Refactoring 101

William C. WakeWilliam.Wake@acm.org

2-2-2000

Refactoring

Improving the design of

existing code, without changing

its observable behavior.

Agenda

“Code smells” Solid set of unit tests Refactorings Demo Questions/Discussion

Code Smells“If it stinks, change it.”

Duplicate code Switch

statements Long method Large class Data class

(“data bag”)

Long parameter list

Primitive obsession

Temporary field etc.

Unit Tests

Automatic Self-checking Run often (almost constantly) “If you want to refactor, the

essential precondition is having solid tests.” -- M. Fowler

JUnit

Unit testing tool Supports creating and running test

suites Available for Java & several other

languages http://xprogramming.com

Refactorings

Fowler’s book has a catalog There are many more Often reversible Pay attention to the mechanics Let the compiler tell you Small steps with constant tests

Extract Method

BEFOREvoid f() { …. // Compute score score = a * b + c; score -= discount;}

AFTERvoid f() { …. computeScore();}

void computeScore() { score = a * b + c; score -= discount;}

Encapsulate Field

BEFORE

public String name;

AFTERprivate String name;

public String getName() {return name;}

public String setName(String newName) { name = newName;}

Introduce Null Object

BEFORE

if (user == null) plan = Plan.basic();else plan = user.getPlan();

AFTER

Parameterize Method

BEFORE AFTER

WebService WebService

handleGet(…)

handlePut(…)

handle(serviceType,…)

Replace Constructor with Factory Method

BEFOREpublic User(int type) { this.type = type;}

AFTERUser(int type) { this.type = type;}

public static UsercreateUser(int type) { return new User(type);}

Replace String with Stringbuffer

BEFORE

String a, b, c; …return a + b + c;

AFTER

String a, b, c; …Stringbuffer result = new StringBuffer(a);result.append(b);result.append(c);return new String(result);

(Not in Fowler’s catalog)

Demo

Hand out real code seeded with refactoring opportunities

Audience takes 5 minutes to identify smells

Audience helps refactor in IDE Observe the motion back and forth

between test and code

Discussion

Questions?

More Information

Refactoring, by Martin Fowler Extreme Programming Explained,

by Kent Beck Anything by Jon Bentley -

Programming Pearls, More Programming Pearls, Writing Efficient Programs