I recently started with Hibernate and in most of the examples I've seen so far they do something like:

but never

I noticed that for other fields they use int and not Integer. So my question: Why do they use Long instead of long and does it actually matter?

For example Integer is a pure java object (Wrapper class). The benefit is, that you can use some methods on this object and convert it easily to other types without any casting. See here: http://www.janeg.ca/scjp/pkglang/wrapper.html

if u use primitive it means that colum in db to which is property mapped has to be not null

if it can be null u should use object Long

in case of id it doesnt matter cause id can not be null in db