The "blank final variable" of the Mindprod website you quoted is an intermediate stage; a variable (better called a field) has been declared butnot yet initialised, so it must be initialised by every constructor.
A final field may point to a reference type, but cannot be altered. The state of the object which is that reference type might however be altered. that is probably what the Head First quote means. You can "change the channel" as Benm Souther has told you.