In this chapter, we learned about some of the more advanced concepts of object-oriented programming and why it is better and more convenient than other programming languages. We started by exploring the concept of sealed classes and enumeration. We then moved on to discussing composition, aggregation, and nested classes. After that, we had a detailed discussion about singletons, objects, and companion object classes. We introduced a few more concepts to do with properties and looked at the get and set functions provided by Kotlin. We ended this chapter by exploring Kotlin's built-in delegates, which can make our lives easier. In the next chapter, we will discuss a very important and completely different topic, called data collection, iteration, and filters.