•A very important
aspect of OOP is to exploit similarities between classes.
•This often happens
when one class is a specialized version of another class – where most operations are
identical or similar, but a few differences do exist.
•For example, a system
administrator of an electronic mail system. They receive messages and manage their
message collections in the same way as all other users do. In
addition, they have special ability to create new accounts (and remove accounts). The
set of all administrators is a subset of the set of all users