Kotlin in plain terms
Kotlin is a modern, statically typed language. That means the compiler checks at build time whether the code is trying to, say, add a number to a string or access data that might not exist. Errors like these never reach users.
The language runs on the Java Virtual Machine, so it has access to every library written for Java over the decades. Kotlin can also compile to native code for iOS and to JavaScript, and that's the foundation of the Kotlin Multiplatform approach.
Where Kotlin is used
- Android apps. The main use case: Google's newer tools, including Jetpack Compose, are designed primarily for Kotlin.
- Backend. APIs and services are built in Kotlin with the Ktor and Spring Boot frameworks.
- Shared code for mobile platforms. Business logic is written once and used on both Android and iOS, while each platform keeps a native UI.
- Internal tools. Build scripts, developer utilities, small desktop apps.
Advantages of the language
- Less code. What takes a dozen lines in Java often fits into a few in Kotlin. The code is easier to read and review.
- Null safety. The language forces you to declare explicitly where data may be missing. This eliminates one of the most common causes of app crashes.
- Java interoperability. Nothing has to be thrown away: an old Java library or module keeps working alongside new code.
- Coroutines. Data loading, server requests, and other long-running operations are simple to write, without tangled callback chains.
- Backed by Google and JetBrains. The language is actively developed, and Android documentation shows Kotlin examples first.
Drawbacks and limitations
- Slower builds. In large projects, Kotlin compilation can take longer than equivalent Java code. This affects developer productivity, not users.
- Many ways to do the same thing. Without team conventions, the language's flexibility turns into inconsistent styles. That's why we set up automated code checks from the start.
- Kotlin Multiplatform has a younger ecosystem. Some libraries aren't available for it yet, and iOS developers need time to get used to a shared module.
- A smaller backend ecosystem. Compared with Java, PHP, or Node.js, there are fewer ready-made solutions and specialists, so choosing Kotlin for the backend should be a deliberate decision.
When another tool is a better fit
Kotlin isn't the answer to everything. We'll honestly recommend something else if:
- you need a single codebase for both logic and UI on iOS and Android. In that case Flutter or React Native is simpler;
- a large, stable Java project is already running on the server. Rewriting it in Kotlin just to follow a trend makes no sense; new modules can be added gradually;
- your team writes JavaScript or PHP and doesn't plan to learn a new language. For the backend, it's better to stay on the stack you know.
Migrating from Java to Kotlin
Rewriting an entire app in one go is risky and expensive. We migrate projects gradually:
- all new code is written in Kotlin from the start;
- parts that change often or break often are rewritten first;
- stable legacy modules stay in Java until their turn comes;
- every step is covered by tests so the app's behavior doesn't change.
Android Studio can convert Java files to Kotlin automatically, but the result almost always needs manual refinement. Mechanically converted code works, but it doesn't take advantage of the language.
Kotlin Multiplatform: shared logic for Android and iOS
This approach makes sense when you already have a strong native Android app and need an iOS version. The shared module holds everything that doesn't depend on the screen: server communication, data models, calculations, validation rules.
Each platform still builds its own UI: Jetpack Compose on Android and SwiftUI on iOS. Users get apps that feel native to their platform, while prices, discounts, and validations work the same everywhere because they're written once.
If you're starting from scratch and both platforms matter equally, we'll compare this option with cross-platform development and recommend the one that's cheaper to maintain.
New project or improving an existing one
In a new project, we lay down a structure that's easy to grow from day one: separate layers for UI, logic, and data, automated tests for key scenarios, and code style checks on every change. That way the app won't need a rewrite a year or two down the road.
With an existing project, we start with an audit. We look at how background tasks are organized, whether there are memory leaks, which libraries are no longer maintained, and where the app crashes most often. You get a clear, prioritized list of issues and a recommendation on what to fix first.
What affects cost
- Type of work: a new app, migration from Java, a shared iOS module, or backend code.
- Size and condition of existing code: how many modules, whether there are tests, how outdated the libraries are.
- Integrations: backend, payments, notifications, third-party SDKs, and hardware.
- Quality requirements: test coverage, documentation, automated checks in the build pipeline.
After an audit or requirements review, we break the work into phases and explain what each one includes.
Frequently asked questions
Will the app run faster after switching to Kotlin? The language itself won't make the app noticeably faster for users. The gains are stability, fewer crashes, and faster development of new features.
Can part of the code stay in Java? Yes. Both languages work fine in the same project, so there's no need to convert everything.
Is Kotlin Multiplatform a good fit for a new product? Yes, if a native UI on each platform matters to you. If the priority is one codebase for everything, a cross-platform framework is usually the simpler choice.
Will you take over support for an app built by another team? Yes. Before changing anything, we review the code and identify which parts are fragile and which ones are solid.
Why Promodex
Since 2015, Promodex has built a portfolio of 750+ projects, including mobile work such as the Telcell Wallet app. Our designers and developers are an in-house team, so we don't outsource work or lose context between phases. We explain technical decisions in plain language, and after release we keep supporting and evolving the app.
