XP and TDD. Extreme Programming and Test Driven Development. Bertrand Meyer, Manuel Oriol Andreas Leitner. Chair of Software Engineering ETH Zurich

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1 XP and TDD Extreme Programming and Test Driven Development Bertrand Meyer, Manuel Oriol Andreas Leitner ETH Zurich October 27, 2006

2 Outline Development Processes Overview Extreme Programming Test Driven Development

3 Outline Development Processes Overview Extreme Programming Test Driven Development

4 Development Processes Overview Traditional Methods Waterfall model V model Spiral model Prototype model Agile Methods Extreme Programming Test Driven Development

5 Waterfall model Figure from: Wikipedia

6 V model

7 Defect Cost 70 Relative cost to correct a defect Requirements Design Code DevelopmentAcceptance Testing Testing Operation Source: Barry W. Boehm, Economics, Prentice Hall, 1981

8 Spiral model Figure from: Ghezzi, Jazayeri, Mandrioli,, 2nd edition, Prentice Hall

9 Project Management Programming competence varies greatly 1:10 in a single group (Sackman, Erikson, Grant) Who introduces more bugs? Experienced Developers Beginners

10 Outline Development Processes Overview Extreme Programming Test Driven Development

11 XP: Motivation Schedule slips Project canceled Systems go sour Defect rate Doesn t solve actual problem Business changes False feature rich Staff turnover

12 XP: Cost of Change

13 XP: Rules The planning game Small Releases Metaphor Simple Design Testing Refactoring Pair programming Collective Ownership Continuous Integration 40h-Week On-Site Customer Coding Standards

14 XP: Programming in the Wild Is XP like programming in the wild?

15 XP: Kinds of Testing Unit testing Integration testing System testing Acceptance testing Regression testing

16 XP: Unit testing 1/2 Tools SUnit Smaltalk (first one) JUnit Java ( cppunit C++ PyUnit Python...

17 Outline Development Processes Overview Extreme Programming Test Driven Development

18 Neil McGovern) (from

19 Neil McGovern) (from

20 Neil McGovern) (from

21 TDD: Overview Evolutionary approach to development Combines Test-first development Refactoring Primarily a method of software design Not just method of testing

22 TDD: The Process

23 TDD = TFD + Refactoring Apply test-first development Refactor whenever you see fit (before next functional modification) Kent Beck s rule: Write new business code only when a test case fails Eliminate any duplication you find

24 TDD and Extreme Programming Easy to give in and skip some test cases Pair-programming can help Writing testable code helps

25 TDD: Consequences Incremental development Development environment must provide rapid response to small changes Components are designed highly cohesive, loosely coupled Developers learn to write good unit tests: Run fast Run in isolation Use data that makes test case easy to read Each test case is step towards overall goal

26 TDD & Documentation Programmers often do not read documentation Instead, they look for examples and play with them Good unit tests can serve as Examples Documentation

27 TDD: pros and cons Pros Reduce gap between decision and feedback Encourage developers to write code that is easily tested Creates a thorough test bed Drawbacks Time taken away from core development Some code is difficult to test

28 References Kent Beck: Agile software development: principles, patterns, and practices. Addision Wesley, 2003 Astels: Test Driven Development: A Practical Guide, Prentice Hall, 2003 Kent Beck: Extreme Programming Explained, Addision Wesley, 2000 Bertrand Meyer: Practice to perfect: the quality first model, IEEE Computer, 30, 5, pages , , 1997 Andrew Hunt: The Pragmatic Programmer: from journeyman to master. Addision Wesley, 2000 Kent Beck: Extreme Programming explained. Addision Wesley, 2000