Kotlin Cheat Sheet

Kotlin syntax, null safety, data classes, coroutines, and extension functions for concise, safe JVM development.

2 PagesIntermediateMar 30, 2026

Basic Syntax

Variables, control flow, and printing.

kotlin

fun main() {    val age = 30              // immutable (val)    var count = 0              // mutable (var)    val name = "Ada"    val pi: Double = 3.14159    if (age >= 18) {        println("$name is an adult")    }    for (i in 0 until 5) {        println("Count: $i")    }}

Null Safety & Data Classes

Compile-time null checks and value objects.

kotlin

data class User(val name: String, val age: Int)fun greet(user: User?) {    val name = user?.name ?: "Guest"   // safe call + Elvis operator    println("Hello, $name")}val user: User? = User("Ada", 30)user?.let { println(it.age) }          // scope function, runs if non-nullval u2 = user!!                        // force-unwrap (throws if null)

Coroutines

Lightweight concurrency with suspend functions.

kotlin

import kotlinx.coroutines.*suspend fun fetchData(): String {    delay(1000)          // non-blocking suspension    return "data"}fun main() = runBlocking {    val job = launch {        val result = fetchData()        println(result)    }    job.join()    val deferred = async { fetchData() }    println(deferred.await())}