[Rxtx] loopback puzzle

[Dr. Stephen Judd]

I am trying to understand basic things about serial ports on the Mac.
The SerialDemo.java program from Sun's javax.CommPort program
displays 2 panels: characters going out to a port, and characters
coming in from it. Question: if I have nothing attached to the port,
why does anything get echoed from the outgoing panel to the incoming
one? My mental model says nothing should happen until there is
some hardware attached to the port.

Nothing I have looked for in the code tells me there is any software
connection between the outgoing and incoming streams.
Please tell me what is broken in my brain.
sj