Mobile
Kotlin
Null-safe, concise JVM engineering — the default for Android — built by engineers who run what they ship.
Overview
Kotlin is a modern, statically typed language from JetBrains that runs on the Java Virtual Machine and interoperates completely with Java. Its headline feature is null safety baked into the type system: a variable is either allowed to be null or it is not, and the compiler enforces the distinction. That single decision eliminates a whole class of NullPointerExceptions — the single most common crash in Java systems — not by catching them in testing but by refusing to compile code that could produce them. On top of that, Kotlin is markedly more concise than Java: data classes, extension functions, smart casts, and type inference remove the ceremonial boilerplate that Java makes you write by hand, so the code you read is the code that matters.
Since 2019, Kotlin has been Google’s preferred language for Android, and for new Android work it is now simply the default — the tooling, the official libraries, and the documentation all assume it. But Kotlin is not an Android-only language. It is a strong server-side choice too: Spring supports Kotlin as a first-class option, Ktor is JetBrains’ own lightweight asynchronous framework, and Kotlin’s coroutines give you structured, readable concurrency without the callback tangle that async code usually descends into. Kotlin Multiplatform pushes this further, letting you share business logic — networking, validation, data models — across Android, iOS, and the backend while keeping each platform’s UI native.
We reach for Kotlin when you are building for Android, when you want a JVM backend that is safer and terser than Java without leaving the JVM ecosystem, or when you have an existing Java codebase you want to modernise incrementally. Because Kotlin and Java compile to the same bytecode and call each other freely, you do not rewrite anything — you add Kotlin file by file and let the two coexist. We are equally clear about where the choice barely matters: on a mature Java team with no Android work and no appetite for a second language, the day-to-day gains are real but modest, and Java remains a perfectly defensible answer.
Best for — Native Android apps, and JVM backends that want Java’s ecosystem with null safety, coroutines, and far less boilerplate — including Java codebases being modernised one file at a time.
Why teams choose Kotlin
NullPointerExceptions ruled out at compile time
Kotlin’s type system distinguishes nullable from non-null references and forces you to handle the difference before the code compiles. The most common crash in Java systems is not caught in testing — it simply cannot be written. That is a measurable drop in production incidents, not a stylistic preference.
Less code to read, review, and maintain
Data classes, extension functions, smart casts, and type inference remove the boilerplate Java demands. A model class that is forty lines of Java is one line of Kotlin. Less code means fewer places for bugs to hide and less to keep in your head during review.
Adopt it without a rewrite
Because Kotlin and Java share the same bytecode and call each other freely, you add Kotlin to an existing Java project incrementally — a new feature here, a converted class there — with no big-bang migration and no throwaway rewrite. The risk of adoption is close to zero.
Why businesses choose Kotlin
- You are building for Android and want to be on the platform’s default, best-supported language rather than fighting the tooling.
- You want fewer production crashes and less boilerplate on the JVM without abandoning Java’s libraries, runtime, or hiring pool.
- You have Java you want to improve gradually, and you value a migration path with no rewrite and no flag day.
- You want the option to share domain logic across Android, iOS, and your backend from one codebase as your product grows.
What we build with Kotlin
The capabilities this technology is genuinely strong at — and what we most often build with it.
Null safety and the type system
Nullable and non-null types are distinct at compile time, and the safe-call (?.), Elvis (?:), and smart-cast operators make handling absence explicit and terse. We use this deliberately — modelling what genuinely can be null rather than defaulting everything to nullable — so the compiler carries the weight that runtime checks carry in Java.
Coroutines for structured concurrency
Kotlin’s coroutines let you write asynchronous and concurrent code that reads top to bottom, with structured concurrency ensuring child jobs are cancelled and cleaned up with their parent. No callback nesting, no manual thread pools for I/O-bound work — and on the server this is how we keep high-concurrency services both fast and readable.
Data classes and immutability
A single data class declaration generates equals, hashCode, toString, and copy, making value objects and DTOs trivial. Combined with val and read-only collections, this makes immutable-by-default design the path of least resistance — which is exactly how we prefer to model domain state.
Extension functions
Kotlin lets you add methods to existing types — including ones you do not own — without inheritance or wrapper classes. Used with restraint this keeps call sites clean and domain-specific; used carelessly it hides logic in surprising places, so we treat it as a scalpel, not a default.
Jetpack Compose for Android UI
Compose is the modern, Kotlin-first, declarative UI toolkit for Android, replacing XML layouts with composable functions and state-driven rendering. We build new Android UIs in Compose, using its state model and effect handling correctly rather than porting old imperative patterns into it.
Kotlin Multiplatform for shared logic
KMP compiles shared Kotlin to run on Android, iOS, and the JVM backend, so networking, validation, and domain models live in one place while each platform keeps a native UI. We use it for the logic layer where the payoff is clear, and are honest about where the shared-UI story is not yet worth it.
Use cases
Native Android applications
Consumer and enterprise Android apps built in Kotlin with Jetpack Compose, from the data layer up — the platform’s default stack, done by people who know its lifecycle and threading sharp edges.
JVM backend services
APIs and services on Spring or Ktor, using coroutines for high-concurrency I/O, with null safety cutting the crash rate you would carry in an equivalent Java service.
Modernising a Java codebase
Introducing Kotlin incrementally into an existing Java project — new features and tests in Kotlin, converting hot spots as we go — with no rewrite and no interruption to delivery.
Shared logic across platforms
A Kotlin Multiplatform module holding the domain, networking, and validation logic shared by an Android app, an iOS app, and the backend, so a rule is written and fixed once.
When Kotlin is the right choice
- You are building a native Android application. Kotlin is Google’s recommended language and the default for new Android work — Jetpack Compose, the modern Android UI toolkit, is Kotlin-first, and the entire ecosystem now assumes it. Choosing Java for greenfield Android would be swimming against the current for no benefit.
- You want a JVM backend that keeps Java’s ecosystem — the libraries, the JVM’s mature runtime and observability, the deep hiring pool — while writing safer, shorter code. Spring and Ktor both run Kotlin well, and coroutines make high-concurrency services readable.
- You have an existing Java codebase and want to modernise without a rewrite. Kotlin’s complete interoperability lets you introduce it incrementally: new features and tests in Kotlin, old code untouched, the two calling each other in the same project until you are ready to convert more.
- You want to share logic across Android, iOS, and a server. Kotlin Multiplatform lets you write networking, validation, and domain models once and run them everywhere, while each platform keeps a fully native UI — a genuine middle ground between a shared codebase and duplicated logic.
- Wrong for: a settled Java team with no Android work, no plans for one, and no wish to run a second language on the JVM. The benefits are real but incremental, and the cost of a mixed-language codebase, a smaller talent pool, and sometimes longer compile times may not repay itself. It is also the wrong tool where the JVM itself is a poor fit — very short-lived serverless functions penalised by JVM startup, or hard real-time systems that cannot tolerate garbage-collection pauses.
Kotlin: pros and cons
Strengths
- Null safety built into the type system eliminates a whole category of NullPointerException crashes before the program runs.
- Concise, expressive syntax — data classes, extension functions, coroutines — removes Java’s boilerplate without sacrificing the JVM ecosystem.
- Complete two-way interoperability with Java means you can adopt Kotlin incrementally in an existing codebase and keep every Java library you already use.
- It is the default for Android and a first-class server language, so one language and one team can cover Android, backend, and shared Multiplatform logic.
Trade-offs
- The ecosystem and hiring pool are smaller than Java’s — growing quickly, and every Java developer can pick Kotlin up, but the pure-Kotlin talent market is still narrower.
- Compile times can be longer than equivalent Java, particularly on large modules and clean builds, which is felt most in day-to-day Android development.
- Kotlin Multiplatform is production-ready for shared logic but still maturing at the edges — tooling, library support, and especially shared-UI stories are less settled than single-platform native.
- On the JVM you inherit the JVM: startup time and memory footprint that make it a poor fit for very short-lived serverless functions or tightly memory-constrained environments.
Architecture
On Android we build in clear layers — UI, presentation, domain, and data — with unidirectional state flow into Compose and coroutines carrying work off the main thread. We keep the domain layer plain Kotlin with no Android dependencies, which makes it testable in isolation and, when it matters, liftable into a Multiplatform module later. Dependency injection, a repository boundary over data sources, and immutable state objects are the load-bearing decisions we settle early, because retrofitting them into a screen-first codebase is where Android projects calcify.
On the server the same discipline applies: coroutines and suspending functions for I/O, a clean boundary between web layer and domain, and null safety used to make illegal states unrepresentable rather than defended against at runtime. When Kotlin Multiplatform is in play, we draw the shared boundary deliberately — logic that is genuinely platform-agnostic goes in the shared module, and anything touching a platform API stays native. Drawing that line well is the whole game with KMP; drawn badly, the shared module leaks platform concerns and stops being shareable.
Performance
Kotlin runs on the JVM, so at steady state its performance is Java’s — the same just-in-time compilation, the same mature garbage collector, the same runtime characteristics. The abstractions that make Kotlin pleasant are largely zero-cost or compiler-optimised: inline functions remove lambda overhead on hot paths, and coroutines are far cheaper than threads for concurrent I/O. We profile with the same JVM tooling we would use for Java and treat coroutine dispatchers, not raw threads, as the lever for I/O-bound throughput.
The honest costs are at the edges. Compile times can be longer than Java, which is a developer-experience tax rather than a runtime one, though incremental compilation and build tuning keep it manageable. And you inherit the JVM’s startup time and memory footprint — fine for long-running Android apps and services, poor for very short-lived serverless functions where the runtime never warms up. Where that matters, we say so before you build, rather than tuning around a fundamentally wrong fit.
Security
Kotlin inherits the JVM’s mature security posture and the same hardened libraries the Java ecosystem has refined for two decades — we are not trading a battle-tested runtime for novelty. Null safety removes an entire class of defensive checks and the crashes that come from missing them, which is a reliability and a security benefit, since crashes are often where error handling and, with it, access control quietly break down. Beyond the language, the discipline is ordinary and unglamorous: validate input at the boundary, keep secrets out of the codebase and out of the client, and enforce authorisation on the server rather than in the app.
On Android specifically, we treat the device as untrusted: no secrets baked into the APK, sensitive data stored through the platform’s encrypted storage and keystore, and network traffic pinned and encrypted. We keep the dependency tree lean and current on both Android and server, because every third-party library is attack surface, and we watch for known vulnerabilities as a standing task rather than a one-off audit.
Scalability
On the server, Kotlin scales the way any well-built JVM service scales: coroutines let a single instance handle large numbers of concurrent I/O-bound requests without a thread per request, and the standard JVM horizontal-scaling and observability story applies unchanged. The language does not change the shape of the problem — your database, caches, and service boundaries still decide how far you go — but coroutines make high-concurrency code you can actually read, which matters when the system needs to grow and be understood by more people.
Scalability of the codebase and team matters just as much. Null safety, immutability by default, and concise, expressive code keep a growing Kotlin codebase coherent as engineers join, and the plain-Kotlin domain layer we insist on stays testable at any size. On Android, the same layered structure is what lets a feature-heavy app keep shipping without every change rippling across screens — and it is what makes a later move to Kotlin Multiplatform an addition rather than a rewrite.
Kotlin integrations & ecosystem
The technologies we most often pair with it — each links to how we work with it.
How we work
On a new project we settle the structure first — layers, state flow, the domain boundary, and whether Multiplatform is in scope — then build in thin vertical slices: one real feature, end to end, in the hands of users or on a device, rather than a scaffold that demos well and does nothing. On an existing Java codebase we start by proving the interop path with a small, real conversion, agree where Kotlin adds value and where Java should stay, and let the two languages coexist rather than forcing a rewrite nobody asked for.
The engineers who design the system are the ones who write it and keep it running. That is what we mean by operating what we build: the person choosing coroutines over threads, or Compose state patterns, or where the KMP boundary sits, is the one who will live with that choice at eighteen months. So we favour boring, durable decisions over whatever is new, we are blunt about where Kotlin’s gains are marginal, and we hand you typed, tested code your own team can extend.
The service behind it
Delivered throughMobile App DevelopmentWhat we build with Kotlin
The disciplines this technology most often shows up in — from a first build to taking over and stabilising an existing one.
How we deliver
- 01
Discover
We map the system, the constraints and the business it serves — including the parts nobody documented.
Architecture brief
- 02
Architect
Decisions get made, written down and defended before a line of production code exists.
Decision records
- 03
Build
Short cycles against working software. You see progress in the product, not in a status deck.
Shipping increments
- 04
Operate
Monitoring, incident response and iteration. The system is alive, so the engagement is too.
Runbooks & SLOs
Industries we use Kotlin in
Domain knowledge changes what gets built. A few of the sectors we know before the first meeting.
Also in Mobile
Why teams choose us for Kotlin
Senior engineers only
Kotlin’s sharp edges are in the details — coroutine cancellation, Compose recomposition, the KMP boundary, Android lifecycle and threading. The people making those calls on your project have made them before and seen how they age. There are no juniors learning on your codebase.
We operate what we build
We run the systems we ship, so we design for the eighteen-month maintenance reality: null safety used deliberately, immutable state, a testable domain layer, and conservative library choices your team can actually maintain — not the demo-friendly version.
Honest about Kotlin versus Java
For new Android work Kotlin is the clear default and we will say so. For a settled Java team with no Android work, we will tell you when the gains are marginal and Java remains the right answer, rather than selling you a migration you do not need.
Typical timeline
- 01
Discovery and architecture
One to two weeks agreeing platform targets, the layer structure, state and concurrency approach, and — where relevant — the interop or Multiplatform boundary, so the foundations are settled before code volume grows.
- 02
First vertical slice
Two to three weeks delivering one real feature end to end, on a device or in production, proving the architecture and the toolchain against reality rather than a plan.
- 03
Iterative build
Feature-by-feature delivery in short cycles, each shippable, with null-safety discipline, tests on the load-bearing paths, and performance checked as we go rather than bolted on later.
- 04
Hardening and handover
Final profiling, crash and error monitoring wired in, coverage on the critical paths, and documentation so your team can own and extend the codebase.
How pricing works
- Fixed-scope builds for well-defined deliverables — an Android app, a specific backend service — quoted once we understand the platform targets, the data, and the integrations.
- Monthly senior engagement for ongoing product work, where scope evolves and you want continuity across Android and backend rather than a single fixed deliverable.
- Focused audits and modernisation for existing Java or Kotlin codebases — a crash-rate problem, an incremental Kotlin migration, or a stalled Android project — priced by the assessment.
Hire Kotlin engineers
Need Kotlin capacity on your own team? We embed named senior engineers into your existing team — reporting to your leads, working in your rituals — so you add capacity without a hiring cycle.
Hire Kotlin engineersCommon questions
Should we choose Kotlin or Java?
For new Android work, Kotlin — it is Google’s default, and the modern tooling and libraries assume it. For a JVM backend it depends on your team: Kotlin gives you null safety, coroutines, and far less boilerplate while keeping Java’s entire ecosystem, which is a real win on a new service or a team open to it. For a settled Java team with no Android work and no appetite for a second language, the day-to-day gains are modest, and Java stays a perfectly defensible choice. We will tell you honestly which side of that line your project falls on.
Can we add Kotlin to our existing Java codebase without rewriting it?
Yes — this is one of Kotlin’s strongest points. Kotlin and Java compile to the same bytecode and call each other freely, so you introduce Kotlin incrementally: new features and tests in Kotlin, existing Java untouched, the two coexisting in one project. There is no big-bang migration and no throwaway rewrite. You convert more over time only where it pays off, and you keep every Java library you already depend on.
What actually is null safety, and why does it matter?
Kotlin’s type system distinguishes references that can be null from those that cannot, and the compiler forces you to handle the difference before the code will build. In practice this rules out NullPointerExceptions — the single most common crash in Java systems — at compile time rather than in testing or production. It is not a lint rule you can ignore; it is enforced by the language, and it measurably lowers the crash rate of the systems we ship.
Is Kotlin Multiplatform ready for production?
For sharing logic — networking, validation, domain models — across Android, iOS, and a backend, yes: teams run KMP in production today and it is a genuine middle ground between duplicated logic and a fully shared codebase. It is still maturing at the edges, particularly the shared-UI story and some library support, so we use it for the logic layer where the payoff is clear and keep each platform’s UI native. We will be honest about whether your specific case is one where it earns its cost.
What are coroutines, and do we need them?
Coroutines are Kotlin’s way of writing asynchronous and concurrent code that reads top to bottom instead of as nested callbacks, with structured concurrency ensuring work is cancelled and cleaned up correctly. On Android they keep the main thread free without the old callback tangle; on the server they let one instance handle many concurrent I/O-bound requests without a thread per request. If your app does network calls, database access, or any concurrency — which is almost all of them — you will use them, and using them correctly is part of what we bring.
Building on Kotlin?
A technical conversation with the engineers who would do the work. If we are not the right fit, we will say so on the call.