Introduction toExtreme Programming

William C. WakeWilliam.Wake@acm.org2-2-2000

XP is...

A lightweight developmentmethodology that emphasizes: ongoing user involvement testing pay-as-you-go design

Background: Cost of Changes

t

$

t

$

“Then” (exponential) “Now” (flattened)

Background: Cost of Money

t

$“Up-front design”

“Pay as you go”

$$

XP Principles

Rapid feedbackAssume simplicityIncremental changeEmbrace ChangeQuality Work

XP Practices

Planning GameMetaphorTestsRefactoring Pair programming

Small releases

Simple designCollective

ownershipContinuous

integrationOpen workspace40-hour week

Rights

Manager and Customer:

You have the right.. to an overall plan, to know what can be accomplished, when, and at what cost.

etc.

Developer:

You have the right.. to know what is needed, via clear requirement stories, with clear declarations of priority.

etc.

The XP Cycle (in the small)

analysis

test

code

design

Planning Game

User stories = lightweight use cases2-3 sentences on a file card that: The customer cares about Can be reasonably tested Can be estimated & prioritized

Planning Game

Users write storiesDevelopers estimate themUsers split, merge, & prioritizePlan overall release (loosely) and the

next iteration

Tests

Functional TestsUnit Tests

Functional Tests

Specified by the userImplemented by users, developers,

and/or test teamAutomatedRun at least dailyPart of the specification

Unit Tests

Written by developersWritten before and after codingAlways run at 100%Support design, coding, refactoring,

and quality.

Test Metrics

Tests failed

Tests passed

1 2 3 4 5 6 7

Key:

Design

Pay as you goSpike when necessary“You aren’t gonna need it”“Simplest thing that could possibly

work”“Once And Only Once”

Refactor (Mercilessly)

Refactor = to improve the structure of code without affecting its external behavior

Done in small stepsSupported by unit tests, simple

design, and pair programmingSeek “once and only once”

Refactoring Example

Replace Magic # w/Symbolic Constant:

return 32.5 * miles_traveled;

static final double MILEAGE_RATE = 32.5;…return MILEAGE_RATE * miles_traveled;

Separate Queryfrom Modifier:

Stack: Object getTopAndPop();

Object getTop(); void pop();

Adopting XP

Some practices can be done solo, others by team, others require users to help.

User involvementFunctional tests and unit testsSimple design & refactoringPair programming

Other Approaches

UML: XP uses it on the whiteboard (if at all)

Rational Unified Process: XP has many fewer roles & documents; XP emphasizes team over artifacts

SCRUM: XP compatible

Who, Us?

And other questions

More Information

Extreme Programming Explained, by Kent Beck

Refactoring, by Martin Fowlerhttp://www.xprogramming.comhttp://c2.com/cgi/wiki?Extreme

ProgrammingRoadmap