XP is a set of principles and practices that guide software development. It is an agile process in that it makes every effort to eliminate unnecessary work, instead focusing on tasks that deliver value to the customer. XP is built upon four principles: simplicity, communication, feedback, and courage, all described in Chapter 2. The four XP principles have nothing to do with programming languages and tools. Although this book shows a set of Java tools that work nicely with XP, you are not limited to Java and these tools. XP is a language-independent software development approach.
 Check out http://www.agilealliance.com to learn more about agile processes.
While XP works with any language, we believe it works well with Java for a few reasons. Most important is the speed with which Java compiles. XP relies on test-first development in which programmers write tests for code before they write the code. For each new feature, you should write a test and then watch the test run and fail. You should then add the feature, compile, and watch the test run successfully. This implies that you must write a little code, compile, and run the tests frequently, perhaps dozens of times each day. Because Java compiles quickly, it is well suited to the test-first approach.
The second reason Java is a good choice for XP development is Java's wealth of tools supporting unit testing and continuous integration. JUnit, covered in Chapter 4, provides a lightweight framework for writing automated unit tests. Ant, the premier build tool for Java, makes continuous integration possible even when working with large development teams. You will also find more specialized testing tools such as Cactus and HttpUnit for server-side testing.
Java's power and simplicity also make it a good language when writing code using XP. Many features of the tools outlined in this book, such as Ant tasks and JUnit's test suites, are built upon Java's reflection capability. Java's relatively easy syntax makes it easier to maintain code written by other team members, which is important for XP's concepts of pair programming, refactoring, and collective code ownership.