XP wants systems that are well positioned for current, actual change.
XP’s goals are simple. Software should be well-tested and expressive. It should have no unnecessary features. Developers should be confident in their ability to meet future needs. Customers should receive the software they really need. XP can make it easier to meet these goals, but really trusting that they’re achievable is hard.
XP developers use several catchphrases to remind themselves of the goal of flexibility through simplicity. These sayings are Do the Simplest Thing that Could Possibly Work, You Aren’t Gonna Need It, and Once and Only Once. Understanding and applying these ideas will help you to produce code that can be changed to meet future needs.