Kotlin Cheatsheet - Kotlin Syntax & Coroutine Reference

This reference is for Kotlin developers — on Android, a JVM backend, or experimenting with scripts — and covers the five things you touch constantly: compiling and running with kotlinc/Gradle, null-safety idioms (safe-call, elvis, the non-null assertion), collection operators like filter/map/groupBy, and coroutines for structured concurrency. Unlike a generic Kotlin grammar dump, entries are grouped by the problem each snippet solves, from "avoid the NPE" to "run I/O off the main thread". After reading, you should be able to write null-safe chains without !! except where intended, transform collections fluently, and launch coroutines scoped so cancellation propagates correctly.

Languages·47 commands·Last updated 2026-07-21

Compile & Run 5

kotlinc main.kt -include-runtime -d main.jar

Compile to an executable jar

java -jar main.jar

Run the compiled jar

kotlinc -script script.kts

Run a Kotlin script

kotlinc-jvm

Enter the REPL

kotlin main.kt

Run directly (no compile)

Gradle Build 5

./gradlew build

Build the project

./gradlew run

Run the main class

./gradlew test

Run tests

./gradlew clean

Clean the build dir

./gradlew dependencies

Show dependencies

Null Safety 5

var name: String? = null

Nullable type declaration

name?.length

Safe call (null -> null)

name ?: "default"

Elvis operator (default on null)

name!!.length

Non-null assertion (throws on null)

val len = name?.length ?: 0

Combine the two

Collections 10

list.filter { it > 5 }

Filter

list.map { it * 2 }

Map

list.flatMap { it.split(",") }

FlatMap

list.groupBy { it.category }

GroupBy

list.sortedBy { it.age }

SortedBy

list.distinct()

Distinct

list.take(5)

Take first 5

list.drop(3)

Drop first 3

list.firstOrNull { it.id == 1 }

Find first match

list.partition { it > 0 }

Partition by predicate

Coroutines 6

runBlocking { ... }

Blocking coroutine (tests)

launch { ... }

Launch (no return value)

async { ... }

Async (returns Deferred)

delay(1000)

Non-blocking delay

withContext(Dispatchers.IO) { ... }

Switch context

coroutineScope { ... }

Create a coroutine scope

Classes & Objects 8

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

Data class (auto equals/hashCode/toString/copy)

sealed class Result

Sealed class (exhaustive when)

object Singleton { val x = 1 }

Singleton object

companion object { val MAX = 100 }

Companion object (class-level)

class User(val name: String)

Primary ctor declares property

open class Animal; class Dog : Animal()

Inheritance (parent must be open)

interface Drawable { fun draw() }

Interface definition

enum class Color { RED, GREEN, BLUE }

Enum class

Scope Functions & Delegation 8

obj.let { it.foo() }

let: it receiver, returns lambda result

obj.apply { this.x = 1 }

apply: this receiver, returns the object (init)

obj.also { it.log() }

also: it receiver, returns object (side-effects)

obj.run { compute() }

run: this receiver, returns lambda result

with(obj) { foo() }

with: this receiver, returns lambda result

val lazyVal by lazy { init() }

Lazy init delegation (computed on first access)

var x by Delegates.observable(0) { _, old, new -> }

Observable property delegation

val v by map

Map delegation (keyed access)

Tips

Kotlin null safety is a compile-time check - avoid !! when you can.
Coroutine launch returns a Job; async returns a Deferred you can await.
Collection chaining is elegant, but watch intermediate-collection memory on large data.
data class auto-generates copy/equals/hashCode/toString - ideal for data models.
Scope functions: use apply to init, let/let it to transform, also for side-effects.
sealed class + when lets the compiler verify exhaustiveness - no else needed.

Official References

Each command links to its official documentation below, so you can verify the latest usage and read deeper.

Official Docs

Maintained by LaoHand

Publicly updated on Jul 21, 2026, continuously proofread against official docs.

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