The question of whether to choose Kotlin or Java for new development has been coming up a lot in the Android community since the Google I/O announcement, although people were already asking the question in February 2016 when Kotlin 1.0 shipped. The short answer is that Kotlin code is safer and more concise than Java code, and that Kotlin and Java files can coexist in Android apps, so that Kotlin is not only useful for new apps, but also for expanding existing Java apps.
The only cogent argument I have seen for choosing Java over Kotlin would be for the case of complete Android development newbies. For them, there might be a barrier to surmount given that most Android documentation and examples are in Java. On the other hand, converting Java to Kotlin in Android Studio is a simple matter of pasting the Java code into a Kotlin file.
For almost anyone else doing Android development, the advantages of Kotlin are compelling. The typical time quoted for a Java developer to learn Kotlin is a few hours—a small price to pay to eliminate null reference errors, enable extension functions, support functional programming, and add coroutines.
Next: Kotlin for Android