Extreme Programming

Extreme Programming (XP) is a method in or approach to software engineering, formulated by Kent Beck, who wrote the first book on the topic, "Extreme Programming Explained: Embrace Change" (ISBN 0201616416). It is one of several agile processes.

Fundamental characteristics of the method are:

• Iterative and Incremental development - small improvements after small ones
• continuous, often repeated automated unit test, regression testing. See JUnit.
• pair programming
• user interaction in the programming team
• fixing all known bugs before adding functionality
• refactoring
• sharing ownership

These characteristics are only derivatives of principles that are known to be good, and are taken into extreme:

1. Interaction between developers and customers is good. Therefore, an XP team is supposed to have a customer on site, who can answer questions as soon as they arise.
2. If learning is good, take it to extremes: Reduce the length of feedback cycles. Test early;
3. Simple code is more likely to work. Therefore, extreme programmers only write code to meet actual needs in the present project;
4. If simple code is good, re-write code when it becomes complex;
5. Code reviews are good. Therefore XP programmers work in pairs, sharing one screen and keyboard (which also improves communication) so that all code is reviewed as it is written;
6. Testing code is good. Therefore, in XP, test are written before the code is written. The code is considered complete when it passes the tests. The system is periodically, or immediately tested using all pre-existing automated tests to assure that it works. (See test-driven development[?])

In general, Extreme Programming is believed to be useful for small teams under 10 persons. Some think it can be useful for larger teams while some consider the RUP process is more appropriate in that case.

Controversial Aspects

• Detailed specifications are not written
• A customer representative is attached to the project
• Programmers work in pairs

Most of the design activity takes place on the fly and incrementally, starting with "the simplest thing that could possibly work" and adding complexity only when it's required by failing tests.

See also: Crystal Light Methods[?]

References & External Links

• Kent Beck, Extreme programming explained, Addison Wesley, 2000
• Ward Cunningham's website, http://www.c2.com/cgi/wiki?ExtremeProgramming - For more information on this and related topics