This document provides an introduction to patterns and pattern languages. It discusses that patterns describe recurring problems and proven solutions, and that a pattern language connects related patterns. It outlines Christopher Alexander's definition of patterns and quality without a name. It then summarizes the history and use of patterns in software, including design patterns and organizational patterns. The document presents examples of strategies and factories patterns. Finally, it discusses writing patterns and connecting them into a pattern language.

1. Introduction to Patterns
Hironori Washizaki
Waseda University
Twitter: @Hiro_Washi washizaki@waseda.jp
http://www.washi.cs.waseda.ac.jp/

2. Agenda
• Patterns and Pattern Languages
• Software Patterns
• Writing Patterns
2

3. PATTERNS AND PATTERN
LANGUAGES
3

4. 4
Stockholm, Sweden
Alaior, Spain

5. Repetition, and, not a coincidence.
• Small public squares
• Street cafes
• Live! Active! Positive!
5

6. What makes this repetition?
6
• Settings were common
– Planning city structure and environment
• Problems were common
– Open and attractive city
– Having places for people gathering and sitting lazily
• Considerations were common
– Not too large space
– Not closed.
=> Common solution! SolutionSolution
ProblemProblem
ContextContext
ForcesForces

7. Pattern form
• Context: when to consider?
• Problem: what should be solved and
when?
• Forces: why the problem is hard?
• Solution: what to do to solve problem?
• Resulting context: both positive and
negative 7
SolutionSolution
ProblemProblem
ContextContext
ForcesForces
Adapted from Joseph Yoder, "AsianPLoP Pattern Bootcamp 2011"

8. SMALL PUBLIC SQUARES
• … this pattern forms the core which makes
an ACTIVITY NODE … it can also help to
generate a PROMENADE, …, through the
action of the people who gather there…
• A town needs public squares; they are the
largest, most public rooms, that the town
has. But when they are too large, they
look and feel deserted.
• Make a public square much smaller than
you would at first imagine…
8
SolutionSolution
ProblemProblem
ContextContext
ForcesForces
Christopher Alexander, et al., “A Pattern Language,“ Oxford University Press, 1977

9. Abstraction and concretization
9
Abstraction
Concretization

10. Alexander’s definition of patterns
• Describes a problem that occurs over
and over again in our environment
• Describes the core of the solution to
that problem
• In such a way that you can use this
solution a million times over without
ever doing it the same way twice.
• Both a process and a thing
– both a description of a thing which is alive
– and a description of the process which
will generate that thing
10
Christopher Alexander, et al., “A Pattern Language,“ Oxford University Press, 1977
Christopher Alexander , “The Timeless Way of Building,” Oxford University Press, 1979
Adapted from Joseph Yoder, "AsianPLoP Pattern Bootcamp 2011"

11. Quality Without A Name (QWAN)
• “There is a central quality which is the root criterion
of life and spirit in a man, a town, a building, or a
wilderness. This quality is objective and precise,
but it cannot be named.”
– Christopher Alexander
• Message from C. Alexander
11
Adapted from Joseph Yoder, "AsianPLoP Pattern Bootcamp 2011"

12. Thinking and communications by
patterns
… OK, so, to attract many
people to our city, SMALL
PUBLIC SQUAREs should be
located in the center. At the
SMALL PUBLIC SQUARE, make
STREET CAFES be OPNENING
TO THE STREET...
12
BuildingBuilding
Street
Public square
Cafe

13. Pattern Language
• “A collection of patterns and
the rules to combine them
into an architectural style.”
– James O. Coplien
• “Each pattern then, depends
both on the smaller patterns
it contains, and on the larger
patterns within which it is
contained.”
– Christopher Alexander
13
SMALL
PUBLIC
SQUARE
STREET CAFES
OPENING TO
THE STREET
DIFFERENT
CHAIRS
ACTIVITY
NODES
Adapted from Joseph Yoder, "AsianPLoP Pattern Bootcamp 2011"

14. SOFTWARE PATTERNS
14

15. History of patterns and Japan
C. Alexander: A Pattern Language for Building
K. Beck and W. Cunningham: Application of pattern language
to software at OOPSLA
E. Gamma: Doctoral thesis on object-oriented design patterns
E. Gamma et al.: Design Patterns
PLoP conference started
(Many books on patterns)
Japan PLoP started as study meetings
IPSJ SIGSE Patterns WG
1979
1987
1990
1995
1994
2003
AsianPLoP conference started2010
1999
(OOPSLA workshops)
Adapted from Takeshi Inoue, “Introduction to Patterns”, IPSJ SIGSE Patterns WG, 2003

16. 16
Software pattern
• A pattern is a proven solution to a problem
in a software context.
• What software community adopted
– Tool for knowledge transfer and communication
– Pattern form: context-problem-solution
– Pattern catalog (and partially language..)
– Especially object-orientation community
• What software community did NOT adopt
– Common language among stakeholders

17. 17What’s the problem?
class Mathematic {
public Data sort(Data data){
switch(settings) {
case QUICK:
return quickSort(data);
case BUBBLE:
return bubbleSort(data);
default: ...
}
}
class Loan {
public double capital() {
if(expiry == null &&
maturity != null)
return ...;
if(expiry != null &&
maturity == null) {
...
}

18. 18
class Mathematic {
public Data sort(Data data){
switch(settings) {
case QUICK:
return quickSort(data);
case BUBBLE:
return bubbleSort(data);
default: ...
}
}
class Loan {
public double capital() {
if(expiry == null &&
maturity != null)
return ...;
if(expiry != null &&
maturity == null) {
...
}
class Mathematic {
Sorter sorter;
public Data sort(Data data){
return sorter.sort(data);
}
abstract class Sorter {
public abstract Data sort(Data);
class QuickSorter extends Sorter {
public Data sort(Data) { ... }
class Loan {
CapitalCalc capitalCalc;
public double capital(){
return capitalCalc.calc(this);
}
Interface CapitalCalc {
double calc(Loan l);
class TermCapital implements ...{
double calc(Loan l) { ... }

19. 19STRATEGY
ContextContext StrategyStrategy
algorithmInterface()algorithmInterface()
ConcreteStrategyAConcreteStrategyA
algorithmInterface()algorithmInterface()
ConcreteStrategyBConcreteStrategyB
algorithmInterface()algorithmInterface()
・・・・・・
・・・・・・
contextInterface()contextInterface()
Structure
Motivation
If there are hard-wiring line breaking algorithms, clients get
bigger and harder to maintain. Moreover it becomes difficult to add
new algorithms and vary existing ones…
Applicability
Many related classes differ only in their behavior.
You need different variants of an algorithm…
Consequences
Benefits: families of algorithms , elimination of conditional statements…
Drawbacks: Communication overhead…
SolutionSolution
ProblemProblem
ContextContext
ForcesForces
E. Gamma, et al. “Design Patterns: Elements of Reusable
Object-Oriented Software,” Addison-Wesley, 1994.

20. 20
Applying STRATEGY
MathematicMathematic SorterSorter
sort(Data)sort(Data)
QuickSorterQuickSorter
sort(Data)sort(Data)
BubbleSorterBubbleSorter
sort(Data)sort(Data)
・・・・・・
・・・・・・
ClientClient
sort(Data)
setSorter(Sorter)
sort(Data)
setSorter(Sorter)
Context
Strategy
ConcreteStrategy
class Client {
Mathematic math;
void init() {
math.setSorter(
new QuickSorter());
}
void calc() {
data = math.sort(data);
}
class Mathematic {
Sorter sorter;
public Data sort(Data data){
return sorter.sort(data);
}
abstract class Sorter {
public abstract Data sort(Data);
class QuickSorter extends Sorter {
public Data sort(Data) { ... }

21. 21Benefit of patterns and
pattern form
• Reuse
– Solution
– Problem
• Communication
• Understanding
• Way of thinking
• Generative. New ideas!

22. 22What’s going on?
interface MessageStrategy { public class HelloWorld {
public void sendMessage(); public static void main(String[] args) {
} MessageBody mb =
new MessageBody();
abstract class AbstractStrategyFactory { mb.configure(“Hello World!”);
public abstract MessageStrategy
createStrategy(MessageBody mb); AbstractStrategyFactory asf
= DefaultFactory.getInstance();
class MessageBody { MessageStrategy strategy
object payload; = asf.createStrategy(mb);
public Object getPayload() { mb.send(strategy);
return payload; }
} }
public void configure(Object obj) {
payload obj;
}
public void send(MessageStrategy ms) {
ms.sendMessage();
}
}
class DefaultFactory extends AbstractStrategyFactory {
private DefaultFactory() {}
static DefaultFactory instance;
public static AbstractStrategyFactory getInstance() {
if(instance == null) instance = new DefaultFactory();
return instance;
}
public MessageStrategy createStrategy(final MessageBody mb) {
return new MessageStrategy() {
MessageBody body = mb;
public void sendMessage() {
Object obj = body.getPayload();
System.out.println(obj);
} }; } }
Joshua Kerievsky, "Refactoring to Patterns," Addison-Wesley, 2004.

23. 23
Pitfall of software patterns
• “Only solution is important.”
– Context, problem and forces are most important!
• “Should use as it is.”
– There could be variants.
• “Always beneficial.”
– Misuse leads to bad complexity and defects.
• “Should use at the beginning.”
– Simple design at the beginning, and refactor it!

24. 24
E.g. Replace Conditional Logic with STRATEGY
MathematicMathematic SorterSorter
sort(Data)sort(Data)
QuickSorterQuickSorter
sort(Data)sort(Data)
BubbleSorterBubbleSorter
sort(Data)sort(Data)
・・・・・・
・・・・・・
ClientClient
sort(Data)sort(Data)
Replace Conditional with
Polymorphism
MathematicMathematicClientClient
sort(Data)sort(Data)
MathematicMathematicClientClient
sort(Data)sort(Data)
SorterSorter
sort(Data)sort(Data)
Move method
if ...
else ...
if ...
else ...
Joshua Kerievsky, "Refactoring to Patterns," Addison-Wesley, 2004.

25. Pattern catalogs and languages
• Product patterns
– “Analysis patterns” (M. Fowler)
– “Pattern-Oriented Software Architecture” (Bushmann et al)
– “Design Patterns: Elements of Reusable Object-Oriented Software”
(Gamma et al.)
– “Implementation patterns” (Beck)
– “xUnit Test Patterns” (Meszaros)
– “Object-Oriented Reengineering Patterns” (Nierstrasz et al)
• Process and organizational patterns
– “EPISODE” (Cunningham)
– "A Generative Development-Process Pattern Language” (Coplien)
– "Organizational Patterns of Agile Software Development“ (Coplien and
Harrison)
• Links to catalogs
– “Pattern almanac” (Linda Rising)
– Portland Pattern Repository (Cunningham http://c2.com/ppr/) 25

26. Network in Portland Pattern
Repository
Pattern name
N.
patterns
referred
by the
pattern
N.
patterns
referring
to the
pattern
ModelViewController 11 12
AdapterPattern 6 15
HandleBodyPattern 9 10
SynchronizationStrategies 9 9
VisitorPattern 7 11
SceneGraph 6 11
ValueObject 3 14
ScapeGoat 6 10
CompositePattern 4 12
StrategyPattern 5 11
26
Hironori Washizaki, Masashi Kadoya, Yoshiaki Fukazawa and Takeshi Kawamura, “Network Analysis
for Software Patterns including Organizational Patterns in Portland Pattern Repository,” Agile 2014
Conference (to appear)

27. 27
ENGAGE CUSTOMERS
• ...an organization is in place,
and its Quality Assurance
function has been generally
shaped and chartered…
• It's important that the
development
organization ensures and
maintains customer
satisfaction by
encouraging communication
between customers and key
development organization
roles…
• Closely couple the Customer
role to the Developer and
Architect, not just to QA or
marketing…James O. Coplien, Neil B. Harrison, "Organizational Patterns
of Agile Software Development", Prentice Hall, 2004.
James O. Coplien, "A
Development Process
Generative Pattern
Language," PLoPD

28. From patterns to Agile development
Pattern languageTakeuchi
The New New
Development
Game, 1986
Design patterns
OO patterns
A Generative Development-
Process Pattern Language EPISODE
XPScrum
Kenji Hiranabe: From Software Patterns to Agile Movements
http://www.infoq.com/jp/articles/AlexanderFestaReport

29. WRITING PATTERNS
29

30. Light-weight pattern writing
1. Individual: write down good experiments or
important things.
2. Team: brainstorming
– Grouping, relating
– Add, modify
1. Team: write in pattern form from important
groups
– (1) Write context and resulting context
– (2) Write context, problem and solution
– (3) Identify forces
– (4) Name it! 30

31. E.g. Self continuing
education
I can study while
commuting because
of no interruption..
I always carry short
literature for little
vacant time…
Smartphone
for free
time…
Study group
meetings on
Fridays…
Wake up early
to …
Plan reading
groups in my
company…
I set concrete
goals in 1, 5, 10
years…
CARRYING SHORT LITERATURE
Context
You want to enrich your knowledge
in an additional and unfamiliar area
by reading some literatures.
Problem
You are too busy to make time for
studying at your home and office.
Forces
-Making time specific for study will
sacrifice your family considerations
and business.
-There are a number of discrete
short times during your
commuting…
Solution
Select short literatures and carry
them at all times so that you could
read them even in short time
during commuting.
Resulting Context
You are now enriching your
knowledge continuously!

32. 32
From patterns to pattern languages
• Connect related
patterns
– X is similar to Y.
– X uses Y in its
solution.
– X can be combined
with Y.
• Identify surrounding
patterns
ACCUMULATION OF
COMMUTING HOURS
SPLITTING FAT
BOOKS
CARRYING
SHORT
LITERATURE
CARRYING
E-BOOK
READER
BOOK
SCANNING

33. 33
Summary
• Pattern: a proven solution to a problem in a
software context.
• Pattern form: context, problem, forces,
solution, resulting context
• Pattern Language: individual patterns are
useful, but they are most powerful when
combined into a language. (Alexander)
• Benefit and pitfall of patterns
• Various software patterns: design patterns and
organizational patterns
• Writing your own patterns: easy and fun!