Not trying to diminish the achievement, but this is really old and I’d be surprised if it’s still relevant for a significant number of people, seeing that native iOS and macOS development has moved to Swift for quite some time now, and using Java as language for some kind of proto-codebase feels like a niche case.
It's useful if you want to share code across apps. This is particularly useful for business logic and such, not as much for UIs.
The other cross-platform alternatives are C++ (potentially faster but unsafe by default with footguns galore) and JavaScript (incredibly slow - have to RPC everything into a webview).
Rust might also be a good fit for something like this, but I'm not sure what the story is for compiling Rust for Android JNI or with Objective-C/Swift.
