GObject (GTK4 and similar): https://github.com/gircore/gir.core (significantly better and faster than Java alternatives, this is just one example among many)
Young: first OSS version was released 8 years ago
Solaris: might as well say "it runs COBOL but not .NET"
It's funny that everyone missed the initial context of the question and jumped onto parroting the same arguments as years ago, without actually addressing the matter at hand or saying anything of substance. Unsurprising show of ignorance by Java community. Please stay this way - will help the industry move on faster.
The premise is always the same - if something is missing in {technology I don't like}, it's a deal-breaker, and when it's not or was always there - it never mattered, or is harmful actually, that is, until {technology I like} gets it as well.
Since the open .NET is pretty young, and they still have trouble with community perception due to their past actions, finding high quality FOSS libraries may pose a problem depending on what you're doing. Pretty much everything from MS is open and high quality, but they don't provide everything under the sun.
And with Java you always have alternative runtimes in case this Oracle deal goes sideways for any reason.
So you're all good, don't worry about it.