Why are the header classes final?

[email protected]
Newsgroups gmane.comp.voip.nist-sip
Message-ID <[email protected]> 
Forgive me if this is a stupid question:  I'm new to java.

Why are the header classes final?  I have a situation where
I want to subclass the Contact header and modify its output
slightly.  I don't see any way to do that other than to
find a copy of the source file for Contact and modify _that_.

Am I missing something obvious?

--David
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