  Kotlin is a novel language that represents an alternative to Java, and has
been recently adopted as a first-class programming language for Android
applications. Kotlin is achieving a significant diffusion among developers, and
several studies have highlighted various advantages of the language when
compared to Java.
  The objective of this paper is to analyze a set of open-source Android apps,
to evaluate their transition to the Kotlin programming language throughout
their lifespan and understand whether the adoption of Kotlin has impacts on the
success of Android apps.
  We mined all the projects from the F-Droid repository of Android open-source
applications, and we found the corresponding projects on the official Google
Play Store and on the GitHub platform. We defined a set of eight metrics to
quantify the relevance of Kotlin code in the latest update and through all
releases of an application. Then, we statistically analyzed the correlation
between the presence of Kotlin code in a project and popularity metrics mined
from the platforms where the apps were released.
  Of a set of 1232 projects that were updated after October 2017, near 20%
adopted Kotlin and about 12% had more Kotlin code than Java; most of the
projects that adopted Kotlin quickly transitioned from Java to the new
language. The projects featuring Kotlin had on average higher popularity
metrics; a statistically significant correlation has been found between the
presence of Kotlin and the number of stars on the GitHub repository.
  The Kotlin language seems able to guarantee a seamless migration from Java
for Android developers. With an inspection on a large set of open-source
Android apps, we observed that the adoption of the Kotlin language is rapid
(when compared to the average lifespan of an Android project) and seems to come
at no cost in terms of popularity among the users and other developers.
