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. 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 in question—reactive programming. Sorry, pals; I know that bookkeeping and accounting are fun, but this is not the book for those topics.
Reactive programming is an old (well, what is old in computer science?) approach that has come recently to Java. Java 9 was the first release that supports some of the aspects of reactive programming in the standard JDK. To sum it up in one sentence, reactive programming is about focusing more on how the data...