Kotlin vs Java for Android: Which Should Your Company Choose?

Updated September 2026

For new Android projects, Kotlin is the language recommended by Google and adopted by most of the market. Java still makes sense for large legacy apps and for teams with deep expertise in the language. According to Google, over 60% of professional Android developers already use Kotlin, and 95% of the top 1,000 apps on Google Play contain Kotlin code.

The decision, however, goes beyond technical preference. The language you choose affects app stability, maintenance costs, delivery speed and how easily you can build your team. In this article, you will see the differences between Kotlin and Java, when each one is the best option and how to migrate a Java app to Kotlin without rewriting everything.

Kotlin vs Java: the short answer

The choice between Kotlin and Java depends on where your project starts. In short:

• New app: Kotlin. It is Google’s priority language for Android, requires less code to maintain and is mandatory for using Jetpack Compose, the modern UI toolkit.
• Existing, large and stable Java app: keep Java and adopt Kotlin gradually, starting with new features.
• Enterprise back end on the JVM: both languages work well (including with Spring); your stack and your team’s experience should decide.

What is Kotlin and what is Java?

Java

Java was released in 1995 and became one of the most widely used languages in the enterprise world. For many years, it was the main language for Android. It runs on the JVM (Java Virtual Machine) and has a huge library ecosystem and a mature community, which explains its presence in banking systems, ERPs and mission-critical applications.

Kotlin

Kotlin was created by JetBrains and introduced in 2011. In 2017, Google began officially supporting it on Android, and in May 2019 it announced Kotlin as the preferred language for Android development. It also runs on the JVM and is fully interoperable with Java: a single project can have classes in both languages calling each other without issues.

Kotlin vs Java: comparison table

The main difference between Kotlin and Java lies in code safety and conciseness; in runtime performance, the two languages are equivalent.

In practice, Kotlin delivers the same performance as Java with less code and a lower risk of failures; Java makes up for it with maturity and the sheer number of existing professionals and systems.

Advantages of Kotlin for Android apps

Null safety and fewer crashes

Kotlin reduces app crashes because it prevents, at compile time, a variable from receiving a null value without handling it. This feature is called null safety. It directly tackles the NullPointerException, which is, according to Google, the leading cause of crashes on Google Play.

var email: String = "[email protected]"
email = null // compile error: the code never even runs

var phone: String? = null // the "?" means the value can be null
val length = phone?.length ?: 0 // handling is mandatory and explicit

The results show up in the numbers. Among the top 1,000 apps on Google Play, those that use Kotlin have 20% fewer crashes per user. The Google Home team, for example, saw a 33% decrease in NullPointerException crashes over one year after adopting the language (Android Developers).

For a company, fewer crashes mean fewer negative store reviews, fewer support tickets and fewer hours spent on fixes. The language helps, but final quality depends on process: in NextAge’s software projects with a dedicated squad, all code goes through AI-assisted code review before reaching the pipeline, preventing failures from reaching users.

Less code, less maintenance

Kotlin lets you express the same logic in far fewer lines. Compare a simple class representing a customer:

// Java
public class Customer {
    private final String name;
    private final String email;

    public Customer(String name, String email) {
        this.name = name;
        this.email = email;
    }

    public String getName() { return name; }
    public String getEmail() { return email; }

    // equals(), hashCode() and toString() still need to be implemented
}

// Kotlin
data class Customer(val name: String, val email: String)

The Kotlin version already includes equals, hashCode, toString and object copying. Less code means fewer points of failure, faster reviews and cheaper maintenance over the product’s lifetime. The Cash App team, for example, cut up to 25% of their code in some cases, and 67% of Android developers say Kotlin has increased their productivity (Android Developers).

Coroutines: simpler asynchronous operations

Coroutines are Kotlin’s feature for running long tasks (such as API calls or database reads) without freezing the app’s interface. In Java, the same result requires callbacks, threads or additional libraries, which makes code harder to read and test. With coroutines, asynchronous flows are written sequentially, reducing errors in critical areas such as login, payments and data synchronization.

Jetpack Compose: modern interfaces, Kotlin only

Jetpack Compose is Google’s official, modern toolkit for building Android interfaces declaratively. It works exclusively with Kotlin. This means an app that stays 100% in Java is stuck with the old XML layout system and falls further behind the new features Google releases for Android.

When Java is still the best choice?

Java remains a good choice when a company already has a large, stable app written in Java and there is no clear business reason to change. Rewriting a system that works consumes budget and brings risks that do not always pay off.

Some scenarios where keeping Java makes sense:

• Large, critical codebase: apps with hundreds of thousands of lines, tested and in production for years.
• Senior Java team: in the short term, an experienced team’s productivity may outweigh the gains of switching languages.
• Integration with enterprise systems: internal libraries, partner SDKs and modules shared with a back end written in Java.
• Tight deadline: an urgent delivery is not the time to introduce a new language to the team.

Even in these cases, remember that Kotlin and Java coexist in the same project. Keeping legacy code in Java does not prevent new features from being written in Kotlin.

Kotlin or Java: how to decide for your project

The best way to decide between Kotlin and Java is to start from the project’s context, not the team’s preference. These questions help guide the choice:

1. Is the app new or does it already exist? If it is new, Kotlin is the natural path.
2. How large and healthy is the current Java code? Well-structured, tested code allows gradual migration; fragile code should be stabilized first.
3. Does the team know Kotlin? If not, consider training or reinforcement with experienced professionals.
4. Will the app use Jetpack Compose? If so, Kotlin is mandatory for that layer.
5. Is there a plan to launch an iOS version? Kotlin Multiplatform lets you share business logic across both platforms.
6. What is the product’s time horizon? Apps that will evolve for many years benefit most from the lower maintenance Kotlin offers.

If the decision is still open, the scope planning stage of NextAge’s Software Projects maps the technical requirements, defines the stack and structures the backlog before the first sprint, so the language choice is based on business needs rather than assumptions.

How to migrate from Java to Kotlin without rewriting your app

Migrating from Java to Kotlin does not require rewriting your app. Thanks to interoperability between the languages, migration can be done in stages, with the app running normally throughout the process.

A safe path:

1. Ensure good test coverage. Tests are the safety net that confirms the app’s behavior has not changed after each conversion.
2. Write new features in Kotlin. This is the cheapest way to start, since it does not touch what already works.
3. Convert simple classes first. Data models and utility classes are good candidates. Android Studio includes an automatic Java to Kotlin converter.
4. Review the converted code. Automatic conversion produces Kotlin with a Java structure, without taking advantage of features such as null safety and data classes. Every converted file needs a human review.
5. Migrate module by module, by business priority. Start with the areas that change most often or generate the most crashes.
6. Adopt Jetpack Compose on new screens. Compose coexists with XML screens, so the interface can also be modernized gradually.

The main risks of a poorly managed migration are longer build times, a temporary drop in team productivity and bugs introduced by unreviewed conversions. All of them can be controlled with planning and professionals who master both languages.

This is exactly the kind of project where a dedicated team makes a difference. Modernizing a Java app requires module-by-module planning, continuous testing and developers experienced in both Kotlin and Java, which is hard to balance with the internal team’s backlog. NextAge builds a dedicated squad for app development and modernization projects, with scope, timeline and SLA defined in the contract, plus guaranteed replacement of professionals so the project never stops.

Kotlin in 2026: Compose, Multiplatform and what comes next

In 2026, Kotlin is no longer just the Android language; it has become a multiplatform development option. The latest stable version, Kotlin 2.4.0, was released in June 2026, and Kotlin Multiplatform has been stable since November 2023.

Kotlin Multiplatform lets you share business logic (rules, validations, data access) across Android, iOS, desktop and server, while keeping native interfaces on each platform. For companies that maintain apps in both stores, this can reduce code duplication and the cost of keeping business rules in sync across two teams.

Combined with Jetpack Compose and Google’s ongoing investment, everything indicates that Kotlin will remain the standard for the Android ecosystem in the coming years, while Java stays strong in back-end and enterprise systems.

Frequently asked questions about Kotlin and Java

Will Kotlin replace Java on Android?

In practice, it already has for new projects: Google has treated Android as Kotlin-first since 2019. Java, however, is still supported and present in many existing apps, and there is no indication that it will stop working on the platform.

Which is faster, Kotlin or Java?

At runtime, performance is equivalent, because both languages compile to JVM bytecode. The difference shows up in build time, which has historically been longer in Kotlin but has been decreasing with the K2 compiler.

Can I mix Kotlin and Java in the same project?

Yes. Kotlin and Java are fully interoperable: classes in both languages can coexist in the same project and call each other. This is what makes a gradual migration possible, without rewriting the app.

Is it worth migrating a Java app to Kotlin?

It is worth it for most apps that will keep evolving, especially to reduce crashes and maintenance costs. The migration should be gradual, starting with new features and the most critical modules, always backed by tests and a review of the converted code.

Does Jetpack Compose work with Java?

No. Jetpack Compose works exclusively with Kotlin. A Java app can keep using the XML layout system, but it needs Kotlin to adopt Compose, even if only for new screens.

Is Kotlin harder to learn than Java?

No. For developers who already know Java, the transition to Kotlin is usually quick, since the concepts are the same and the syntax is leaner. The main challenge is learning to make good use of features such as null safety and coroutines.

Which language is better for enterprise apps?

For new enterprise Android apps, Kotlin is the best choice, as it offers stronger protection against failures and lower maintenance costs. For enterprise systems already consolidated in Java, the best strategy is usually to keep the legacy code and adopt Kotlin gradually.

Conclusion

Kotlin vs Java is no longer an even contest on Android: for new projects, Kotlin is the safest, most productive choice and the one most aligned with the platform’s future. Java remains relevant in legacy systems and on the back end, and both languages can coexist in the same project while the migration takes place.

What defines an app’s success, however, is execution: well-defined scope, reviewed code and a team that delivers on time. With over 19 years in the market and more than 600 projects delivered, NextAge builds dedicated squads to create or modernize Android apps with a guaranteed SLA and AI-validated quality.

Have an Android app to build or modernize? Talk to a NextAge specialist, at no cost and with no commitment.