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

Most people run into Kotlin through Android. That’s where it got popular. The language itself goes further than that, though. It’s statically typed, general-purpose, and it compiles to the Java Virtual Machine, JavaScript, and native code from one shared codebase.

JetBrains builds and maintains it. The trademark belongs to the Kotlin Foundation, a nonprofit that keeps the language’s direction from depending on any single vendor.

You’ll find it in Android apps, on backend servers, and in web projects, usually competing with Java on the same virtual machine.

Kotlin Multiplatform, the cross-platform layer built on top of the language, now runs in production at over 20,000 companies worldwide, according to JetBrains’ Kotlin Multiplatform case studies (2025).

What Is Kotlin?

On paper it looks like a lot of modern languages. Static types, type inference, a compiler that complains early. What sets it apart is how many places the compiled output can go.

The Java Virtual Machine is the default target. The same source code can also produce JavaScript, native binaries, and WebAssembly.

That range is the reason Kotlin exists in its current form. Most languages pick one runtime and stay there. Kotlin was designed so a single team could cover Android, backend servers, the browser, and iOS without learning a new language for each one.

Some things you’ll notice right away:

• Types are static, but inference means you rarely write them out yourself
• Null safety lives in the type system, not in a linter rule someone added later
• It calls Java code and gets called from Java code on the same JVM
• Open source, so adopting it in a commercial product costs nothing

Teams use it for web, backend, and mobile software development. Android is just where it earned its reputation first.

Who Created Kotlin and Who Maintains It Today

JetBrains created Kotlin and still pays for most of its day to day development.

Governance is a separate matter. That sits with the Kotlin Foundation, which JetBrains and Google set up to hold the trademark and keep the language from following one company’s roadmap.

The first stable release landed on February 15, 2016. From that version on, JetBrains committed to backward compatibility. The Foundation came two years later, in 2018, announced at KotlinConf that October with JetBrains and Google as its original members.

Licensing is simple. Kotlin uses the Apache License 2.0, so you can ship it in commercial products and owe no royalty.

The current major line is Kotlin 2.4. Every target now runs on the K2 compiler, which became the default in the Kotlin 2.0 release of May 2024.

Membership has grown since the early days. Gradle, Touchlab, and Shopify joined as Foundation members in 2023, and Uber came on board as a Silver Member in 2024 (Kotlin Foundation).

None of that changes who writes most of the compiler. JetBrains engineers still do the bulk of it. What the Foundation really does is stop the language from getting pulled toward one company’s product line.

What Are Kotlin’s Core Language Features

The design goal is less boilerplate and more mistakes caught at compile time instead of in production. Null safety and coroutines get most of the attention. Data classes and sealed classes do quieter work that you end up relying on just as much.

Null Safety and the Type System

Nullability is part of the type. Declare a variable as String and it can’t hold null. Declare it as String? and it can, but the compiler won’t let the code build until you deal with that case.

You get a few tools for dealing with it. The safe call operator ?. short-circuits instead of throwing. The Elvis operator ?: gives you a fallback value in one line. And after a null check, smart casts narrow the type for you, so you don’t cast by hand.

The effect shows up in real numbers. Among the top 1,000 Android apps on Google Play, apps built with Kotlin show 20% fewer crashes than comparable Java code, and Google credits much of that gap to null pointer exceptions getting caught before they ship (Android Developers, Google).

Coroutines for Asynchronous Work

Coroutines let you write async code that reads top to bottom. If you’ve maintained async Java with callbacks nested four levels deep, you’ll see the appeal fast. Kotlin calls this structured concurrency.

They’re cheap, too. A suspended coroutine doesn’t block its thread, so launching thousands of them on a single thread is fine.

The Kotlin coroutines library, kotlinx.coroutines, ships separately from the standard library. It covers channels, flows, and dispatchers for different thread pools.

Data Classes and Extension Functions

Put the data modifier on a class and Kotlin generates equals, hashCode, toString, and copy for you.

That’s forty lines of boilerplate gone from a class that only holds a few fields. The Kotlin data classes feature comes up constantly when teams explain why they moved their model layer off Java.

Extension functions are a different kind of convenience. They let you add a method to a class you don’t own, without subclassing it or touching its source.

A typical case is adding a custom formatting function to Kotlin’s own String type. At the call site it looks like a native method, even though it’s defined somewhere else entirely.

Smart Casts and Sealed Classes

A sealed class limits a type hierarchy to a closed set of subclasses, all declared in the same file.

Because the compiler knows every possible subtype, a when expression over a sealed class doesn’t need an else branch once you’ve covered each case. Inside each branch, smart casts kick in automatically.

Small feature on paper. In practice it’s why Kotlin’s when feels closer to pattern matching than to a plain switch statement.

What Platforms Does Kotlin Compile To?

The target you choose decides what your code turns into: JVM bytecode, JavaScript, a native binary, or a WebAssembly module. A lot of the source can be shared between them.

Kotlin/JVM is the oldest and most mature path. It runs on the same virtual machine as ordinary Java code, which explains a lot about how tight the interop between the two languages is.

Kotlin/Native reached production stability in November 2023. It compiles straight to machine code through LLVM, so the target device doesn’t need a JVM at all.

Then Kotlin 2.0 shipped in May 2024 with the rewritten K2 compiler, which JetBrains says compiles code up to twice as fast as the old one (JetBrains).

That matters more than it sounds. Faster feedback loops are a big part of why Kotlin keeps spreading into cross-platform app development instead of staying an Android thing.

How Does Kotlin Interoperate With Java?

Directly, in both directions. There’s no wrapper layer and no translation step.

Both languages compile to the same bytecode format and run on the same JVM. A call from Kotlin into Java is just a method call.

• Any Java class, library, or framework works from Kotlin without an adapter
• Java can call Kotlin functions and classes too, although nullable types need extra annotations to stay safe
• .kt and .java files can live in the same module and build together
• You can migrate a class or a package at a time

That last point is the one teams care about most. Nobody has to rewrite the whole project on day one, and plenty of developers keep a Java cheat sheet open next to the Kotlin docs while both languages share a repository.

It isn’t flawless. Checked exceptions don’t exist in Kotlin. Some Java libraries built around loose nullability assumptions also need small annotation fixes before Kotlin can treat them safely, which is annoying the first time you hit it.

What Is Kotlin Multiplatform?

Kotlin Multiplatform (KMP) keeps an app’s business logic in one codebase and compiles it separately for Android, iOS, desktop, and web.

Networking, data parsing, and business rules usually go in the shared module, written once in Kotlin and compiled through Kotlin/JVM, Kotlin/Native, and Kotlin/JS. The interface can stay fully native on each platform. If you’d rather share the UI as well, Compose Multiplatform sits on top as an optional layer.

Anything that needs a native API, like camera access or push notifications, stays outside the shared code.

KMP and Kotlin/Native went stable together in November 2023, after several years in alpha and beta.

McDonald’s uses KMP to share complex code, in-app payments included, across its Global Mobile App. That single codebase now handles over 6.5 million monthly purchases (Kotlin Multiplatform case studies, JetBrains).

If you’re deciding between fully native and shared logic, the Kotlin Multiplatform guide goes deeper into setup and the trade-offs.

What Tools and Build Systems Support Kotlin Development?

The tooling list is short. That’s mostly because the company behind the language also makes its main editor.

Android Studio has had official Kotlin support since Android Studio 3. For Android apps (as opposed to server code) it’s still the default pick.

Dokka generates documentation from source comments. Detekt handles static analysis and flags style and complexity problems before code review.

On the career side, the JetBrains State of Developer Ecosystem 2024 survey of more than 23,000 developers put Kotlin among the highest-paying languages, alongside Scala, Go, and C.

You don’t need any of this, technically. A plain text editor and the command-line compiler work fine. Most people give that up once IntelliJ’s inspections start catching their mistakes for free, though. While you’re learning, keep a Kotlin cheat sheet close by. The standard library has more shorthand than Java developers expect.

What Is Kotlin Used For?

Android first. It was the original use case and it’s still the biggest by a wide margin.

After that you’ll see it in backend services (Ktor, Spring Boot), frontend web code compiled through Kotlin/JS, and cross-platform or desktop apps built with Kotlin Multiplatform.

Most Kotlin work still revolves around Android development, which makes sense given that’s where the language spent its first decade building momentum.

Backend adoption is growing, though. Ktor gives teams a Kotlin-native way to build RESTful APIs without a heavier framework.

Kakao Pay, a South Korean fintech company, runs a meaningful share of its backend on Kotlin with Spring. Its insurance team has talked publicly about how the language’s features make stable backend services simpler to run (Kotlin case studies, JetBrains).

Why Did Google Make Kotlin the Preferred Language for Android?

Google gave Kotlin official standing at Google I/O in 2017. On May 7, 2019, it went further and named Kotlin the preferred language for Android app development.

Part of it was plain convenience. Adapters, listeners, and view binding all took far less code in Kotlin. Jetpack Compose, Android’s modern UI toolkit, also shipped with Kotlin-first APIs from the start, so the direction was already set.

The bigger reason was null safety, because null pointer crashes had been hurting Android apps for years. And this wasn’t theoretical. The Google Home team saw a 33% decrease in Null Pointer Exception crashes over one year after moving their codebase to Kotlin (Android Developers, Google).

With a result like that coming from a team inside Google, the preferred-language call wasn’t hard to defend.

Kotlin vs Java: How Do They Compare?

Same JVM, same project if you want. The syntax, safety model, and approach to concurrency are where they split.

There’s data on development speed as well. A JetBrains telemetry study of roughly 320,000 IntelliJ IDEA developers found Kotlin-oriented projects finish everyday coding tasks 15 to 20% faster than comparable Java projects (JetBrains Research, 2026).

Java isn’t going anywhere, to be clear. The two overlap a lot and just solve things differently. If you’re choosing for an Android project specifically, read the Kotlin or Java breakdown before you commit.

Which Companies Use Kotlin in Production?

On Android, Kotlin is everywhere. Backend is a slower story, but it’s moving.

Google’s own numbers show how far the Android side has gone. 95% of the top 1,000 Android apps on Google Play contain Kotlin code, and over 60% of professional Android developers now use it as their primary language (Android Developers, Google). Across all platforms, Kotlin counts roughly 2.5 million developers worldwide (JetBrains, 2025).

ING is a good backend example. Kotlin now appears in roughly 8% of the bank’s active internal repositories, out of more than 90,000 total, and that share has grown steadily in the five years since the first team took it to production.

I find that number more convincing than the Android stats. Nobody forced ING’s roughly 20,000 tech staff to use Kotlin on the backend. Teams picked it up repository by repository, which tells you more than a mandate would.

You see similar adoption in finance, e-commerce, and consumer apps. That spread is part of why long-term library support hasn’t really worried teams adopting it.

Where Kotlin Falls Short

It’s not the right choice for every project.

The strengths are covered above: fewer null crashes, easier async code, and interop that lets you migrate gradually. The weaknesses get less airtime.

Hiring is the first one. The pool of Kotlin developers is smaller than Java’s in most regions and industries. Large mixed Java and Kotlin builds can also compile slower than pure Java equivalents, and Java simply has more decades of enterprise support contracts behind it.

The TIOBE Index, a widely cited measure of language popularity, shows the hiring gap. Java has held a top five spot for years. Kotlin climbed as high as 15th in late 2023 but has more typically sat closer to 20th (TIOBE).

The speed gap is real, just narrower than people assume. JetBrains’ own research found that on extra-large, multi-day features, Kotlin’s advantage over Java became statistically uncertain, with results ranging from an 11% improvement to a slight regression (JetBrains Research, 2026).

Plain Java still makes sense in a regulated environment tied to long-term Java-only vendor support contracts. Same for a team with zero Kotlin experience and a deadline that leaves no room for a learning curve.

A very large, mature monorepo is the trickier case. If the build tooling was never tuned for mixed-language compilation, adding Kotlin can hurt more than it helps, at least at first.

Before you decide, run a quick gap analysis of what the team actually knows about Kotlin.

Add a short risk assessment matrix for the migration. The decision usually gets settled faster that way than by arguing about it in a meeting.

How Do You Write and Run Your First Kotlin Program?

You don’t need Android Studio for this. If you just want to see the language run, the setup is small.

1. Install the Kotlin compiler, or open an IDE that bundles it, like IntelliJ IDEA
2. Create a file named Main.kt
3. Write a minimal entry point: fun main(), then println("Hello, Kotlin") inside the braces
4. Compile it from the command line with kotlinc, or press Run inside the IDE
5. Run the compiled output with java -jar, or read the result in the IDE console

The command-line route is where people get stuck, and it’s nearly always the same flag.

If you forget -include-runtime when compiling to a jar, the Kotlin standard library gets left out. The program then crashes the moment it touches any standard library class. This is the most common first-run error by far.

An old JDK comes second. Kotlin needs a JDK on the system path to compile at all, and an outdated one throws confusing errors that have nothing to do with your code.

FAQ on Kotlin

Where does the name Kotlin come from?

It’s named after Kotlin Island, near Saint Petersburg, where the original JetBrains development team was based.

There’s a nod to Java in there, since Java is also named after an island, the Indonesian one known for coffee. The name has nothing to do with any programming concept or acronym.

Is Kotlin a programming language or a framework?

A programming language. It has its own compiler, syntax, and standard library.

Spring and Ktor are frameworks. They run on top of Kotlin and don’t replace it.

Is Kotlin good for beginners to learn?

Yes, especially if you’re starting with Android.

The syntax is cleaner than Java’s, and null safety catches mistakes early. JetBrains also publishes Kotlin Koans, a free set of short guided exercises for learning the syntax.

Is Kotlin free to use?

Yes. It’s licensed under the Apache License 2.0.

You can build, modify, and redistribute Kotlin-based software, commercial or open source, without paying JetBrains a licensing fee. The compiler, standard library, and most official tooling use the same license.

What mistakes do Java developers commonly make when switching to Kotlin?

Overusing nullable types out of habit is a big one. It throws away the null safety they switched for.

Many also keep writing Java-style loops instead of using Kotlin’s collection functions, or write out full classes where a data class would do.

What Should You Do First After Learning What Is Kotlin?

Start with a small, contained pilot. One module or a handful of files, built and tested, before anyone commits to migrating an entire Java codebase.

Measure real build times on that pilot against your Java baseline. Once null safety feels natural to the team, bring in coroutines. Shared cross-platform code should come last, after the core language is second nature.

The downside of going slowly is that you’ll be running two build toolchains, Java and Kotlin, side by side for longer than a clean rewrite would take.

Once that’s in place, most people move on to how Kotlin flows handle streams of asynchronous values, since coroutines rarely stay simple for long in production code.

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Bogdan Sandu

Bogdan Sandu is a front-end developer at TMS Outsource with 8+ years of experience in web technologies. He writes about developer tools, software platforms, and web workflows based on daily hands-on use.

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