Chair ofSoftware Engineering

XP and TDDExtreme Programming and Test Driven Development

Bertrand Meyer, Manuel OriolAndreas Leitner

Chair of Software EngineeringETH Zurich

October 27, 2006

Chair ofSoftware Engineering

Outline

Development Processes Overview

Extreme Programming

Test Driven Development

Chair ofSoftware Engineering

Outline

Development Processes Overview

Extreme Programming

Test Driven Development

Chair ofSoftware Engineering

Development Processes Overview

I Traditional MethodsI Waterfall modelI V modelI Spiral modelI Prototype model

I Agile MethodsI Extreme ProgrammingI Test Driven Development

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Waterfall model

Figure from: Wikipedia

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V model

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Defect Cost

Relative cost to correct a defect

0

10

20

30

40

50

60

70

Requirements Design Code DevelopmentTesting

AcceptanceTesting

Operation

Source: Barry W. Boehm, Software Engineering Economics, Prentice Hall, 1981

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Spiral model

Figure from: Ghezzi, Jazayeri, Mandrioli, Software Engineering, 2nd edition, Prentice Hall

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Project Management

I Programming competence varies greatlyI 1:10 in a single group (Sackman, Erikson, Grant)

I Who introduces more bugs?I Experienced DevelopersI Beginners

Chair ofSoftware Engineering

Outline

Development Processes Overview

Extreme Programming

Test Driven Development

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XP: Motivation

I Schedule slipsI Project canceledI Systems go sourI Defect rateI Doesn’t solve actual problemI Business changesI False feature richI Staff turnover

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XP: Cost of Change

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XP: Rules

I The planning gameI Small ReleasesI MetaphorI Simple DesignI TestingI RefactoringI Pair programmingI Collective OwnershipI Continuous IntegrationI 40h-WeekI On-Site CustomerI Coding Standards

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XP: Programming in the Wild

I Is XP like “programming in the wild”?

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XP: Kinds of Testing

I Unit testingI Integration testingI System testingI Acceptance testingI Regression testing

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XP: Unit testing 1/2

I ToolsI SUnit – Smaltalk (first one)I JUnit – Java (www.junit.org)I cppunit – C++I PyUnit – PythonI ...

Chair ofSoftware Engineering

Outline

Development Processes Overview

Extreme Programming

Test Driven Development

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(fromNeil McGovern)

Chair ofSoftware Engineering

(fromNeil McGovern)

Chair ofSoftware Engineering

(fromNeil McGovern)

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TDD: Overview

I Evolutionary approach to developmentI Combines

I Test-first developmentI Refactoring

I Primarily a method of software designI Not just method of testing

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TDD: The Process

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TDD = TFD + Refactoring

I Apply test-first developmentI Refactor whenever you see fit (before next functional

modification)I Kent Beck’s rule:

I Write new business code only when a test case failsI Eliminate any duplication you find

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TDD and Extreme Programming

I Easy to give in and skip some test casesI Pair-programming can helpI Writing testable code helps

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TDD: Consequences

I Incremental developmentI Development environment must provide rapid response to

small changesI Components are designed highly cohesive, loosely

coupledI Developers learn to write good unit tests:

I Run fastI Run in isolationI Use data that makes test case easy to readI Each test case is step towards overall goal

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TDD & Documentation

I Programmers often do not read documentationI Instead, they look for examples and play with themI Good unit tests can serve as

I ExamplesI Documentation

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TDD: pros and cons

I ProsI Reduce gap between decision and feedbackI Encourage developers to write code that is easily testedI Creates a thorough test bed

I DrawbacksI Time taken away from core developmentI Some code is difficult to test

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References

I Kent Beck: Agile software development: principles,patterns, and practices. Addision Wesley, 2003

I Astels: Test Driven Development: A Practical Guide,Prentice Hall, 2003

I Kent Beck: Extreme Programming Explained, AddisionWesley, 2000

I Bertrand Meyer: Practice to perfect: the quality first model,IEEE Computer, 30, 5, pages 102-103, 105-106, 1997

I Andrew Hunt: The Pragmatic Programmer: fromjourneyman to master. Addision Wesley, 2000

I Kent Beck: Extreme Programming explained. AddisionWesley, 2000