Day-to-day developer work related not only with writing new functionality but updating existing ones. When a developer tasked with adding a new feature to an existing application, the goal is to extend the functionality of that application with new behaviors. Extending software is the introduction of a new behavior by the addition of code. Some applications are flexible to this kind of change, whereas others may fight you tooth and nail!

Flexibility is how easily software can adapt to shifting requirements.

In an ideal extensible system, adding new behavior involves strictly adding new code without changing existing code. Adding new behavior to an extensible system means adding new classes, methods, functions, or data that encapsulate the new behavior. But because real systems are rarely ideal, you’ll still find yourself needing to make changes to existing code regularly.

One of the things that help developers write clean, well-structured, and easily-maintainable code is SOLID principles.