u can plug a keyboard switcher to your microsoft pc or your linux server.
And u 'll be a client for your computers.

Keyboard is your interface  to both machines. They think different, run different
but both can be used over a keyboard.

OR u can use one monitor on different machines having different graphic cards.
DAC(digital to analog converter) on your graphics card both have standart
outputs to your
monitor. DAC is the interface your graphic processor will be the core code of
your class.

Or PCI is a common interface for your pc. modems, tv cards blabla both can be
run on
the same PCI slot.

Servlet has the interface for POST and GET operations. U can write a servlet
with
implementing generic servlet interface. Then just say, 'this as servlet run my
tomcat run'.
U dont need to write any http protocol code. tomcat will call post or get method
of your servlet with the information u need, youyr servlet will process the
request. Tomcat has no idea about your servlet cause its only a servlet
interface.