There is no Java in core LibreOffice, it just has some weird Java-based extension system because of its Sun history.

LibreOffice uses an extremely dated, also messy, homegrown UI toolkit and has resisted the idea of switching to something last (really) updated this millennium (sic).

I am currently trying to wrangle this UI. It is called VCL and it was initially created in the early 1990s.

If you want to see my efforts, I have a number of branches on github:

Phase 5 can be located here:

https://github.com/chrissherlock/libreoffice-experimental/tr...

Check the source here:

https://github.com/chrissherlock/libreoffice-experimental/tr...

in particular keep an eye on:

https://github.com/chrissherlock/libreoffice-experimental/tr...