[qopp]

This is uncommon, but in this case you can use your email address or github.io, etc.

[TeMPOraL]

It's more common than not to not have a domain. Most programmers I know don't have a personal domain they own, and not all code is written for work (most open-source code isn't, in particular).

[tokenizerrr]

So put your open-source code on github and use something like <githubUserName>.github.io. And if it's for work you use your work groupId. If it's personal but not public, who cares what groupId you use? Just pick something.

What's the problem?

[TeMPOraL]

And when you move your code off github or change your username, are you supposed to change the package name too?

If not, or if we're going to put whatever we want in there anyway, then why hold to the idea it must correspond to a domain name anyway?