Guide to Migrating from Java to Kotlin for Android Development

At Google I/O last May, Google announced a Kotlin‑first approach, making Kotlin the preferred language for Android developers, and has since expanded its Kotlin ecosystem with free online courses and internal case studies.

The migration from Java to Kotlin is recommended to be done "from point to surface," addressing both people and code aspects.

On the people side, teams should appoint a Kotlin "advocate" who becomes an expert, defines development processes, participates in code reviews, organizes learning groups, and gathers feedback for the community.

On the code side, developers should start by writing tests in Kotlin while the project still uses Java, then write new features in Kotlin, and finally convert existing Java code to Kotlin.

Android Studio provides a built‑in Java‑to‑Kotlin conversion tool; simply select Code → Convert Java File to Kotlin File from the menu.

Migration does not need to happen all at once; a project can contain both .java and .kt files, though a single file cannot mix the two languages, and Kotlin code can freely use any Java library.

Images illustrating the conversion tool and settings are included in the original article.

When calling Java from Kotlin, developers should annotate with @Nullable/@NotNull, use property prefixes (getName(), setName(), isActive()), and avoid Kotlin reserved keywords such as when and is.

Google engineers view Kotlin as a JVM language that is highly interoperable with Java, allowing both to coexist in the same project; many Google apps, like Google Home, adopt this blended approach, using Kotlin for new features while retaining Java where appropriate.

The article concludes that there is no definitive answer to "Kotlin or Java"; the choice depends on the specific needs and preferences of the development team.