Tim Moores wrote:Java. As Tim H said, you can always convert Java to Kotlin if necessary (whereas the reverse is not generally true). And there are many more Android books and examples out there than Kotlin ones, so it'll be easier to get started with.
Tim Holloway wrote:Welcome to the Ranch, Sami!
Unfortunately, since Android doesn't adhere as strongly to Java's backwards-compatibity mindset as "real" Java does and mutates radically quite frequently, it's pretty likely that even Kotlin examples can be out of date. Especially anything that comes to asynchronous processing.