Question

The convention for packages is your domain name. Dashes are sometimes not allowed in package names. So if your domain name has a dash what should you use?

Answer 1

The “official” convention in Java is to replace it with an underscore.

From (an older version of) the spec:

In some cases, the internet domain name may not be a valid package
name. Here are some suggested conventions for dealing with these
situations:

If the domain name contains a hyphen, or any other special character
not allowed in an identifier (§3.8), convert it into an underscore. If
any of the resulting package name components are keywords (§3.9) then
append underscore to them. If any of the resulting package name
components start with a digit, or any other character that is not
allowed as an initial character of an identifier, have an underscore
prefixed to the component.