Both Java and Kotlin are awesome, and it is totally a choice of yours.
  Keep in mind that you should use programming language as a tool.
  So, learn both
  but explicitly
  Kotlin also typically has fewer codes, whereas Java adheres more to a 
  traditional approach to making everything verbose. One advantage of 
  Kotlin over Java is Kotlin's flexibility – it can choose to do everything 
  in the traditional OOP approach, or it can go a different way.