Kotlin Cheat Sheet

Updated 2026-04-29

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Kotlin is a modern, statically-typed programming language developed by JetBrains that runs on the Java Virtual Machine (JVM), compiles to native binaries, and WebAssembly, and is officially supported for Android development. Since Google announced Kotlin as a first-class language for Android in 2017, it has become the preferred choice for millions of developers due to its concise syntax, null safety guarantees, and seamless Java interoperability. The K2 compiler — stable since Kotlin 2.0 — brings significantly faster compilation and unified behaviour across all platforms. Understanding Kotlin's type system and its convention-based approach (where language features like operator overloading and property delegation are implemented through specific naming conventions) is key to writing idiomatic, expressive code across Android, server-side, and Kotlin Multiplatform projects.

What This Cheat Sheet Covers

This topic spans 22 focused tables and 167 indexed concepts, 156 flashcards. Below is a complete table-by-table outline of this topic, spanning foundational concepts through advanced details.

Table 1: Null Safety FeaturesTable 2: Coroutines & Async ProgrammingTable 3: Extension Functions & PropertiesTable 4: Scope FunctionsTable 5: Data Classes & Sealed HierarchiesTable 6: Collections & Functional OperationsTable 7: Lambdas & Higher-Order FunctionsTable 8: Properties & DelegationTable 9: Type System & GenericsTable 10: Control Flow & ExpressionsTable 11: Strings & TemplatesTable 12: Inline Functions & ModifiersTable 13: Destructuring & ConventionsTable 14: Sequences vs ListsTable 15: Object-Oriented FeaturesTable 16: SAM Conversions & Functional InterfacesTable 17: Interoperability AnnotationsTable 18: Advanced Type FeaturesTable 19: DSLs & Type-Safe BuildersTable 20: Kotlin ContractsTable 21: Context Parameters (Kotlin 2.2)Table 22: Kotlin Multiplatform (KMP)

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Table 1: Null Safety Features

Kotlin's headline feature: the type system tracks whether a value can be null, turning a whole class of runtime crashes into compile errors. The operators here are how you work with that distinction — declaring nullable types with ?, navigating them safely with ?. and the Elvis ?:, and only resorting to the !! assertion when you're truly certain a value is present.

Feature	Example	Description
Nullable types	`var name: String? = null`	• Append `?` to any type to allow `null` values • compiler enforces explicit handling before access.
Safe call operator	`val length = name?.length`	• Returns `null` if the receiver is `null` • chains safely without throwing `NullPointerException`.
Elvis operator	`val len = name?.length ?: 0`	Provides default value when the left-hand expression evaluates to `null`.
Smart casts	`if (x != null) x.length`	Compiler automatically casts to non-nullable type after a null check — no explicit cast needed.
let function	`name?.let { print(it.length) }`	• Executes lambda only if receiver is non-null • useful for null-safe transformations.
filterNotNull	`listOf(1, null, 2).filterNotNull()`	• Returns a list with all `null` elements removed • result type is `List<T>` (non-nullable).
Safe casts	`val x: String? = obj as? String`	Returns `null` instead of throwing `ClassCastException` when the cast fails.
Not-null assertion	`val len = name!!.length`	• Throws `NullPointerException` if `null` • use sparingly and only when absolutely certain value is non-null.

Table 2: Coroutines & Async Programming

Coroutines are how Kotlin does concurrency without callback hell — suspend functions pause and resume without blocking a thread. Beyond the basic builders (launch, async, runBlocking) you'll find the structured-concurrency machinery that keeps work tidy (scopes, jobs, dispatchers, cancellation) and the Flow family for reactive streams, where the key split is cold flows versus the hot StateFlow/SharedFlow that drive modern Android UI state.

Concept	Example	Description
suspend function	`suspend fun fetchData(): String`	• Function that can be paused and resumed • callable only from other suspend functions or coroutine builders.

The rest of this section is part of the full cheat sheet

Table 3: Extension Functions & Properties

Extensions let you add new functions and properties to a class you don't own — even String or List — without subclassing it. They're resolved statically and called with ordinary dot notation, and the variations here (nullable receivers, generics, companion-object extensions) cover the cases that make this one of Kotlin's most-used features.

Feature	Example	Description
Extension function	`fun String.lastChar() = this[length - 1]`	• Adds a new function to an existing class without inheritance • called with dot notation.

The rest of this section is part of the full cheat sheet

Table 4: Scope Functions

Five small functions — let, apply, also, run, and with — that run a block of code in the context of an object. They differ on just two axes: whether the object is referred to as it or this, and whether the block returns that object or its own result. Once those two distinctions click, picking the right one for configuration, null-checks, or side effects becomes second nature.

Function	Example	Description
let	`val result = obj?.let { it.transform() }`	• Object is `it` • returns lambda result • ideal for null-safe transformations.

The rest of this section is part of the full cheat sheet

Table 1: Null Safety Features

Feature	Example	Description
Nullable types	`var name: String? = null`	• Append `?` to any type to allow `null` values • compiler enforces explicit handling before access.
Safe call operator	`val length = name?.length`	• Returns `null` if the receiver is `null` • chains safely without throwing `NullPointerException`.
Elvis operator	`val len = name?.length ?: 0`	Provides default value when the left-hand expression evaluates to `null`.
Smart casts	`if (x != null) x.length`	Compiler automatically casts to non-nullable type after a null check — no explicit cast needed.
let function	`name?.let { print(it.length) }`	• Executes lambda only if receiver is non-null • useful for null-safe transformations.
filterNotNull	`listOf(1, null, 2).filterNotNull()`	• Returns a list with all `null` elements removed • result type is `List<T>` (non-nullable).
Safe casts	`val x: String? = obj as? String`	Returns `null` instead of throwing `ClassCastException` when the cast fails.
Not-null assertion	`val len = name!!.length`	• Throws `NullPointerException` if `null` • use sparingly and only when absolutely certain value is non-null.

Table 2: Coroutines & Async Programming

Concept	Example	Description
suspend function	`suspend fun fetchData(): String`	• Function that can be paused and resumed • callable only from other suspend functions or coroutine builders.

Table 3: Extension Functions & Properties

Feature	Example	Description
Extension function	`fun String.lastChar() = this[length - 1]`	• Adds a new function to an existing class without inheritance • called with dot notation.

Table 4: Scope Functions

Function	Example	Description
let	`val result = obj?.let { it.transform() }`	• Object is `it` • returns lambda result • ideal for null-safe transformations.