In this chapter, we presented the main programming paradigms used in Java. We have learned that two different paradigms, such as imperative programming and functional programming, can coexist in the same language; and we have learned how Java went from pure, imperative object-oriented programming to integrating functional programming elements.

Although Java introduced new functional elements, starting from version 8, it is at its core still an object-oriented language. In order to write solid and robust code that is easy to extend and maintain, we learned about the fundamental principles of object-oriented programming languages.

An important part of developing software is designing the structure and the desired behavior of the components of our programs. This way, we can work on large systems, in large teams, sharing our object-oriented designs within or between teams. In order to be able to do this, we highlighted the main UML diagrams and concepts relevant to object-oriented design and programming. We also use UML extensively in our book to describe the examples.

After introducing the class relationships and showing how to represent them in diagrams, we dove into the next section, where we described what the object-oriented design patterns and principles are, and we presented the main principles.

In the next chapter, we will move on to presenting the group of design patterns dealing with object creation in such a way that our code is robust and extendable.