If you have a function which invokes a different object (or more commonly, a different network service), you pass that as a parameter instead of making a call i

If you have a function which invokes a different object (or more commonly, a different network service), you pass that as a parameter instead of making a call inside the function. foo() { service = someRemoteService(authn, authz); http://lookingglass.internal.adm.bol.com/?page=TAM+Middleware+PRO+Nonworkhours&showclock=1&showsoft=1&showacks=1&showzzz=1&showmute=1&showmenu=0 http://lookingglass.internal.adm.bol.com/?page=TAM+Middlewar... } is replaced by foo(service) { do_something_with_service(service) } It makes replacing service with fake_service_object easier, thus simplifying the test process. Common use case: If your service was a remote database, you can replace the database handle with a mock object which pretends to be a DB but is actually a simple object returning hardcoded values.