In this chapter, we will develop a sample program that does the inventory management part of the company we created the order handling code for. Do not expect a fully developed, ready-to-use, professional application, and also, do not expect that we will get into the details of accounting and bookkeeping. That is not our aim. We will focus more on the programming technique that is of our interest—reactive programming. Sorry pals, I know that bookkeeping and accounting is fun, but this is not that book.

Reactive programming is an old (well, what is old in computer science?) approach that has come recently to Java. Java 9 is the first release that supports some of the aspects of reactive programming in the standard JDK. In one sentence, reactive programming is about focusing more on how the data flows and less on how the implementation handles the data flow. As you may recall, this is also a step towards describing what...