In this chapter, you learned the basic concepts of object-oriented programming: class and methods and how they are connected by generic functions in R through method dispatch. You learned how to create S3, S4, RC, and R6 classes and methods. These systems share similar ideas but are distinct in implementation and usage. Hadley Wickham gives some nice suggestions in picking a system (http://adv-r.had.co.nz/OO-essentials.html#picking-a-system).
After getting familiar with R's most important features, we will discuss more practical topics in the subsequent chapters. In the next chapter, you will learn about the packages and techniques used to access popular databases. You will gain necessary knowledge and techniques to connect R to relational databases such as SQLite and MySQL as well as the upcoming non-relational databases such as MongoDB and Redis.
