Oct 22, 2025·8 min
How Kotlin Modernized the JVM and Won Android Development
Kotlin added safer syntax, better tooling, and Java interop, helping the JVM evolve and making Android apps faster to build and easier to maintain.
Kotlin added safer syntax, better tooling, and Java interop, helping the JVM evolve and making Android apps faster to build and easier to maintain.
Kotlin is a modern programming language created by JetBrains that compiles to JVM bytecode. That means it runs anywhere Java runs: backend services, desktop apps, and—most visibly—Android. It can also target JavaScript and native platforms through Kotlin Multiplatform, but its “home turf” is still the JVM.
Kotlin didn’t replace Java; it raised the baseline for what JVM development can feel like. In practice, “improvement” meant:
Android already depended heavily on Java APIs, tooling, and libraries. Kotlin’s seamless interoperability let teams introduce it file-by-file: call Java from Kotlin, call Kotlin from Java, and keep the same build system and runtime.
Just as important, Kotlin fit naturally into Android Studio and Gradle workflows, so adopting it didn’t require a new toolchain or a rewrite. Teams could start with a small module, reduce risk, and expand once they saw productivity gains.
Kotlin often pays off when you’re building or maintaining a sizable Android codebase, especially where correctness and readability matter. The trade-offs are real: build times can increase, APIs offer multiple ways to do the same thing, and mixed Java/Kotlin projects need consistent style and conventions.
This article covers the practical wins, the gotchas, and when Kotlin is the right choice for your Android app and JVM projects.
Kotlin didn’t succeed just because it added shiny new syntax. It targeted a specific set of frustrations that JVM and Android teams had been living with for years—problems that got worse as apps, codebases, and organizations grew.
Early Android development leaned heavily on Java patterns that were fine on the server, but awkward on mobile. Everyday tasks routinely turned into long stretches of boilerplate: getters/setters, builders, callbacks, and repetitive “plumbing” code to move data around.
Null handling was another constant source of bugs. A single unexpected null could crash an app at runtime, and defensive checks (if (x != null)) spread everywhere—making code noisy and still not fully safe.
As Android apps became “real products” (multiple screens, offline support, analytics, experiments, feature flags), teams needed code that stayed readable under pressure. More contributors meant more review overhead and a higher cost when APIs were unclear.
In that environment, a language that encouraged concise, predictable code stopped being a nice-to-have—it directly affected shipping speed and defect rates.
Mobile apps are inherently asynchronous: network calls, databases, sensors, UI events. Java-era Android often relied on nested callbacks, custom thread handling, or ad-hoc abstractions. The result was “callback spaghetti,” tricky error propagation, and code that was hard to cancel, test, or reason about.
Kotlin’s rise aligned with the need for safer defaults: patterns that make it harder to block the UI thread, leak work past a screen’s lifecycle, or silently drop failures.
Crucially, Kotlin couldn’t demand a clean-slate rewrite. The JVM ecosystem represents decades of investment: existing libraries, build systems, and teams with Java expertise.
So Kotlin was designed to fit into the world developers already had—compiling to JVM bytecode, working inside Android Studio and Gradle, and interoperating with Java so teams could adopt it file-by-file instead of betting everything on a big migration.
Kotlin’s fastest path into the JVM ecosystem was simple: it didn’t ask teams to abandon Java. Kotlin compiles to standard JVM bytecode, uses the same libraries, and can live in the same module as Java files. That “100% interoperability” message lowered adoption risk because existing code, dependencies, build tools, and developer skills stayed relevant.
In a real Android codebase, it’s common to call Java from Kotlin and Kotlin from Java in the same feature. Kotlin can consume Java classes as-is:
val user = UserRepository().findById("42") // UserRepository is Java
And Java can call Kotlin, including top-level functions (via generated *Kt classes) and regular classes:
String token = AuthKt.generateToken(userId); // generateToken is a Kotlin top-level function
This mixing is what made gradual migration practical: a team could start by writing new screens in Kotlin, then convert small leaf components, then move deeper layers over time—without requiring a “big rewrite” milestone.
Interop is excellent, but not magic. The main friction points tend to be:
String! and can still trigger NullPointerException unless you validate or wrap them.@Nullable/@NonNull (or JSpecify). Without them, Kotlin can’t enforce null safety.Interop didn’t just make Kotlin compatible—it made adoption reversible, incremental, and therefore realistic for production teams.
Kotlin’s appeal wasn’t a single headline feature—it was the steady removal of small, recurring sources of defects and noise. Everyday code got shorter, but also more explicit about intent, which made it easier to review and safer to change.
Kotlin distinguishes between nullable and non-nullable types: String is different from String?. That simple split moves a whole class of “forgot to check for null” issues from runtime into compile time.
Instead of sprinkling defensive checks everywhere, you’re guided into clear patterns like ?. (safe call), ?: (Elvis operator), and let { } when you truly want to handle a missing value.
A few features compound quickly:
equals(), hashCode(), toString(), and copy() automatically, reducing hand-written code (and inconsistencies) in models.Extension functions let you add utility methods to existing types without modifying them. This encourages small, discoverable helpers (often close to where they’re used) and avoids “Utils” classes full of unrelated functions.
Default arguments eliminate constructor and method overloads that exist only to supply common values. Named parameters make calls self-documenting, especially when multiple arguments share the same type.
Taken together, these features reduce “ceremony” in pull requests. Reviewers spend less time validating repetitive plumbing and more time checking business logic—an advantage that compounds as teams and codebases grow.
Kotlin made code feel more modern while still compiling to standard JVM bytecode and fitting into typical Java-based build and deployment setups.
A major shift is treating functions as values. Instead of writing small, named “listener” classes or verbose anonymous implementations, you can pass behavior directly.
This is especially noticeable in UI and event-driven code: lambdas make intent obvious (“do this when it finishes”) and keep related logic close together, reducing the mental overhead of jumping between files to understand a flow.
Some Kotlin patterns would be expensive or awkward in plain Java without extra plumbing:
parse<T>() or findView<T>()-style helpers without forcing callers to pass Class<T> everywhere.Many apps model “states” such as Loading/Success/Error. In Java, this is often done with enums plus extra fields, or inheritance with no guardrails.
Kotlin’s sealed classes let you define a closed set of possibilities. The payoff is that a when statement can be exhaustive: the compiler can warn you if you forgot to handle a state, preventing subtle UI bugs when new cases are added later.
Kotlin can infer types from context, removing repetitive declarations and making code less noisy. Used well, it improves readability by emphasizing what the code does over how it’s typed.
The balance is to keep types explicit when inference would hide important information—especially at public API boundaries—so the code stays understandable to the next person reading it.
Async work is unavoidable on Android. The UI thread must stay responsive while apps fetch data over the network, read/write storage, decode images, or call location and sensors. Coroutines made that everyday reality feel less like “thread management” and more like straightforward code.
Before coroutines, developers often ended up with callback chains that were hard to read, harder to test, and easy to break when errors happened mid-flow. Coroutines let you write asynchronous logic in a sequential style: do the request, parse the result, update state—while still running off the main thread.
Error handling also becomes more consistent. Instead of splitting success and failure across multiple callbacks, you can use normal try/catch and centralize retries, fallbacks, and logging.
Coroutines aren’t just “lighter threads.” The big shift is structured concurrency: work belongs to a scope, and scopes can be cancelled. On Android that matters because screens and view models have lifecycles—if the user navigates away, the related work should stop.
With scoped coroutines, cancellation propagates automatically, helping prevent wasted work, memory leaks, and “update UI after it’s gone” crashes.
Many Android libraries expose coroutine-friendly APIs: networking, databases, and background work can offer suspend functions or streams of values. Conceptually, that means you can compose operations (fetch → cache → display) without glue code.
Coroutines shine in request/response flows, parallelizing independent tasks, and bridging UI events to background work. Misuse happens when heavy CPU work stays on the main thread, when scopes outlive the UI, or when developers launch “fire-and-forget” jobs without clear ownership or cancellation.
Kotlin didn’t spread on syntax alone—it spread because it felt “native” in the tools developers already used. Strong editor support turns adoption into a series of low-risk steps rather than a disruptive rewrite.
Android Studio and IntelliJ shipped Kotlin support that was more than basic highlighting. Autocomplete understood Kotlin idioms, quick-fixes suggested safer patterns, and navigation worked smoothly across mixed Java/Kotlin projects. Teams could introduce Kotlin file-by-file without slowing day-to-day work.
Two features removed a lot of fear:
The converter isn’t perfect, but it’s great for getting 70–80% of a file migrated quickly, then letting a developer clean up style and nullability with IDE hints.
Many teams also adopted the Gradle Kotlin DSL because it brings autocompletion, safer refactors, and fewer “stringly-typed” mistakes to build scripts. Even if a project keeps Groovy, Kotlin DSL often wins for larger builds where readability and tooling feedback matter.
Tooling maturity showed up in CI: incremental compilation, build caching, and better diagnostics made Kotlin builds predictable at scale. Teams learned to watch compile times, enable caching where appropriate, and keep dependencies tidy to avoid unnecessary recompiles.
Kotlin works cleanly with JUnit and popular mocking libraries, while making tests easier to read (clearer naming, less boilerplate setup). The result is not “different testing,” just faster-to-write tests that are easier to maintain.
Kotlin existed before Google endorsed it, but official Android support changed the decision from “interesting option” to “safe default.” For many teams, that signal mattered as much as any language feature.
Official support meant Kotlin was treated as a first-class citizen in Android’s core workflow: Android Studio templates, Lint checks, build tooling, and platform guidance assumed Kotlin would be used—not merely tolerated.
It also meant clearer documentation. When Android’s own docs and samples show Kotlin by default, teams spend less time translating Java examples or guessing best practices.
Once Kotlin became the recommended path, it stopped being a niche skill. Candidates could point to standard Android docs, official codelabs, and widely used libraries as proof of experience. Companies benefited too: onboarding got easier, reviews became more consistent, and “who knows this language?” stopped being a risk factor.
Android’s endorsement also implied compatibility and long-term support expectations. Kotlin’s evolution emphasized pragmatic change, strong tooling, and backward compatibility where it matters—reducing the fear that a new language version would force a painful rewrite.
Plenty of JVM languages are technically capable, but without platform-level backing they can feel like a bigger bet. Official Android support lowered that uncertainty: clearer upgrade paths, fewer surprises, and confidence that libraries, samples, and tooling would keep pace.
Kotlin didn’t just make Android code nicer—it nudged Android’s APIs and libraries toward being more expressive, safer, and easier to read. As adoption grew, the platform team and library authors increasingly designed with Kotlin’s strengths in mind: extension functions, default parameters, named arguments, and strong type modeling.
Android KTX is essentially a set of Kotlin extensions that make existing Android and Jetpack APIs feel natural in Kotlin.
Instead of verbose patterns (builders, listeners, utility classes), KTX leans on:
The high-level impact is “less scaffolding.” You spend fewer lines setting things up and more lines describing what you actually want the app to do.
Jetpack libraries increasingly assume Kotlin usage—especially in how they expose APIs.
Lifecycle-aware components, navigation, and paging tend to pair well with Kotlin’s language features: concise lambdas, strong typing, and better modeling of “states” and “events.” This doesn’t just reduce boilerplate; it also encourages cleaner app architecture because the libraries reward explicit, well-typed flows of data.
Jetpack Compose is where Kotlin’s influence is most obvious. Compose treats UI as a function of state, and Kotlin is a great fit for that style:
Compose also shifts where complexity lives: away from XML files and view wiring, toward Kotlin code that’s easier to refactor, test, and keep consistent.
Kotlin encourages state-driven UIs with explicit models:
When UI state is modeled this way, you reduce “impossible states,” which is a common source of crashes and weird UI behavior.
With KTX + Jetpack + Compose, Kotlin pushes Android development toward declarative, state-driven UI and library-guided architecture. The result is less glue code, fewer edge-case nulls, and UI code that reads more like a description of the screen than a set of instructions for wiring it together.
Kotlin didn’t stop at making Android apps nicer to write. It also strengthened the wider JVM ecosystem by giving teams a modern language that still runs anywhere Java runs—servers, desktop apps, and build tools—without forcing a “rewrite the world” moment.
On the JVM, Kotlin is often used for backend services alongside Java libraries and frameworks. For many teams, the organizational win is significant: you can standardize on one language across Android and server code, share conventions, and reuse skills—while continuing to rely on the mature Java ecosystem.
Kotlin Multiplatform lets you write certain parts of an app once and use them in multiple targets (Android, iOS, desktop, web), while still building a native app for each platform.
Think of it as sharing the “brain” of the app—not the entire app. Your UI stays native (Android UI on Android, iOS UI on iOS), but shared code can cover:
Because Android already runs on the JVM, KMP can feel like a natural extension: you keep JVM-friendly code where it makes sense, and only branch where platforms truly differ.
KMP can save time, but it adds complexity:
KMP is a good fit if you have parallel Android + iOS apps, shared product rules, and a team willing to invest in shared architecture. Stay Android-only if your roadmap is Android-first, your app is UI-heavy with little shared logic, or you need a broad set of platform-specific libraries immediately.
Kotlin is a big productivity win, but it isn’t “free.” Knowing where the sharp edges are helps you keep code readable, fast, and easy to maintain—especially during a Java-to-Kotlin transition.
In most apps, Kotlin performance is comparable to Java because it compiles to JVM bytecode and uses the same runtime. Differences tend to come from how you write Kotlin:
Rule of thumb: write idiomatic Kotlin, then measure. If something is slow, optimize the specific bottleneck rather than “avoiding Kotlin.”
Kotlin encourages concise code, which can tempt teams into “puzzle Kotlin.” Two common issues:
let, run, apply, also, with) until control flow becomes hard to follow.Prefer clarity: break complex expressions into named variables and small functions.
Interop is great, but watch for:
@Nullable/@NonNull) or wrap unsafe calls.@Throws when exposing Kotlin to Java callers.Migrate incrementally:
Agree early on style and review norms: when to use scope functions, naming conventions, null-handling patterns, and when to prefer explicit types. A short internal guide plus a few training sessions will save months of churn.
If you’re coordinating a migration across multiple repos or squads, it can help to standardize on a lightweight “planning mode” workflow (migration checklist, module boundaries, rollback steps). Teams that want a more guided approach sometimes use platforms like Koder.ai to draft implementation plans, generate scaffolding for related services (often a web dashboard in React or a backend in Go + PostgreSQL), and keep snapshots/rollback points while iterating—without forcing a full pipeline overhaul.
Kotlin won Android not by replacing the JVM world, but by making it feel modern without forcing a clean break. Teams could keep their existing Java code, Gradle builds, and library stack—then steadily add Kotlin where it delivered immediate value.
Start small and keep the experiment measurable:
If you want more practical guides and migration stories, browse /blog. If you’re evaluating tooling or support for teams adopting Kotlin at scale, see /pricing.
Kotlin raised the developer experience baseline on the JVM by removing common boilerplate (e.g., data classes, properties, smart casts) and adding safer defaults like null-safety—while still compiling to standard JVM bytecode and using the same Java libraries and tooling.
Because it’s interoperable with Java at the source and bytecode level. Teams can introduce Kotlin file-by-file, keep existing libraries and Gradle builds, and avoid a high-risk “big rewrite.”
Common friction points include:
String!) where Java nullability is unknown@Throws for Java callers)It splits types into nullable (T?) and non-null (T) and forces you to handle missing values explicitly. Practical tools include:
?. safe calls?: (Elvis) defaults/fallbackslet {} for scoped handlingThis shifts many crashes from runtime into compile-time feedback.
Yes—often significantly. Use data classes for models and UI state because they generate equals(), hashCode(), toString(), and copy() automatically. That reduces hand-written code and makes state updates more explicit and consistent.
They let you add functions/properties to existing types (including Java/Android classes) without modifying those classes. This encourages small, discoverable helpers and avoids oversized “Utils” classes—especially when paired with Android KTX extensions.
Coroutines let you write async code in a sequential style using suspend functions, with normal try/catch error handling. The bigger win is structured concurrency: work runs in a scope, cancellation propagates, and lifecycle-aware cancellation helps prevent leaks and “update UI after it’s gone” bugs.
Most teams feel Kotlin improves readability, but compile times can increase. Mitigations typically include:
Prefer readability over cleverness. Common traps include:
let/run/apply/also/with) until control flow is unclearWhen in doubt, split expressions, name intermediate values, and measure performance before optimizing.
A practical approach is:
This keeps risk low while building Kotlin fluency across the team.