What Is Kotlin? A Beginner’s Guide to the Language

Google backed it. JetBrains built it. And over 60% of professional Android developers now write in it daily.

So what is Kotlin, and why has it taken over so much of the programming landscape in just a few years? Kotlin is a statically typed, open-source programming language that runs on the Java Virtual Machine, compiles to JavaScript, and produces native binaries. It handles everything from mobile apps to server-side systems and cross-platform projects.

This guide breaks down how Kotlin works, where it fits in modern software development, how it compares to Java, and what its real-world limitations look like. Whether you’re picking your first language or evaluating a new tech stack, this covers what you actually need to know.

What is Kotlin

Kotlin is a statically typed, open-source programming language developed by JetBrains. It runs on the Java Virtual Machine, compiles to JavaScript, and produces native binaries.

JetBrains first unveiled it in 2011. Version 1.0 shipped in February 2016. By 2019, Google had declared Kotlin the preferred language for Android development.

The language is licensed under Apache 2.0, which means anyone can use it, modify it, and distribute it without paying fees. Its source code lives on GitHub, and contributions come from both JetBrains employees and the broader community.

So what makes it different from every other language targeting the JVM? Conciseness, mostly. Kotlin eliminates a massive amount of repetitive code that Java developers have been writing for decades. Data classes, type inference, extension functions, and built-in null safety all cut boilerplate dramatically.

According to Google’s Android team, over 60% of professional Android developers now use Kotlin, and apps written in it are 20% less likely to crash compared to those written purely in Java.

But Kotlin isn’t just an Android thing. It handles server-side applications, cross-platform app development, and even browser-based projects through Kotlin/JS. The language keeps expanding into areas where Java either can’t go or goes awkwardly.

Kotlin vs Java

Every developer learning Kotlin eventually asks the same question. How does it compare to Java?

The short answer: Kotlin is fully interoperable with Java. You can mix them in the same project, call Java code from Kotlin and vice versa. Both compile to JVM bytecode, so runtime performance is comparable.

The longer answer gets more interesting.

Kotlin reduces boilerplate in ways that actually matter. A Java POJO requiring 50+ lines of getters, setters, equals, hashCode, and toString becomes a single-line data class in Kotlin. Type inference means you declare fewer types explicitly. Smart casts remove tedious instanceof checks.

Java has a much larger legacy codebase across enterprise environments. It has been around since 1995. Kotlin respects that history by not forcing an all-or-nothing migration. Teams can adopt it file by file, converting gradually.

The JetBrains Developer Ecosystem Survey 2024 found that 75% of Kotlin users express satisfaction with the language. At the same time, roughly 49% of developers use Kotlin as their main language alongside Java.

If you’re curious about the specific tradeoffs between these two for mobile work, there’s a detailed breakdown of Kotlin or Java for Android projects.

Null Safety in Kotlin Compared to Java

NullPointerException. If you’ve written Java for any length of time, you’ve seen this crash your app more than once. Tony Hoare called null references his “billion-dollar mistake.” Kotlin took that seriously.

Kotlin bakes nullability into the type system at the compiler level. A regular String can never hold null. A String? can, but the compiler forces you to handle that possibility before accessing it.

Java relies on optional annotations like @Nullable and @NonNull. These are hints, not enforcement. The compiler doesn’t stop you from ignoring them.

Real-world impact: Google’s Home team saw a 30% reduction in crashes caused by null pointer exceptions during the year they migrated new feature development to Kotlin, according to Android Developers documentation.

The safe call operator (?.), the Kotlin elvis operator, and smart casts all work together to make null handling readable instead of painful. Took me a while to get used to the syntax, honestly. But once it clicks, going back to Java’s null handling feels clumsy.

Kotlin for Android Development

This is where Kotlin made its name.

Google announced official support for Kotlin on Android at Google I/O 2017. Two years later, they went further and declared it the preferred language for Android development. That’s not a suggestion. It’s a strong signal that new Android tooling, documentation, and libraries are designed Kotlin-first.

The numbers back this up. Google estimates that 70% of the top 1,000 apps on the Play Store are written in Kotlin. JetBrains puts it even higher, claiming 95% of those top apps include at least some Kotlin code.

Over 70 of Google’s own apps use Kotlin internally, including Maps, Drive, Play, Home, and Messages. This isn’t a company telling developers to use something it won’t touch itself.

Android Studio, the official IDE for mobile application development on Android, has deep Kotlin support baked in. It includes a Java-to-Kotlin converter tool, Kotlin-specific debugging, and code completion that actually understands Kotlin idioms.

The mobile app development process on Android has shifted significantly. New Jetpack libraries ship with Kotlin-first APIs. Some, like Jetpack Compose, only work with Kotlin.

Jetpack Compose and Kotlin

Jetpack Compose changed how Android UIs get built. It replaced the old XML layout system with a declarative, code-first approach, and it runs exclusively on Kotlin.

You can’t use Compose with Java. Full stop.

The framework lets you describe what the UI should look like based on application state. When state changes, the UI updates automatically. No manual view binding, no findViewById calls, no XML inflation.

Google rolled out Compose Multiplatform (originally just Jetpack Compose) alongside Kotlin 2.0, extending it beyond Android to iOS, web, and desktop. If you follow Material Design principles, Compose makes implementing them straightforward with built-in Material 3 components.

The shift to Compose is a big deal for anyone evaluating tech stack choices for app development. It ties the future of Android UI directly to Kotlin.

Kotlin Multiplatform

Kotlin Multiplatform (KMP) lets you share business logic across Android, iOS, desktop, and web while keeping the UI native to each platform.

JetBrains announced KMP as Stable in November 2023. At Google I/O 2024, Google added official support for using KMP to share business logic between Android and iOS. That’s two major companies backing the same technology.

According to JetBrains’ Developer Ecosystem data, KMP usage jumped from 7% to 18% in a single year (2024 to 2025). That’s more than double.

The approach differs from Flutter or React Native. Those frameworks share the UI layer too. KMP shares only the logic (networking, data models, caching, business rules) and leaves UI to native implementations. This means your iOS app still feels like an iOS app.

Forbes shares over 80% of its logic across iOS and Android using KMP. McDonald’s, Netflix, and Google Workspace have all adopted it for production apps.

Stone, a Brazilian fintech company, achieved 61% code sharing with KMP and reported 40% faster delivery of new features at KotlinConf 2025.

Organizations adopting KMP report up to a 30% reduction in development and maintenance costs compared to maintaining separate native teams. That stat alone explains why enterprise interest keeps growing.

If you’re weighing KMP against other options, the comparison of native or hybrid or cross-platform apps covers the broader tradeoffs. And for a deeper look at what KMP actually does under the hood, there’s a dedicated guide on Kotlin multiplatform.

Kotlin Syntax and Language Features

Kotlin’s syntax is concise without being cryptic. That balance is what pulls developers in.

The language supports both object-oriented and functional programming. You can write classes with inheritance and interfaces, then turn around and chain lambdas and higher-order functions in the same file. Most real-world Kotlin code mixes both styles freely.

Extension functions let you add methods to existing classes without subclassing or using the decorator pattern. You can extend String, List, or any third-party class with your own utility functions. It feels like the method was always there.

Sealed classes restrict class hierarchies to a known set of subclasses, which makes when expressions exhaustive. The compiler tells you if you forgot to handle a case. Pattern matching in Kotlin (via when) replaces Java’s verbose switch statements with something much more readable.

Kotlin data classes generate equals, hashCode, toString, and copy functions automatically from the properties you declare. One line replaces what Java needs 40+ lines for.

Type inference handles much of the type declaration work. You write val name = "Kotlin" instead of String name = "Kotlin". The compiler figures it out. But you can still be explicit when clarity matters, which I tend to prefer for function return types.

Kotlin lambda functions and higher-order functions make collection processing clean. Filtering, mapping, and reducing lists reads almost like natural language. If you’ve used streams in Java 8+, imagine that but without the ceremony.

Kotlin Coroutines

Coroutines are Kotlin’s answer to asynchronous programming. They let you write non-blocking code that looks and reads like sequential code.

Traditional threading creates a new OS-level thread for each concurrent task. That works, but threads are expensive in terms of memory. Coroutines run on a smaller number of threads, suspended and resumed by the Kotlin runtime.

Structured concurrency is the key concept. Every coroutine runs inside a scope, and when that scope cancels, all child coroutines cancel too. No orphaned threads, no leaked resources. It’s way cleaner than manually managing thread pools.

The Kotlin coroutines library is used heavily in both Android and server-side applications. On Android, coroutines handle network calls, database queries, and UI updates without callback nesting. On the server side, frameworks like Ktor are built entirely around coroutine-based I/O.

JetBrains and Azul benchmarked Kotlin server applications in 2025, finding that optimized JVM runtimes reduced latencies by 23.9% and improved throughput by up to 30.5% for coroutine-heavy workloads.

For a deeper look at reactive patterns with coroutines, the guide on Kotlin flows covers the streaming side of things.

Kotlin for Server-Side Development

Kotlin on the server doesn’t get as much attention as Kotlin on Android, but it’s growing fast.

Two frameworks dominate: Spring Boot and Ktor.

Spring Boot has supported Kotlin officially since Spring Framework 5. According to JetBrains, 27% of Spring developers have used Kotlin, and that number is climbing. Spring Boot 4 is getting deeper Kotlin-first improvements through a direct collaboration between JetBrains and the Spring team announced at KotlinConf 2025.

Ktor is JetBrains’ own framework, built from scratch for Kotlin. It’s lightweight, modular, and fully asynchronous using coroutines. No annotation magic, no hidden configuration. You wire up exactly what you need.

When to pick which:

• Spring Boot – large enterprise apps, existing Java infrastructure, teams that want a massive ecosystem of integrations and community support
• Ktor – microservices, lightweight APIs, teams that want full control and minimal overhead, projects where coroutines are central to the architecture

Micronaut also supports Kotlin and works well for cloud-native Microservices Architecture setups. It competes with Spring Boot on startup time and memory footprint.

Companies like Amazon, Uber, and Atlassian run Kotlin on their backends. The back-end development landscape has shifted enough that listing Kotlin as a server-side option is no longer controversial.

Kotlin’s server-side story ties into the broader software development process. Teams building RESTful API services or backend logic can share code between their server and mobile apps through KMP, cutting duplication across the entire app lifecycle.

Job postings mentioning Kotlin expertise surged 30% year-over-year in 2024, with server-side roles making up a growing share of that demand.

Kotlin for Web and Desktop Applications

Kotlin compiles to more than just JVM bytecode. Two additional targets, Kotlin/JS and Kotlin/Native, push the language into browsers and desktop operating systems.

Kotlin/JS compiles Kotlin code to JavaScript. You can use it to build browser-based applications, interact with the DOM, and work with existing JavaScript libraries. It’s been available for a while and powers production web apps at several companies.

Kotlin/Native compiles directly to native binaries without needing a JVM at all. It targets macOS, Windows, Linux, and even embedded platforms. Performance is close to C-level for computation-heavy tasks.

Then there’s Compose Multiplatform for desktop. JetBrains extended the Compose UI framework to run on Windows, macOS, and Linux with hardware-accelerated rendering. It’s the same declarative approach used in Jetpack Compose on Android, just on a bigger screen.

Kotlin/Wasm is the newest target. It compiles to WebAssembly for browser-based apps with better performance than JavaScript. Compose for Web uses Kotlin/Wasm by default, with a Kotlin/JS fallback for older browsers.

Wrike, a project management platform, has been running Compose Multiplatform features in production since its beta. Markaz, Pakistan’s second-largest e-commerce platform with over 5 million downloads, built its entire app (100+ screens) using Compose Multiplatform.

The JVM target remains the most mature. Kotlin/JS and Kotlin/Native are stable but still catching up on library support and tooling. If your project needs broad platform coverage beyond front-end development and mobile, Kotlin gives you a single language across all of them.

How to Start Learning Kotlin

You don’t need to install anything to write your first Kotlin program.

Kotlin Playground (play.kotlinlang.org) runs in the browser. Type code, hit run, see results. Good for quick experiments and getting a feel for the syntax before committing to a full setup.

For structured learning, three paths stand out:

• Kotlin Koans: Interactive exercises built as failing unit tests. Your job is to write code that makes them pass. Available online or as a plugin inside IntelliJ IDEA and Android Studio
• JetBrains Academy: Step-by-step courses that walk you through building real applications while learning Kotlin fundamentals
• Official Kotlin documentation at kotlinlang.org, which is surprisingly well-written compared to most language docs

For IDEs, IntelliJ IDEA Community Edition is free and has the deepest Kotlin support (JetBrains made both). Android Studio works too if your goal is mobile.

Which path depends on where you’re headed. Android developers should start with Compose tutorials and the Android basics course. Backend-focused developers are better off with Ktor or Spring Boot quick-start guides.

The Kotlin Slack community has over 60,000 members. That’s a lot of people willing to answer beginner questions at 2am. Which matters more than most people realize when learning a new language.

If you’re coming from a background in software development with Java, the transition is smoother than you’d expect. Most developers report being productive within a few weeks.

Kotlin Adoption and Industry Use

The Stack Overflow Developer Survey 2024 ranked Kotlin as the 4th most loved language at 58.2% developer satisfaction. JetBrains’ own Developer Ecosystem survey shows 75% satisfaction among Kotlin users.

On the TIOBE Index, Kotlin reached 15th place in late 2023, though it has fluctuated since. The TIOBE CEO predicted Kotlin could reach top 10 status. As of early 2025, it’s been moving between positions 20-25, partly because cross-platform frameworks are competing for the same developer attention.

The PYPL Index, which tracks tutorial search volume, placed Kotlin 13th with a 1.76% share.

Industry numbers tell a clearer story than index rankings.

The number of Kotlin users with 4+ years of experience has tripled since 2021, according to JetBrains data. That’s not just new adoption. It’s retention.

Companies running Kotlin in production include Netflix, Pinterest, Uber, Atlassian, Amazon, and McDonald’s. These aren’t experimental side projects. They’re using Kotlin for core products that millions of people interact with daily.

Kotlin’s biggest growth area beyond Android is server-side development. The language has become a common choice for teams building GraphQL API services, backend microservices, and data processing pipelines on the JVM.

Job postings mentioning Kotlin expertise surged 30% year-over-year in 2024, and Kotlin roles pay roughly 15% more than positions requiring only traditional JVM languages.

Limitations and Trade-offs of Kotlin

Kotlin is not without problems. Anyone telling you otherwise is selling something.

Compilation speed has been the most consistent complaint. Kotlin historically compiled slower than Java, especially on larger projects. The K2 compiler (shipped with Kotlin 2.0) improved this dramatically, cutting build times by over 40% for some projects, according to JetBrains’ KotlinConf 2025 benchmarks. But Kotlin/Native builds for iOS targets remain noticeably slower than JVM compilation.

Community and ecosystem size still trails Java’s decades-old ecosystem by a wide margin. When you hit a niche problem in Java, someone posted a Stack Overflow answer about it in 2012. With Kotlin, you might be the first person to ask.

Kotlin/Native and Kotlin/JS are less mature than Kotlin/JVM. Library availability is thinner, debugging tools aren’t as polished, and some APIs are still experimental. Build speed on Kotlin/Native is something JetBrains is actively working on, with build optimization listed as a top priority in their 2025 roadmap.

Learning curve for non-JVM developers: If you’re coming from Python or JavaScript, Kotlin’s type system, coroutines, and Gradle build configuration can feel like a lot at once. Gradle itself is honestly the biggest friction point. A build automation tool shouldn’t need its own learning journey, but here we are.

The TIOBE Index shows Kotlin’s general popularity rankings have slipped slightly in 2025, with TIOBE’s CEO noting that Kotlin and Swift face pressure from cross-platform frameworks entering their niche. That said, the Flutter or React Native comparison doesn’t fully account for Kotlin Multiplatform, which competes on a different axis (shared logic rather than shared UI).

Some teams also struggle with source control management when mixing Java and Kotlin in the same Git repository. The interop is seamless at runtime, but code review gets messy when half the team writes Java and half writes Kotlin with very different idioms.

Despite all this, the direction is clear. JetBrains keeps investing. Google keeps endorsing. And developer satisfaction remains high. The trade-offs are real, but for most projects, the benefits still outweigh them.

FAQ on What Is Kotlin

Is Kotlin better than Java?

Kotlin reduces boilerplate, adds null safety at the compiler level, and supports coroutines natively. Java has a larger ecosystem and faster compilation. For new Android projects, Kotlin is the preferred choice. For legacy enterprise systems, Java still holds strong.

Is Kotlin hard to learn?

Developers with Java experience pick up Kotlin within a few weeks. The syntax is more concise and readable. Beginners from non-JVM backgrounds face a steeper curve, mostly around Gradle configuration and the type system rather than the language itself.

Is Kotlin only for Android?

No. Kotlin runs on the Java Virtual Machine for server-side apps, compiles to JavaScript for web projects, and produces native binaries for desktop. Frameworks like Ktor and Spring Boot use Kotlin for backend development across multiple industries.

Is Kotlin free to use?

Yes. Kotlin is open-source under the Apache 2.0 license. JetBrains maintains it, and the source code is publicly available on GitHub. IntelliJ IDEA Community Edition and Android Studio both provide free IDE support for Kotlin development.

What companies use Kotlin?

Google, Netflix, Uber, Pinterest, Amazon, Atlassian, and McDonald’s all run Kotlin in production. Google uses it in over 70 of its own apps, including Maps and Drive. Adoption spans both mobile and server-side applications.

What is Kotlin Multiplatform?

Kotlin Multiplatform lets you share business logic across Android, iOS, desktop, and web while keeping native UI on each platform. Google officially supports it for Android-iOS code sharing. Companies like Forbes and Netflix use it in production apps.

What is the difference between Kotlin and Flutter?

Kotlin is a programming language. Flutter is a UI framework built on Dart. Kotlin Multiplatform shares logic while keeping native UI. Flutter shares both logic and UI through its own rendering engine. They solve cross-platform problems differently.

What are Kotlin coroutines used for?

Coroutines handle asynchronous programming without callback nesting. They manage network calls, database operations, and background tasks using structured concurrency. Coroutines run on shared thread pools, making them lighter than traditional threads for concurrent workloads.

Can Kotlin and Java work together?

Yes. Kotlin is fully interoperable with Java. You can call Java code from Kotlin and vice versa within the same project. Teams adopt Kotlin incrementally, converting files one at a time without rewriting their entire codebase.

What IDE should I use for Kotlin?

IntelliJ IDEA Community Edition offers the deepest Kotlin support since JetBrains develops both. Android Studio works well for mobile-focused projects. Both include built-in Kotlin tools like code completion, debugging, and a Java-to-Kotlin converter.

Conclusion

Understanding what is Kotlin comes down to seeing it in action. It’s a statically typed programming language built by JetBrains that handles Android apps, server-side systems, and multiplatform projects with less code and fewer crashes than Java.

Google’s endorsement made it the default for Android. Kotlin Multiplatform is pushing it into iOS, desktop, and web territory. Coroutines solved asynchronous programming without the callback mess.

The ecosystem is still smaller than Java’s. Compilation speed, while improved by the K2 compiler, isn’t perfect. But developer satisfaction stays high, adoption keeps climbing, and companies like Netflix and Google are betting production systems on it.

If you’re building modern applications, especially anything touching custom app development on Android or shared logic across platforms, Kotlin deserves a serious look. The language has moved well past “promising alternative” into “industry standard” territory.

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Bogdan Sandu

Bogdan Sandu specializes in web design, focusing on creating user-friendly websites, and innovative UI kits.

Many of his resources are available on various design marketplaces and for free on Codepen.

Over the years, he's worked with a range of clients and contributed to design publications like Design Your Way, Designmodo, WebDesignerDepot, WPDean, Speckyboy, and Slider Revolution among others.

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