Abstract: In this paper we introduce a new unit test technique
called déjà-vu object. Déjà-vu objects replace real objects used by
classes under test, allowing the execution of isolated unit tests. A
déjà-vu object is able to observe and record the behaviour of a real
object during real sessions, and to replace it during unit tests,
returning previously recorded results. Consequently déjà-vu object
technique can be useful when a bottom-up development and testing
strategy is adopted. In this case déjà-vu objects can increase test
portability and test source code readability. At the same time they
can reduce the time spent by programmers to develop test code and
the risk of incompatibility during the switching between déjà-vu and
production code.