This post continues the analisys of the SOLID principles started some blog posts ago. This is the turn for the Open Closed Priciple (OCP).
The definition
An object/entity should be open for extension but closed for modification.
What we are basically talking about is to design our modules, classes and functions in a way that when a new functionality is needed, we should not modify our existing code but rather write new code that will be used by existing code.