Per the comments, using the term Java in an online course about the language would almost certainly draw scrutiny from Oracle.
Trademarks | Legal | Oracle says this:
Domain Names Do not use Oracle trademarks or potentially confusing variations in your Internet domain name. This helps prevent Internet
users from being confused as to whether you or Oracle is the source of
the Web site.
That could be viewed as a warning of potential litigation. Oracle is a worldwide company with deep pockets, so as in your example, they will litigate over a business or product name. If your domain is related to software, beware. Poland is a member of the European Union, so EU laws apply in Poland. Trade mark law of the European Union - Wikipedia.