In an object-oriented programming world, there is no shortage of principles, but those we will discuss in the next two chapters are the fundamental design guidelines for a better application. One cannot predict all future requirements in advance, so changes are often needed in enterprise applications. This is why a flexible application that can adopt future requirements easily is considered a better application. This part will review case studies using (and without using) these principles and help you think about their importance. A detailed study of these principles can help you create efficient and flexible applications.