Generally speaking, Kotlin is considered to be a more modern choice – all the newer Android applications are created in this language. Also, Kotlin applications are considered to be smaller, more lightweight and faster compared to the ones written in Java. However, there are still a number of great applications that are written entirely in Java – mostly older, monolith-type ones. Luckily, Kotlin is compatible with Java. So if you already have an app written in Java that needs modernisation, you don’t need to rewrite it entirely to add new features in Kotlin.