There's nothing wrong with Java. It's outdated language by today's standards, a lot of code which takes 100 lines in Java would take 50 lines in Kotlin (and if we'll take autogenerated boilerplate getters/setters, it could be 1000->50), but overall Java is fine.

But I still suggest you to look into Kotlin. If you know Java, you won't spend much time learning it and with awesome IDE support you can be productive in very short time. You might find that you'll just prefer Kotlin over Java for a lot of little things which makes coding more enjoyable.