You don't have to learn Kotlin. It cut down LOC by about 40% by the time I picked it up, but today's Java has become more efficient and supports things like @nullable. Kotlin is nice though, and it leads into more advanced things like Jetpack Compose for UI and Kotlin Flow which is a nice way to do reactive programming.

Reactive programming is useful for front end because the UI will update as soon as the data downloads, so it scales linearly with complexity and API calls, instead of an exponential mess of if-else.

You can use the github repo I linked. I designed it to be complex enough to handle the harder tasks but simple enough for a beginner to reverse engineer. Just download, run to see it work, and hack it into what you need.