The subject of object-oriented programming (OOP from now on) has caused endless debates in the software community. Alan Kay, who coined the term, has often said that what we think of OOP today is not at all what he intended. This is to say that it is a complex topic, and it would be impossible for me to explain everything about this design paradigm in a single chapter. However, for better or worse, Apex is an OOP language, and we must understand advanced OOP techniques if we want to move forward in our journey to practicing clean code.