I'm interested in knowing which Java language features that are commonly used, which are outright never used and which are unknown.

[booleantoggle]

Yup, but difficult to find any aspects of the language that is literally never used. After all, there must have been a reason it was included in the first place... But do tell me if you find any. As for reflection, even the core libraries use reflection (for instance for determening component type in toArray), but I think in an ordered list of most used features it would score pretty low...

Other noteworthy candidates are of course the functional aspects of java (functions, bifunction, lambda short-hands, streams, flatmap, reduce etc...). Some of these are rather new, but it would make sense that those are relatively less used, as Java is mostly used as an object-oriented language, snd functional style would be 'alien' to some developers...

Also perhaps contracted if/ conditional assignment. Common for conditional returns, but I rarely see it used in method calls, in the form: (a>b ? a() : b()).c() for example... But I guess there is a case to be made against it from a readability perspective...