open closed principle

• Open/closed principle

In object-oriented programming, the open/closed principle states "software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification"; that is, such an entity can allow its behaviour to be modified without altering its source code.
code obeying the principle doesn't change when it is extended, and therefore needs no such effort.
http://en.wikipedia.org/wiki/Open/closed_principle