In the previous chapter, we began building the messenger application by designing and implementing a REST application programming interface that the client messenger application will communicate with. Over the course of implementing the backend API, we covered many things, such as working with Spring Boot, RESTful application programming interfaces and how they work, creating databases with PostgreSQL, and deploying Spring Boot web applications to AWS, to name a few.

In this chapter, we will go one step further in our application development journey by implementing the Android Messenger application and integrating it with the RESTful API we created in Chapter 4, Designing and Implementing the Messenger Backend with Spring Boot 2.0. In the process of developing the Messenger Android app, we will learn a vast array of new topics, such as:

• Building MVP Android applications
• Server communication via HTTP
• Working with Retrofit
• Reactive programming
• Using token-based authentication in an Android app

Over the course of this chapter, you will learn firsthand how powerful Kotlin is in the Android application development domain. Let's dive into the development of the Messenger app.