I don't know why you'd care what it's written in, but for the reasons that I'd care (resource usage mostly) Java and Javascript are languages I'd hope it's not
Java and Javascript are in no way related. Compared to an Electron app something written in Java is likely to be fast and RAM efficient. Java might have been slow in the 90s, but the world caught up and overtook it years ago.
I'm not quite going to downvote you for this, just share my observation that I have never, once, used a Java app which has what I would consider a "native look and feel" for macOS.
Better than Electron? Sure. But native? Not even close.
Perhaps it's different on other platforms; I wouldn't know.
Well I'm not a desktop Java developer anymore (thank the Gods) so I'll just take your word for it.
On a practical level, is there some example of a Java app which I could download and try out for native look and feel? See if my custom menu items show up, if I can drag a document window bar icon onto another application, basic stuff like that?
IDEA and related products are written in java and look and feel native to some degree. CyberDuck is java. Most developers don't know its so so easy to just toggle the look and feel to be native. A lot of app designers decide non-native even when it's native code, see adobe products.
You were probably Doing It Wrong. The default L&F is not good, but its one line of code to select the native L&F. I commented on another post with a few example above.
I don't know why you'd care what it's written in, but for the reasons that I'd care (resource usage mostly) Java and Javascript are languages I'd hope it's not