Introduction

In the last lesson, you were introduced to SOLID and its five principles. You learned the first two principles: Single Responsibility and Open-Closed Principle.

You created a new sample system that started as a very messy implementation, then you broke it down into multiple stand-alone pieces to respect the Single Responsibility principle. You also resumed working on the contacts app and improved it to follow the Open-Closed Principle, which is the second SOLID principle.

In this lesson, you’ll cover the remaining three SOLID principles. As you learn more about those principles, you’ll realize that both the contacts app and the sample app you created in the last lesson fall short. You’ll refactor those apps to apply the new principles to them.