Introduction to OOP

Chapter 3: Object-Oriented Design:

Maintainence and Evolution

Software does not remain fixed after the first working version is released.

Errors or bugs can be discovered. Must be corrected.
Requirements may change. Say as a result of government regulations, or standardization among similar products.
Hardware may change.
Users expectations may change. Greater functionality, more features. Often as a result of competition from similar products.
Better documentation may be required.

A good design recognizes the inevitability of change, and plans an accommodation for these activities from the very beginning.