JXTA is used to do P2P in java.  I have read some on it, but never used
it (so don't think I am an expert).  In order to have 2 private
machines on different networks talk, your router has to support some
special features.  You will have to look into that.  Not all routers
will be compatible.  I am not sure if JXTA will even do that type of
P2P, if I remember correctly, it won't, but don't quote me on that.
I would say your best bet is to go to JXTA.org and read up on it there.
I read an older book by Brendon J. Wilson called JXTA and I am using
what I remember from that to answer.  Also, on JXTA's site I copied the
following:

JXTA peers create a virtual network where any peer can interact with
other peers and resources directly even when some of the peers and
resources are behind firewalls and NATs or are on different network
transports.

It says when SOME of the peers are behind firewalls and NATs, the key
word being some.  I don't think it will work if all are behind.