Java Development Services: Is Java Still Relevant for Android Apps?

Java remains relevant for Android applications, as evidenced by its use in many mature applications containing hundreds of thousands of lines of code. Despite the emergence of Kotlin as a strong default for new application code, Java continues to have a significant role. The decision to use one language over another depends on various factors, such as existing components, public APIs, and computational work.

Java’s legacy as an architectural reality should not be overlooked, as it still plays a crucial part in enterprise applications, vendor SDKs, device integrations, and shared JVM libraries. The argument that Java is dead is misleading, as it oversimplifies the decision-making process. When considering a rewrite from Java to Kotlin, it is essential to evaluate whether the existing Java code has technical debt that needs to be addressed.

Technical debt refers to issues like poor boundaries, unsafe assumptions, missing tests, and costly changes, which a Kotlin rewrite may preserve. Existing Java code should not automatically be considered technical debt. Google's Android guidance emphasizes Kotlin, and Jetpack Compose is built around Kotlin. While this makes Kotlin a practical default for new application development, especially when adopting Compose, coroutine-based APIs, and Kotlin-oriented libraries, Java still works in Android projects.

Kotlin-first ecosystems do not require immediate conversion of every Java module. The key question is which parts of the system benefit from changing language—and which parts deserve stability. When deciding between Java and Kotlin, it is crucial to consider architectural implications, such as nullability, asynchronous work, and data models.

Kotlin expresses nullability directly, but it can still fail with null-related errors if unsafe assertions or unannotated Java APIs are present. Coroutines in Kotlin improve composition, allowing readable asynchronous code and structured concurrency. However, they do not automatically make blocking work non-blocking. Java can also implement reliable asynchronous systems, albeit with additional coordination code.

Java can remain the better engineering choice in certain scenarios, such as maintaining stable enterprise components with extensive tests and few defects. Retaining Java in these cases can be appropriate when the component is stable, its responsibilities are clear, dependencies are supported, and the team understands its behavior.

Migration may not always provide measurable benefits, so prioritizing testing, modularity, security, and dependency maintenance should be considered. Existing integration contracts and boundaries can also necessitate the use of Java. Examples include vendor SDK adapters, established JNI facades, public library interfaces, legacy serialization models, and generated source code.

The important property is identifying where Java remains the better choice and avoiding confusing modernization with a complete rewrite.