  Context: During the last years, developers of mobile applications have the
possibility to use new paradigms and tools for developing mobile applications.
For instance, since 2017 Android developers have the official support to write
Android applications using Kotlin language. Kotlin is programming language
fully interoperable with Java that combines object-oriented and functional
features. Objective: The goal of this paper is twofold. First, it aims to study
the degree of adoption of Kotlin language on development of open-source Android
applications and to measure the amount of Kotlin code inside Android
applications. Secondly, it aims to measure the quality of Android applications
that are written using Kotlin and to compare it with the quality of Android
applications written using Java. Method: We first defined a method to detect
Kotlin applications from a dataset of open-source Android applications. Then,
we analyzed those applications to detect instances of code smells and computed
an estimation of quality of the applications. Finally, we studied how the
introduction of Kotlin code impacts on the quality of an Android application.
Results: Our experiment found that 11.26% of applications from a dataset with
2,167 open-source applications have been written (partially or fully) using
Kotlin language. We found that the introduction of Kotlin code increases the
quality (in terms of presence of code smells) of the majority of the Android
applications initially written in Java.
