What are the basic components of Extreme Programming (XP) ?

The basic components of Extreme Programming (XP) are:

Test-First Programming
- Developers write unit tests before coding. It has been noted that this kind of approach motivates the coding, speeds coding and also improves design results in better designs.
- It supports a practice called re-factoring.
- Agile practitioners prefer tests to text for describing system behavior. Tests are more precise than human language and they are also a lot more likely to be updated when the design changes. How many times have you seen design documents that no longer accurately described the current workings of the software? Out-of-date design documents look pretty much like up-to-date documents. Out-of-date tests fall.
- Many open source tools like xUnit have been developed to support this methodology.

Refactoring
- It is the practice changing a software system in such a way that it does not alter the external behavior of the code yet improves its internal structure.
- Traditional development tries to understand how all the code will work together in advance. This is the design. With agile methods, this difficult process of imagining what code might look like before it is written is avoided. Instead, the code is restructured as needed to maintain a coherent design.Frequent refactoring allows less up-front planning of design.
- Agile methods replace high level design with frequent re-design. It also requires a way of ensuring whether the behavior was not changed inadvertently. That's where the tests come in.
- Make the simplest design that will work and add completely only when needed and re-factor as necessary.
- Re-factoring requires unit tests to ensure that design changes do not break existing code.

Acceptance Testing
- Make up the user experiences or user stories which are short descriptions of the features to be coded.
- Acceptance tests verify the completion of user stories.
- Ideally, they are written before coding.