readLine()

"42".toInt()      // String to Int
42.toString()     // Int to String
"3.14".toDouble() // to Double

variable is String    // true/false
variable::class.simpleName

val name = "John"

var count = 0

val age: Int = 25

var name: String? = null

val fruits = listOf("apple", "banana")

val nums = mutableListOf(1, 2, 3)

val ages = mapOf("John" to 30, "Jane" to 25)

5 + 3   // 8 (addition)
5 - 3   // 2 (subtraction)
5 * 3   // 15 (multiplication)
5 / 3   // 1 (integer division)
5.0 / 3 // 1.666... (double division)
5 % 3   // 2 (modulus)

5 == 5  // true (structural equality)
5 === 5 // true (referential equality)
5 != 3  // true (not equal)
5 > 3   // true (greater)
5.compareTo(3) // 1 (compare)

true && false   // false (AND)
true || false   // true (OR)
!true           // false (NOT)

name?.length     // null if name is null
name ?: "default" // Elvis operator
name!!           // assert non-null

1..5      // 1, 2, 3, 4, 5 (inclusive)
1 until 5 // 1, 2, 3, 4 (exclusive)
5 downTo 1 // 5, 4, 3, 2, 1
1..10 step 2 // 1, 3, 5, 7, 9

5 in 1..10     // true
"a" in listOf("a", "b") // true
5 !in 1..3     // true

for (i in 1..5) {
    println(i)  // 1, 2, 3, 4, 5
}

for (i in 0 until 5) {
    println(i)  // 0, 1, 2, 3, 4
}

for (i in 5 downTo 1) {
    println(i)  // 5, 4, 3, 2, 1
}

for (i in 0..10 step 2) {
    println(i)  // 0, 2, 4, 6, 8, 10
}

list.forEach { println(it) }
list.forEachIndexed { i, item -> println("$i: $item") }

while (count < 10) {
    count++
}

do {
    count++
} while (count < 10)

for (i in 1..10) {
    if (i == 5) continue
    if (i == 8) break
    println(i)
}

repeat(5) { index ->
    println("Index: $index")
}

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

when (x) {
    1 -> println("one")
    2, 3 -> println("two or three")
    in 4..10 -> println("4 to 10")
    else -> println("other")
}

val result = when {
    x > 0 -> "positive"
    x < 0 -> "negative"
    else -> "zero"
}

if (x > 0) {
    println("positive")
} else if (x < 0) {
    println("negative")
} else {
    println("zero")
}

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

fun add(a: Int, b: Int) = a + b

fun greet(name: String = "World") = "Hello, $name!"

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

fun String.addExclaim() = this + "!"

val len = name?.length

val len = name?.length ?: 0

val len = name!!.length

name?.let { println(it) }

class Dog(val name: String) {
    fun bark() = "$name says woof!"
}

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

object Singleton {
    fun doSomething() { }
}

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

sealed class Result {
    data class Success(val data: String) : Result()
    data class Error(val msg: String) : Result()
}

list.map { it * 2 }

list.filter { it > 0 }

list.reduce { acc, n -> acc + n }

list.find { it > 10 }

people.groupBy { it.city }

people.sortedBy { it.age }

launch {
    delay(1000)
    println("Done")
}

val result = async { fetchData() }
val data = result.await()

suspend fun fetchData(): String {
    delay(1000)
    return "data"
}