Question

E.g. I have

project X
with class a.b.c.d.AbstractFoo

now I have project Y with an implementation of AbstractFoo

Is there a convention on using package names? Is there an advantage, if say, the package names are the same?

Answer 1

Is there a convention on using package names?

The package name of the implementation of AbstractFoo should make sense for that class, and should not necessarily be the same as the package of AbstractFoo.

I for instance often override / implement JComponent, still I wouldn’t dream of writing package javax.swing in one of my source files.

Is there an advantage, if say, the package names are the same?

No, not really. There is a semantical difference though, and that is due to the default (package level) access modifier. Relying on that the package name of one project matches the package name of another project seems like a really bad idea to me though.