What you need for this book 

To immerse into the content of this book and to soak up most of the skills and knowledge, we assume that you already have some experience with programming. We do not assume too much but hope that you already know what a variable is, that computers have memory, disk, network interfaces, and what they generally are.
In addition to these basic skills, there are some technical requirements to try out the code and the examples of the book. You need a computer—something that is available today and can run Windows, Linux, or OSX. You need an operating system and, probably, that is all you need to pay for. All other tools and services that you will need are available as open source and free of charge. Some of them are also available as commercial products with an extended feature set, but for the scope of this book, starting to learn Java 9 programming, those features are not needed. Java, a development environment, build tools, and all other software components we use are open source.