  Background: Google announced Kotlin as an Android official programming
language in 2017, giving developers an option of writing applications using a
language that combines object-oriented and functional features. Aims: The goal
of this work is to understand the usage of Kotlin features considering four
aspects: i) which features are adopted, ii) what is the degree of adoption,
iii)when are these features added into Android applications for the first time,
and iv) how the usage of features evolves along with applications' evolution.
Method: Exploring the source code of 387 Android applications, we identify the
usage of Kotlin features on each version application's version and compute the
moment that each feature is used for the first time. Finally, we identify the
evolution trend that better describes the usage of these features. Results: 15
out of 26 features are used on at least 50% of applications. Moreover, we found
that type inference, lambda and safe call are the most used features. Also, we
observed that the most used Kotlin features are those first included on Android
applications. Finally, we report that the majority of applications tend to add
more instances of 24 out of 26 features along with their evolution. {\bf
Conclusions:} Our study generates 7 main findings. We present their
implications, which are addressed to developers, researchers and tool builders
in order to foster the use of Kotlin features to develop Android applications.
