Software Design Patterns in Practice
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Yann-Gaël Guéhéneuc This work is licensed under a Creative Commons Attribution-NonCommercial- ShareAlike 3.0 Unported License Some Theory and Practice on Patterns – In Practice NII, Tokyo, Japan 12/02/14
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Following the lecture on quality models and the importance of taking into account good practices, we focus on patterns by providing examples of well-known idioms, design patterns, and architectural patterns. We then go on presenting their history, forms, as well as concrete examples of their uses. We also discuss approaches to identify their occurrences in the source code of OO systems under maintenance.
Transcript of Software Design Patterns in Practice
Yann-Gaël Guéhéneuc
This work is licensed under a Creative Commons Attribution-NonCommercial-
ShareAlike 3.0 Unported License
Some Theory and Practice on Patterns – In Practice
NII, Tokyo, Japan12/02/14
2/213
Patterns
Patterns document reusable solutions to recurring problems– Architecture
• Architectural styles– Design
• Design patterns• Design anti-patterns
– Implementation• Idioms
3/213
Examples
Do you know– C++?– Java?– Lisp?– Prolog?– Smalltalk?
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C++
class Dog {string name;
Dog(const Dog* dog) : name(dogname) {}}class Kennel { Dog* dog; string name; }
if (&kennel != this) {
thisdog =new Dog(kennel.dog);thisname = kennel.name;
}return *this;
Bruce Eckel ; Thinking in C++ ; Volume 2, pages 551–553, Planet PDF, 2nd Edition, 2000.
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C++
class Dog {string name;
Dog(const Dog* dog) : name(dogname) {}}class Kennel { Dog* dog; string name; }
if (&kennel != this) {
thisdog =new Dog(kennel.dog);thisname = kennel.name;
}return *this;
Bruce Eckel ; Thinking in C++ ; Volume 2, pages 551–553, Planet PDF, 2nd Edition, 2000.
?
6/213
C++
class Dog {string name;
Dog(const Dog* dog) : name(dogname) {}}class Kennel { Dog* dog; string name; }
if (&kennel != this) {
thisdog =new Dog(kennel.dog);thisname = kennel.name;
}return *this;
Bruce Eckel ; Thinking in C++ ; Volume 2, pages 551–553, Planet PDF, 2nd Edition, 2000.
?Overriding of operator =
7/213
Java
final Object oldListOfEntities =this.listOfEntities();
this.fireVetoableChange("RemoveEntity",oldListOfEntities,anEntity);
this.removeEntity(anEntity);this.firePropertyChange(
"RemoveEntity",oldListOfEntities,anEntity);
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Java
final Object oldListOfEntities =this.listOfEntities();
this.fireVetoableChange("RemoveEntity",oldListOfEntities,anEntity);
this.removeEntity(anEntity);this.firePropertyChange(
"RemoveEntity",oldListOfEntities,anEntity);
?
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Java
final Object oldListOfEntities =this.listOfEntities();
this.fireVetoableChange("RemoveEntity",oldListOfEntities,anEntity);
this.removeEntity(anEntity);this.firePropertyChange(
"RemoveEntity",oldListOfEntities,anEntity);
?Veto protocol of JavaBeans
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(define (square ls)(if (null? ls)
'()(cons ((lambda(x) (* x x))
(car ls))(square (cdr ls)))))
Lisp
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(define (square ls)(if (null? ls)
'()(cons ((lambda(x) (* x x))
(car ls))(square (cdr ls)))))
Lisp
?
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(define (square ls)(if (null? ls)
'()(cons ((lambda(x) (* x x))
(car ls))(square (cdr ls)))))
Lisp
?Map
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Prolog
checkLt0(LA, LT, LD, NNLA, NNLT, NNLD) :-nextEvent([],E),interpretEvent(E, IE),checkLt1(IE, LA, LT, LD, NLA, NLT, NLD),!,((IE = programEnd,NNLA = NLA,NNLT = NLT,NNLD = NLD)
;checkLt0(NLA, NLT, NLD, NNLA, NNLT, NNLD)).
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Prolog
checkLt0(LA, LT, LD, NNLA, NNLT, NNLD) :-nextEvent([],E),interpretEvent(E, IE),checkLt1(IE, LA, LT, LD, NLA, NLT, NLD),!,((IE = programEnd,NNLA = NLA,NNLT = NLT,NNLD = NLD)
;checkLt0(NLA, NLT, NLD, NNLA, NNLT, NNLD)).
?
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Prolog
checkLt0(LA, LT, LD, NNLA, NNLT, NNLD) :-nextEvent([],E),interpretEvent(E, IE),checkLt1(IE, LA, LT, LD, NLA, NLT, NLD),!,((IE = programEnd,NNLA = NLA,NNLT = NLT,NNLD = NLD)
;checkLt0(NLA, NLT, NLD, NNLA, NNLT, NNLD)).
?Conditional
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Smalltalk
Integer>>+ aNumber^aNumber addInteger: self
Float>>+ aNumber^aNumber addFloat: self
Integer>>addInteger: anInteger<primitive: 1>
Float>>addFloat: aFloat<primitive: 2>
Integer>>addFloat: aFloat^self asFloat addFloat: aFloat
Float>>addInteger: anInteger^self addFloat: anInteger asFloat
Kent Beck ; Smalltalk – Best practice patterns ; Pages 55–57, Prentice Hall, 1997, ISBN 0-13-476904-X.
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Smalltalk
Integer>>+ aNumber^aNumber addInteger: self
Float>>+ aNumber^aNumber addFloat: self
Integer>>addInteger: anInteger<primitive: 1>
Float>>addFloat: aFloat<primitive: 2>
Integer>>addFloat: aFloat^self asFloat addFloat: aFloat
Float>>addInteger: anInteger^self addFloat: anInteger asFloat
Kent Beck ; Smalltalk – Best practice patterns ; Pages 55–57, Prentice Hall, 1997, ISBN 0-13-476904-X.
?
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Smalltalk
Integer>>+ aNumber^aNumber addInteger: self
Float>>+ aNumber^aNumber addFloat: self
Integer>>addInteger: anInteger<primitive: 1>
Float>>addFloat: aFloat<primitive: 2>
Integer>>addFloat: aFloat^self asFloat addFloat: aFloat
Float>>addInteger: anInteger^self addFloat: anInteger asFloat
Kent Beck ; Smalltalk – Best practice patterns ; Pages 55–57, Prentice Hall, 1997, ISBN 0-13-476904-X.
?Double dispatch
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Conclusion on the Examples
The examples showed idioms in the given pieces of source code– These idioms are recurring motifs in a program
source code– These motifs connote a recognized,
acknowledge style of programming
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Outline
Definition Quality Form Example Catalogue Practice Conclusion
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Outline
Definition Quality Form Example Catalogue Practice Conclusion
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Definition
Context– 1977 et 1979: architecture
• Christopher Alexander• A Pattern Language: Towns, Buildings,
Construction and the idea of generative patterns• The Timeless Way of Building and the idea of
perfection in architecture
– 1990: object-oriented design• Erich Gamma, Richard Helm, Ralph Johnson,
and John Vlissides†
• Design Patterns drawn from experience
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Definition
A Pattern Language: Towns, Buildings, Construction – 253 patterns– Generative grammar– “At the core... is the idea that people should
design for themselves their own houses, streets and communities. This idea... comes simply from the observation that most of the wonderful places of the world were not made by architects but by the people.”
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Definition
“Each pattern describes a problem which occurs over and over again in our environment, and then describes the core of the solution to that problem, in such way that you can use this solution a million times over, without ever doing it the same way twice.”
—Christopher Alexander, 1977
“Each pattern is a three part rule, which express a relation between a context, a problem, and a solution.”
—Christopher Alexander, 1977
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Definition
Design Patterns: Elements of Reusable OO Software– 23 patterns– Not a language?– “Dynamic, highly parameterized software is
harder to understand and build than more static software.”
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Definition
“The strict modeling of the real world leads to reflect today’s realities but not necessarily tomorrow’s. The abstractions that emerge during design are key to making a design flexible.”
—Erich Gamma, 1994
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JHotDrawhttp://www.jhotdraw.org/
http://www.javaworld.com/article/2074997/swing-gui-programming/become-a-programming-picasso-with-jhotdraw.html
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Definition
A pattern is a general reusable solution to a commonly occurring problem within a given context in software development, operation, and maintenance
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Definition
A pattern is a general reusable solution to a commonly occurring problem within a given context in software development, operation, and maintenance– Patterns have been identified for
• Different phases of software development• Different levels of abstraction• Different technologies• …
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Definition
A pattern is a general reusable solution to a commonly occurring problem within a given context in software development, operation, and maintenance
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Definition
A pattern is a general reusable solution to a commonly occurring problem within a given context in software development, operation, and maintenance– Problem faced by three people at three different
times in a similar context– Particular problems are not included, except if
they occur more than three times…
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Definition
A pattern is a general reusable solution to a commonly occurring problem within a given context in software development, operation, and maintenance
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Definition
A pattern is a general reusable solution to a commonly occurring problem within a given context in software development, operation, and maintenance– Essentially, a solution must describe steps to
solve the problem• Architecture• Design• Implementation
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Definition
A pattern is a general reusable solution to a commonly occurring problem within a given context in software development, operation, and maintenance
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Definition
A pattern is a general reusable solution to a commonly occurring problem within a given context in software development, operation, and maintenance– The solution must not be particular– The solution can be adapted– The solution must be adapted
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Definition
A pattern is a general reusable solution to a commonly occurring problem within a given context in software development, operation, and maintenance– The solution must not be particular– The solution can be adapted– The solution must be adapted
• Forces• Variants
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Recall the C++ Example
class Dog {string name;
Dog(const Dog* dog) : name(dogname) {}}class Kennel { Dog* dog; string name; }
if (&kennel != this) {
thisdog =new Dog(kennel.dog);thisname = kennel.name;
}return *this;
Bruce Eckel ; Thinking in C++ ; Volume 2, pages 551–553, Planet PDF, 2nd Edition, 2000.
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Recall the C++ Example
class Dog {string name;
Dog(const Dog* dog) : name(dogname) {}}class Kennel { Dog* dog; string name; }
if (&kennel != this) {
thisdog =new Dog(kennel.dog);thisname = kennel.name;
}return *this;
Bruce Eckel ; Thinking in C++ ; Volume 2, pages 551–553, Planet PDF, 2nd Edition, 2000.
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Recall the C++ Example
class Dog {string name;
Dog(const Dog* dog) : name(dogname) {}}class Kennel { Dog* dog; string name; }
if (&kennel != this) {
thisdog =new Dog(kennel.dog);thisname = kennel.name;
}return *this;
Bruce Eckel ; Thinking in C++ ; Volume 2, pages 551–553, Planet PDF, 2nd Edition, 2000.
Development, operation, and maintenance
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Recall the C++ Example
class Dog {string name;
Dog(const Dog* dog) : name(dogname) {}}class Kennel { Dog* dog; string name; }
if (&kennel != this) {
thisdog =new Dog(kennel.dog);thisname = kennel.name;
}return *this;
Bruce Eckel ; Thinking in C++ ; Volume 2, pages 551–553, Planet PDF, 2nd Edition, 2000.
Development, operation, and maintenance
Commonly occurring problem within a given context
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Recall the C++ Example
class Dog {string name;
Dog(const Dog* dog) : name(dogname) {}}class Kennel { Dog* dog; string name; }
if (&kennel != this) {
thisdog =new Dog(kennel.dog);thisname = kennel.name;
}return *this;
Bruce Eckel ; Thinking in C++ ; Volume 2, pages 551–553, Planet PDF, 2nd Edition, 2000.
Development, operation, and maintenance
Commonly occurring problem within a given context
General reusable solution
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Outline
Definition Quality Form Example Catalogue Practice Conclusion
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Quality
A means to enhance the reusability– Of the code written using the pattern
+ Its flexibility– Of the problem and its solution
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Quality
A means to enhance the reusability– Of the code written using the pattern
+ Its flexibility– Of the problem and its solution
A means to encapsulate design experience
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Quality
A means to enhance the reusability– Of the code written using the pattern
+ Its flexibility– Of the problem and its solution
A means to encapsulate design experience A common vocabulary among designers
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Quality
A means to enhance the reusability– Of the code written using the pattern
+ Its flexibility– Of the problem and its solution
A means to encapsulate design experience A common vocabulary among designers
A means to have that “quality without a name”
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Quality Without a Name
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Quality Without a Name
“There is a point you reach in tightening a nut, where you know that to tighten just a little more might strip the thread, but to leave it slightly looser would risk having the hut coming off from vibration. If you've worked with your hands a lot, you know what this means, but the advice itself is meaningless.”
—Robert Pirsig, circa. 1972
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Quality Without a Name
“There is a point you reach in tightening a nut, where you know that to tighten just a little more might strip the thread, but to leave it slightly looser would risk having the hut coming off from vibration. If you've worked with your hands a lot, you know what this means, but the advice itself is meaningless.”
—Robert Pirsig, circa. 1972
“[T]oo often software developers spend their days grinding away for pay at programs they neither need nor love. But not in the Linux world - which may explain why the average quality of software originated in the Linux community is so high.”
—Eric Raymond, 1998
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Quality Without a Name
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Quality Without a Name
ニワトリのヒナの雌雄鑑別
(chick sexing)
http://discovermagazine.com/2011/sep/18-your-brain-knows-lot-more-than-you-realize
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Quality Without a Name
ニワトリのヒナの雌雄鑑別
(chick sexing)“The master would stand over the apprentice and watch. The student would pick up a chick, examine its rear, and toss it into one bin or the other. The master would give feedback: yes or no.”
—Eagleman, 2011
http://discovermagazine.com/2011/sep/18-your-brain-knows-lot-more-than-you-realize
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Practice, practiceand practice more
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Quality
Rationale– “Software design is widely recognised as being
a “wicked” or “ill-structured” problem, characterised by ambiguous specifications, no true/false solutions (only ones that are “better” or “worse” from a particular perspective), the lack of any “stopping rule” to determine when a solution has been reached, and no ultimate test of whether a solution meets the requirements.”
—Zhang and Budgen, 2012
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Quality
“Important assumptions– That patterns can be codified in such a way that
they can be shared between different designers– That reuse will lead to “better” designs. There is
an obvious question here of what constitutes “better”, but a key measure is maintainability”
—Zhang and Budgen, 2012 (With minor adaptations)
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Quality
“Advantages:– Using patterns improves programmer productivity and
program quality– Novices can increase their design skills significantly by
studying and applying patterns– Patterns encourage best practices, even for experiences
designers– Design patterns improve communication, both among
developers and from developers to maintainers”—Zhang and Budgen, 2012
(With minor adaptations)
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Outline
Definition Quality Form Example Catalogue Practice Conclusion
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Form
Several books, articles– “Theoretical”– With examples
– Among others…
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Form
Several books, articles– “Theoretical”– With examples
– Among others…
63/213
Form
Several books, articles– “Theoretical”– With examples
– Among others…
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Form
Several books, articles– “Theoretical”– With examples
– Among others…
65/213
Form
Several books, articles– “Theoretical”– With examples
– Among others…
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Form
Several books, articles– “Theoretical”– With examples
– Among others…
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Form
Several books, articles– Amazon.com
• Books › Computers & Technology › Programming ›Software Design, Testing & Engineering › Object-Oriented Design › "patterns"
• 224 results on May 31, 2013
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Form
Several books, articles– Amazon.com
• Exclusion– Unreleased books– Specific to a technology or frameworks
» e.g., MVVM Unleashed by Michael Brown– Process oriented, user-interface, programming languages
» e.g., Process Patterns: Building Large-Scale Systems Using Object Technology by Scott W. Ambler and Barbara Hanscome
– Proceedings of conferences– Unrelated to software engineering
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Form1. Pattern-Oriented Software Architecture, Patterns for Concurrent and Networked Objects: Volume 2 (Wiley Software...
by Douglas C. Schmidt, Michael Stal, Hans Rohnert and Frank Buschmann2. Pattern-Oriented Software Architecture, Patterns for Resource Management: Volume 3 (Wiley Software Patterns
Series... by Michael Kircher and Prashant Jain3. Pattern-Oriented Software Architecture, A System of Patterns: Volume 1 (Wiley Software Patterns Series) by Frank
Buschmann, Regine Meunier, Hans Rohnert and Peter Sommerlad4. Pattern-Oriented Software Architecture For Dummies (For Dummies (Computers)) by Robert Hanmer5. Web Security Patterns by Ramesh Nagappan and Christopher Steel6. Safe C++ by Vladimir Kushnir7. Programming in the Large with Design Patterns by Eddie Burris8. Elemental Design Patterns by Jason McC. Smith9. Java Application Architecture: Modularity Patterns with Examples Using OSGi (Robert C. Martin Series) by Kirk
Knoernschild10. Enterprise Integration Patterns: Designing, Building, and Deploying Messaging Solutions (Addison-Wesley
Signature... by Gregor Hohpe and Bobby Woolf11. Patterns of Enterprise Application Architecture (Addison-Wesley Signature Series (Fowler)) by Martin Fowler12. Cognitive Patterns: Problem-Solving Frameworks for Object Technology by Robert K Konitzer, Bobbin Teegarden,
Alexander Rush and Karen M Gardner13. Service Design Patterns: Fundamental Design Solutions for SOAP/WSDL and RESTful Web Services by Robert
Daigneau14. The ACE Programmer's Guide: Practical Design Patterns for Network and Systems Programming by Stephen D.
Huston, James CE Johnson and Umar Syyid15. Patterns for Parallel Software Design (Wiley Software Patterns Series) by Jorge Luis Ortega-Arjona16. Design Patterns in Object-oriented ABAP by Igor Barbaric17. Object-Oriented Reengineering Patterns by Oscar Nierstrasz, Stéphane Ducasse and Serge Demeyer18. Dependency Injection by Dhanji R. Prasanna19. Object-Oriented Software Engineering Using UML, Patterns, and Java (3rd Edition) by Bernd Bruegge and Allen H.
Dutoit20. J2EE Design Patterns by William Crawford and Jonathan Kaplan21. Applying UML and Patterns: An Introduction to Object-oriented Analysis and Design and Iterative Development by
Craig Larman22. Object-oriented Analysis and Design Using Umlan Introduction to Unified Process and Design Patterns by Mahesh P.
Matha23. C++ Design Patterns and Derivatives Pricing (Mathematics, Finance and Risk) by M. S. Joshi24. Effective Java (2nd Edition) by Joshua Bloch25. Patterns for Fault Tolerant Software (Wiley Software Patterns Series) by Robert Hanmer26. Implementation Patterns by Kent Beck27. Patterns for Computer-Mediated Interaction (Wiley Software Patterns Series) by Till Schummer and Stephan Lukosch28. Pattern Oriented Software Architecture Volume 5: On Patterns and Pattern Languages by Frank Buschmann, Kevlin
Henney and Douglas C. Schmidt29. Object-Oriented Analysis and Design with Applications (3rd Edition) by Grady Booch, Robert A. Maksimchuk, Michael
W. Engle and Bobbi J. Young30. Head First Object-Oriented Analysis and Design by Brett D. McLaughlin, Gary Pollice and Dave West 31. Agile Principles, Patterns, and Practices in C# by Robert C. Martin and Micah Martin32. Design Patterns For Dummies by Steve Holzner33. Pattern Languages of Program Design 5 by Dragos Manolescu, Markus Voelter and James Noble34. Design Patterns in Java(TM) (Software Patterns Series) by Steven John Metsker and William C. Wake35. Object-Oriented Design and Patterns by Cay S. Horstmann
37. Object-Oriented Modeling and Design with UML (2nd Edition) by Michael R. Blaha and James R Rumbaugh38. Remoting Patterns: Foundations of Enterprise, Internet and Realtime Distributed Object Middleware (Wiley
Software... by Markus Völter, Michael Kircher and Uwe Zdun39. Software Factories: Assembling Applications with Patterns, Models, Frameworks, and Tools (Wiley Application
Development... by Jack Greenfield, Keith Short, Steve Cook and Stuart Kent40. Refactoring to Patterns by Joshua Kerievsky41. Architecting Enterprise Solutions: Patterns for High-Capability Internet-based Systems (Wiley Software Patterns...
by Paul Dyson and Andrew Longshaw42. Enterprise Patterns and MDA: Building Better Software with Archetype Patterns and UML by Jim Arlow and Ila
Neustadt43. Data Access Patterns: Database Interactions in Object-Oriented Applications by Clifton Nock44. Domain-Driven Design: Tackling Complexity in the Heart of Software by Eric Evans45. Pattern-Oriented Analysis and Design: Composing Patterns to Design Software Systems by Sherif M. Yacoub,
Hany H. Ammar, Sherif Yacoub and Hany Ammar46. Java Extreme Programming Cookbook by Eric M. Burke and Brian M. Coyner47. J2EE Best Practices: Java Design Patterns, Automation, and Performance (Wiley Application Development
Series) by Darren Broemmer48. Real-Time Design Patterns: Robust Scalable Architecture for Real-Time Systems by Bruce Powel Douglass49. Design Patterns Java¿ Workbook by Steven John Metsker50. EJB Design Patterns: Advanced Patterns, Processes, and Idioms by Floyd Marinescu51. Streamlined Object Modeling: Patterns, Rules, and Implementation by Jill Nicola, Mark Mayfield and Mike Abney52. Design Patterns Explained: A New Perspective on Object-Oriented Design by Alan Shalloway and James Trott53. Small Memory Software: Patterns for systems with limited memory (Software Patterns Series) by James Noble
and Charles Weir54. AntiPatterns in Project Management by William J. Brown, Hays W. "Skip" McCormick III and Scott W. Thomas55. Pattern Languages of Program Design 4 (Software Patterns Series) by Brian Foote, Neil Harrison and Hans
Rohnert56. Testing Object-Oriented Systems: Models, Patterns, and Tools by Robert V. Binder57. Design Patterns and Contracts by Jean-Marc Jezequel, Michel Train and Christine Mingins58. Object-Oriented Software Development Using Java: Principles, Patterns, and Frameworks (1/e) by Xiaoping Jia59. Refactoring: Improving the Design of Existing Code by Martin Fowler, Kent Beck, John Brant and William Opdyke60. More Process Patterns: Delivering Large-Scale Systems Using Object Technology (SIGS: Managing Object
Technology... by Scott W. Ambler61. Pattern Hatching: Design Patterns Applied by John Vlissides62. AntiPatterns: Refactoring Software, Architectures, and Projects in Crisis by William J. Brown, Raphael C. Malveau,
Hays W. "Skip" McCormick and Thomas J. Mowbray63. A Little Java, A Few Patterns (Language, Speech, & Communication) by Matthias Felleisen, Daniel P. Friedman
and Ralph E. Johnson64. Pattern Languages of Program Design 3 (v. 3) by Robert C. Martin, Dirk Riehle and Frank Buschmann65. Object Models: Strategies, Patterns, and Applications (2nd Edition) by Peter Coad, David North and Mark Mayfield66. Analysis Patterns: Reusable Object Models by Martin Fowler67. Patterns of Software: Tales from the Software Community by Richard P. Gabriel68. Pattern Languages of Program Design 2 (v. 2) by John Vlissides, James O. Coplien and Norman L. Kerth69. Software Patterns by James Coplien70. Software Architecture: Perspectives on an Emerging Discipline by Mary Shaw and David Garlan71. Adaptive Object-Oriented Software: The Demeter Method with Propagation Patterns: The Demeter Method with
Propagation... by Karl Lieberherr72. Pattern Languages of Program Design by James O. Coplien and Douglas C. Schmidt
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Form
“Each pattern is a three part rule, which express a relation between a context, a problem, and a solution.”
—Christopher Alexander, 1977
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Form
General form as for the GoFalso inspired by Coplien’s form– Name– Problem(s)– Solution– Consequences
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Form (Extended)
General form as for the GoFalso inspired by Coplien’s form– Name– Problem(s)– Example(s)– Solution– Example(s)– Consequences– (Follow-up)
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Form
General form as for the GoFalso inspired by Coplien’s form– Not formal– Room for interpretation
– But…• UML-like class diagrams• UML-like sequence diagrams• Smalltalk / C++ example code
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Outline
Definition Quality Form Example Catalogue Practice Conclusion
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Example (1/5)
Simplified compiler– Parse files to build an AST– Iterate over the AST
• Build DefaultMutableTreeNodesjavax.swing.tree.DefaultMutableTreeNode
for a graphical representation of the AST
• Bind types• Generate code• …
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Example (1/5)
Simplified compiler– Parse files to build an AST– Iterate over the AST
• Build DefaultMutableTreeNodesjavax.swing.tree.DefaultMutableTreeNode
for a graphical representation of the AST
• Bind types• Generate code• …
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Example (2/5)
AST
Field
generateCode()
Statement
generateCode()
Method
generateCode()
Class
generateCode()
CompilationUnit
generateCode()
Main
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Example (3/5)
package compiler;
import java.util.Set;
public class Method {private Set statements;
public void addStatement(final Statement aStatement) {this.statements.add(aStatement);
}public void removeStatement(final Statement aStatement) {
this.statements.remove(aStatement);}
}
package compiler;
public class Field {/* To be implemented. */
}
package compiler;
public class Statement {/* To be implemented. */
}
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Example (4/5)package compiler;
import java.util.Set;
public class Class {private String name;private Set methods;private Set fields;
public String getName() {return this.name;
}public void addMethod(final Method aMethod) {
this.methods.add(aMethod);}public void removeMethod(final Method aMethod) {
this.methods.remove(aMethod);}public void addField(final Method aField) {
this.fields.add(aField);}public void removeField(final Field aField) {
this.fields.remove(aField);}
}
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Example (5/5)package compiler;
import java.util.Iterator;import java.util.Set;
public class CompilationUnit {private Set classes;
public void addClass(final Class aClass) {this.classes.add(aClass);
}public void removeClass(final Class aClass) {
this.classes.remove(aClass);}public Class getClass(final String aName) {
final Iterator iterator = this.classes.iterator();while (iterator.hasNext()) {
final Class aClass = (Class) iterator.next();if (aClass.getName().equals(aName)) {
return aClass;}
}return null;
}}
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Naïve Implementation (1/7)
How to generate microcode for– Microsoft Windows operating system– Intel Pentium processor
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Naïve Implementation (1/7)
How to generate microcode for– Microsoft Windows operating system– Intel Pentium processor
Add a generateCode()method in each class
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Naïve Implementation (2/7)
public class Method {…
public String generateCode() {String generatedCode = "";/* Do something at the beginning. */final Iterator iterator = this.statements.iterator();while (iterator.hasNext()) {
final Statement aStatement = (Statement) iterator.next();generatedCode += aStatement.generateCode();
}/* Do something at the end. */return generatedCode;
}}
public class Field {…
public String generateCode() {String generatedCode = "";/* Do something. */return generatedCode;
}}
public class Statement {…
public String generateCode() {String generatedCode = "";/* Do something. */return generatedCode;
}}
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Naïve Implementation (3/7)
public class Class {…
public String generateCode() {String generatedCode = "";/* Do something at the beginning. */final Iterator iteratorOnFields = this.fields.iterator();while (iteratorOnFields.hasNext()) {
final Field aField = (Field) iteratorOnFields.next();generatedCode += aField.generateCode();
}final Iterator iteratorOnMethods = this.methods.iterator();while (iteratorOnMethods.hasNext()) {
final Method aMethod = (Method) iteratorOnMethods.next();generatedCode += aMethod.generateCode();
}/* Do something at the end. */return generatedCode;
}}
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Naïve Implementation (4/7)
public class CompilationUnit {…
public String generateCode() {String generatedCode = "";/* Do something at the beginning. */final Iterator iterator = this.classes.iterator();while (iterator.hasNext()) {
final Class aClass = (Class) iterator.next();generatedCode += aClass.generateCode();
}/* Do something at the end. */return generatedCode;
}}
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Naïve Implementation (5/7)
cu : CompilationUnit
c : Class m : Method s : Statementm : Main
generateCode( )
generateCode( )
generateCode( )
generateCode( )
87/213
Naïve Implementation (6/7)
Limitations of the naïve implementation– What about generating code for
• Linux on PowerPC?• Linux on Motorola 68060?• OS/400 on AS/400?
88/213
Naïve Implementation (6/7)
Limitations of the naïve implementation– What about generating code for
• Linux on PowerPC?• Linux on Motorola 68060?• OS/400 on AS/400?
Combinatorial explosion of generateCodeForXXX()
methods in each class
89/213
Naïve Implementation (7/7)
Requirements– Decouple the data structure
• The AST– From algorithms on the data structure
• The generateCodeForXXX() method• And others, including type binding!
90/213
Naïve Implementation (7/7)
Requirements– Decouple the data structure
• The AST– From algorithms on the data structure
• The generateCodeForXXX() method• And others, including type binding!
The Visitor design pattern guides you to do that!
91/213
Visitor (2/13)
Name: Visitor
Intent: “Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates.”
92/213
Visitor (3/13)
Motivation: “Consider a compiler that represents programs as abstract syntax trees. It will need to perform operations on abstract syntax trees for "static semantic" analyses like checking that all variables are defined. It will also need to generate code.”
93/213
Visitor (4/13)
Motivation (cont’d): “[…] It will be confusing to have type-checking code mixed with pretty-printing code or flow analysis code. […] It would be better if each new operation could be added separately, and the node classes were independent of the operations that apply to them.”
94/213
Visitor (5/13)
Motivation (cont’d): “We can have both by packaging related operations from each class in a separate object, called a visitor, and passing it to elements of the abstract syntax tree as it's traversed.”
95/213
Visitor (6/13)
Motivation (cont’d): “When an element accepts the visitor, it sends a request to the visitor that encodes the element's class. It also includes the element as an argument.The visitor will then execute the operation for that element—the operation that used to be in the class of the element.”
96/213
Visitor (8/13)
Applicability– An object structure contains many classes of
objects with differing interfaces…– Many distinct and unrelated operations need to
be performed on objects in an object structure…– The classes defining the object structure rarely
change, but you often want to define new operations over the structure…
97/213
Visitor (9/13)
Structure
98/213
Visitor (10/13)
Participants– Visitor (NodeVisitor)
• Declares a Visit operation for each class…
– ConcreteVisitor(TypeCheckingVisitor)
• Implements each Visit…– Element (Node)
• Defines an Accept operation…
– ConcreteElement(AssignmentNode)
• Implements Accept …– ObjectStructure (Program)
• Can enumerate its elements• May provide a high-level
interface to allow the visitor to visit its elements
• May either be a composite (see Composite) or a collection
99/213
Visitor (11/13)
Collaborations
100/213
Visitor (13/13)
Consequences: … Implementation: … Sample code: … Known uses
– ASTs– Meta-models– …
Related patterns: Composite
101/213
Better Implementation (1/6)
package compiler.visitor;
import compiler.Class;import compiler.CompilationUnit;import compiler.Field;import compiler.Method;import compiler.Statement;
public interface Visitor {void open(final Class aClass);void open(final CompilationUnit aCompilationUnit);void open(final Method aMethod);void close(final Class aClass);void close(final CompilationUnit aCompilationUnit);void close(final Method aMethod);void visit(final Field aField);void visit(final Statement aStatement);
}
102/213
Better Implementation (2/6)public class Method {
…public void accept(final Visitor aVisitor) {
aVisitor.open(this);final Iterator iterator = this.statements.iterator();while (iterator.hasNext()) {
final Statement aStatement = (Statement) iterator.next();aStatement.accept(aVisitor);
}aVisitor.close(this);
}}
public class Field {…
public void accept(final Visitor aVisitor) {aVisitor.visit(this);
}}
public class Statement {…
public void accept(final Visitor aVisitor) {aVisitor.visit(this);
}}
103/213
Better Implementation (3/6)
public class Class {…
public void accept(final Visitor aVisitor) {aVisitor.open(this);final Iterator iteratorOnFields = this.fields.iterator();while (iteratorOnFields.hasNext()) {
final Field aField = (Field) iteratorOnFields.next();aField.accept(aVisitor);
}final Iterator iteratorOnMethods = this.methods.iterator();while (iteratorOnMethods.hasNext()) {
final Method aMethod = (Method) iteratorOnMethods.next();aMethod.accept(aVisitor);
}aVisitor.close(this);
}}
104/213
Better Implementation (4/6)
public class CompilationUnit {…
public void accept(final Visitor aVisitor) {aVisitor.open(this);final Iterator iterator = this.classes.iterator();while (iterator.hasNext()) {
final Class aClass = (Class) iterator.next();aClass.accept(aVisitor);
}aVisitor.close(this);
}}
105/213
Bet
ter i
mpl
emen
tatio
n
(5/6
)
106/213
Bet
ter i
mpl
emen
tatio
n
(5/6
)cu :
CompilationUnitc : Class m : Method s : Statementm : Main
generateCode( )
generateCode( )
generateCode( )
generateCode( )
107/213
Bet
ter i
mpl
emen
tatio
n
(5/6
) m : Main cu : CompilationUnit
c : Class m : Method s : Statement v : IVisitor
accept(IVisitor)
accept(IVisitor)
accept(IVis itor)
accept(IVisitor)open(CompilationUnit)
open(Clas s)
open(Method)
visit(Statement)
close(Method)
close(Class)
close(CompilationUnit)
108/213
cu : CompilationUnit
c : Class m : Method s : Statementm : Main
generateCode( )
generateCode( )
generateCode( )
generateCode( )
m : Main cu : CompilationUnit
c : Class m : Method s : Statement v : IVisitor
accept(IVisitor)
accept(IVisitor)
accept(IVis itor)
accept(IVisitor)open(CompilationUnit)
open(Clas s)
open(Method)
visit(Statement)
close(Method)
close(Class)
close(CompilationUnit)
109/213
Better Implementation (6/6)
By using the visitor design pattern– Decouple data structure and algorithms– Put the traversal in only one place, in the AST– Allow the addition of new algorithms without
changing the data structure
110/213
Better Implementation (6/6)
By using the visitor design pattern– Decouple data structure and algorithms– Put the traversal in only one place, in the AST– Allow the addition of new algorithms without
changing the data structure
Much better, clearer implementation!
111/213
Conclusion on the Example
The Visitor design pattern is useful anywhere you have– A data (stable) structure– Algorithms (infinite) on that data structure
Design patterns provided good solution to recurrent problems!
112/213
Outline
Definition Quality Form Example Catalogue Practice Conclusion
113/213
Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides ;Design Patterns: Elements of Reusable Object-Oriented Software ; Addison-Wesley, 1995
114/213
Catalogue
Design patterns– Development and maintenance– Design/implementation levels– Examples in C++ and Smalltalk
Divided in three categories– Behavioural– Creational– Structural
115/213
Catalogue
116/213
Catalogue
117/213
Catalogue
Abstract Factory (87) – Provide an interface for creating families of
related or dependent objects without specifying their concrete classes
Adapter (139) – Convert the interface of a class into another
interface clients expect. Adapter lets classes work together that couldn't otherwise because of incompatible interfaces
118/213
Catalogue
Bridge (151) – Decouple an abstraction from its implementation
so that the two can vary independently Builder (97)
– Separate the construction of a complex object from its representation so that the same construction process can create different representations
119/213
Catalogue
Chain of Responsibility (223) – Avoid coupling the sender of a request to its
receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it
120/213
Catalogue
Command (233) – Encapsulate a request as an object, thereby
letting you parameterize clients with different requests, queue or log requests, and support undoable operations
Composite (163) – Compose objects into tree structures to
represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly
121/213
Catalogue
Decorator (175) – Attach additional responsibilities to an object
dynamically. Decorators provide a flexible alternative to subclassing for extending functionality
Facade (185) – Provide a unified interface to a set of interfaces
in a subsystem. Facade defines a higher-level interface that makes the subsystem easier to use
122/213
Catalogue
Factory Method (107) – Define an interface for creating an object, but let
subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses
Flyweight (195) – Use sharing to support large numbers of fine-
grained objects efficiently
123/213
Catalogue
Interpreter (243) – Given a language, define a representation for its
grammar along with an interpreter that uses the representation to interpret sentences in the language
Iterator (257) – Provide a way to access the elements of an
aggregate object sequentially without exposing its underlying representation
124/213
Catalogue
Mediator (273) – Define an object that encapsulates how a set of
objects interacts. Mediator promotes loose coupling by keeping objects from referring to each other explicitly, and it lets you vary their interaction independently
Memento (283) – Without violating encapsulation, capture and
externalize an object's internal state so that the object can be restored to this state later
125/213
Catalogue
Observer (293) – Define a one-to-many dependency between
objects so that when one object changes state, all its dependents are notified and updated automatically
Prototype (117) – Specify the kinds of objects to create using a
prototypical instance, and create new objects by copying this prototype
126/213
Catalogue
Proxy (207) – Provide a surrogate or placeholder for another
object to control access to it. Singleton (127)
– Ensure a class only has one instance, and provide a global point of access to it
127/213
Catalogue
State (305) – Allow an object to alter its behaviour when its
internal state changes. The object will appear to change its class
Strategy (315) – Define a family of algorithms, encapsulate each
one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it
128/213
Catalogue
Template Method (325) – Define the skeleton of an algorithm in an
operation, deferring some steps to subclasses. Template Method lets subclasses redefine certain steps of an algorithm without changing the algorithm's structure
129/213
Catalogue
Visitor (331) – Represent an operation to be performed on the
elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates
130/213
Outline
Definition Quality Form Example Catalogue Practice Conclusion
131/213
Practice
“The strict modeling of the real world leads to reflect today’s realities but not necessarily tomorrow’s. The abstractions that emerge during design are key to making a design flexible.”
—Erich Gamma, 1994
132/213
Practice
“The description of communicating objects and classes customized to solve general design problem in a particular context.”
—Erich Gamma, 1994
133/213
Practice
“Each design pattern lets some aspect of system structure vary independently of other aspects, thereby making a system more robust to a particular kind of change.”
—Erich Gamma, 1994
134/213
The following is only complementaryto reading books and practicing
135/213
Practice
Scattered information– Informal text
A suggested example rather than a general rule
Interpreting them all...
136/213
Practice
When encountering complex problems?– Numerous design patterns (is there any
complete list out there?)– Granularity
• Requirements, analysis, architecture• Design, implementation (idioms)• Refactoring, testing• …
Knowing them all...
137/213
Practice
Iterative induction process – From an example to an abstraction to an
application to the abstraction to an application…
– Validation process?
Applying them all...
138/213
Practice
Use known categories– Behavioural – Creational – Structural
Use the Internet
Read and discuss others’ code
139/213
Practice
Use known categories– Behavioural – Creational – Structural
Use the Internet
Read and discuss others’ code
140/213
Read and discuss others’ code
141/213
Practice
Assess the trade-offs– Flexibility– Complexity
142/213
Practice
Assess the trade-offs– Flexibility– Complexity
143/213
Practice
Assess the trade-offs– Flexibility– Complexity
Sometimes it is necessary to remove the solution of a DP used in the code
144/213
Practice
Sometimes it is necessary to remove the solution of a DP used in the code
145/213
Practice
“There is a natural relation between patterns and refactorings. “Patterns are where you want to be; refactorings are ways to get there from somewhere else”
—Josuha Kerievsky citingMartin Fowler, 2004
146/213
147/213
Practice
Refactoring to a Visitor– Previous example of code generation from a
common AST Implementing a variant of the Visitor
– padl.kernel.impl.Constituent.accept(IVisitor)
– padl.kernel.impl.Constituent.accept(IVisitor, String)
– padl.kernel.impl.Constituent.accept(Class, IVisitor, String, boolean)
Refactoring away from the Visitor– ptidej.statement.creator.classfiles.loc.BCELLOCFinder
148/213
Practice
Refactoring to a Visitor– Previous example of code generation from a
common AST Implementing a variant of the Visitor
– padl.kernel.impl.Constituent.accept(IVisitor)
– padl.kernel.impl.Constituent.accept(IVisitor, String)
– padl.kernel.impl.Constituent.accept(Class, IVisitor, String, boolean)
Refactoring away from the Visitor– ptidej.statement.creator.classfiles.loc.BCELLOCFinder
149/213
Practice
Refactoring to a Visitor– Previous example of code generation from a
common AST Implementing a variant of the Visitor
– padl.kernel.impl.Constituent.accept(IVisitor)– padl.kernel.impl.Constituent.accept(IVisitor, String)– padl.kernel.impl.Constituent.accept(Class, IVisitor,
String, boolean)
Refactoring away from the Visitor– ptidej.statement.creator.classfiles.loc.BCELLOCFinder
150/213
Practice
Implementing a variant of the Visitor– Problem when implementing the solution of the
Visitor design pattern• Too many duplicated accept(…) methods• Two cases for visit(…) and open(…)+close(…)
151/213
Practice
Implementing a variant of the Visitor– Problem when implementing the solution of the
Visitor design pattern• Too many duplicated accept(…) methods• Two cases for visit(…) and open(…)+close(…)
• What if the data structure changes?
152/213
Practice
Implementing a variant of the Visitor– Problem when implementing the solution of the
Visitor design pattern• Too many duplicated accept(…) methods• Two cases for visit(…) and open(…)+close(…)
• What if the data structure changes?
153/213
Practice
Implementing a variant of the Visitor– Problem when implementing the solution of the
Visitor design pattern• Too many duplicated accept(…) methods• Two cases for visit(…) and open(…)+close(…)
• What if the data structure changes?
– Solution• Use the introspection mechanism of Java
154/213
Practice
Implementing a variant of the Visitor– Problem when implementing the solution of the
Visitor design pattern• Too many duplicated accept(…) methods• Two cases for visit(…) and open(…)+close(…)
• What if the data structure changes?
– Solution• Use the introspection mechanism of Java
155/213
Prac
tice
U
se th
e in
trosp
ectio
n m
echa
nism
of J
ava
private boolean accept(
final java.lang.Class currentReceiver,
final IVisitor visitor,
final String methodName,
final boolean shouldRecurse) {
acceptClassName = currentReceiver.getName();
java.lang.Class argument = null;
try {
argument = visitor.getClass().getClassLoader().loadClass(acceptClassName);
}
catch (final ClassNotFoundException e) {
visitor.unknownConstituentHandler(methodName, this);
return false;
}
try {
final Method method = visitor.getClass().getMethod(
methodName,
new java.lang.Class[] { argument });
method.invoke(visitor, new Object[] { this });
return true;
}
catch (final Exception e) {
if (e instanceof NoSuchMethodException) {
visitor.unknownConstituentHandler(methodName + '(‘ + argument.getName() + ')', this);
}
else {
throw new RuntimeException(e);
}
}
return false;
}
156/213
Prac
tice
U
se th
e in
trosp
ectio
n m
echa
nism
of J
ava
private boolean accept(
final java.lang.Class currentReceiver,
final IVisitor visitor,
final String methodName,
final boolean shouldRecurse) {
acceptClassName = currentReceiver.getName();
java.lang.Class argument = null;
try {
argument = visitor.getClass().getClassLoader().loadClass(acceptClassName);
}
catch (final ClassNotFoundException e) {
visitor.unknownConstituentHandler(methodName, this);
return false;
}
try {
final Method method = visitor.getClass().getMethod(
methodName,
new java.lang.Class[] { argument });
method.invoke(visitor, new Object[] { this });
return true;
}
catch (final Exception e) {
if (e instanceof NoSuchMethodException) {
visitor.unknownConstituentHandler(methodName + '(‘ + argument.getName() + ')', this);
}
else {
throw new RuntimeException(e);
}
}
return false;
}
final Method method = visitor.getClass().getMethod(
methodName,
new java.lang.Class[] { argument });
method.invoke(visitor, new Object[] { this });
157/213
Prac
tice
U
se th
e in
trosp
ectio
n m
echa
nism
of J
ava
private boolean accept(
final java.lang.Class currentReceiver,
final IVisitor visitor,
final String methodName,
final boolean shouldRecurse) {
acceptClassName = currentReceiver.getName();
java.lang.Class argument = null;
try {
argument = visitor.getClass().getClassLoader().loadClass(acceptClassName);
}
catch (final ClassNotFoundException e) {
visitor.unknownConstituentHandler(methodName, this);
return false;
}
try {
final Method method = visitor.getClass().getMethod(
methodName,
new java.lang.Class[] { argument });
method.invoke(visitor, new Object[] { this });
return true;
}
catch (final Exception e) {
if (e instanceof NoSuchMethodException) {
visitor.unknownConstituentHandler(methodName + '(‘ + argument.getName() + ')', this);
}
else {
throw new RuntimeException(e);
}
}
return false;
}
158/213
Prac
tice
U
se th
e in
trosp
ectio
n m
echa
nism
of J
ava
private boolean accept(
final java.lang.Class currentReceiver,
final IVisitor visitor,
final String methodName,
final boolean shouldRecurse) {
acceptClassName = currentReceiver.getName();
java.lang.Class argument = null;
try {
argument = visitor.getClass().getClassLoader().loadClass(acceptClassName);
}
catch (final ClassNotFoundException e) {
visitor.unknownConstituentHandler(methodName, this);
return false;
}
try {
final Method method = visitor.getClass().getMethod(
methodName,
new java.lang.Class[] { argument });
method.invoke(visitor, new Object[] { this });
return true;
}
catch (final Exception e) {
if (e instanceof NoSuchMethodException) {
visitor.unknownConstituentHandler(methodName + '(‘ + argument.getName() + ')', this);
}
else {
throw new RuntimeException(e);
}
}
return false;
}
acceptClassName = currentReceiver.getName();
java.lang.Class argument = null;
try {
argument = visitor.getClass().getClassLoader().loadClass(acceptClassName);
}
catch (final ClassNotFoundException e) {
visitor.unknownConstituentHandler(methodName, this);
return false;
}
159/213
Prac
tice
U
se th
e in
trosp
ectio
n m
echa
nism
of J
ava
private boolean accept(
final java.lang.Class currentReceiver,
final IVisitor visitor,
final String methodName,
final boolean shouldRecurse) {
acceptClassName = currentReceiver.getName();
java.lang.Class argument = null;
try {
argument = visitor.getClass().getClassLoader().loadClass(acceptClassName);
}
catch (final ClassNotFoundException e) {
visitor.unknownConstituentHandler(methodName, this);
return false;
}
try {
final Method method = visitor.getClass().getMethod(
methodName,
new java.lang.Class[] { argument });
method.invoke(visitor, new Object[] { this });
return true;
}
catch (final Exception e) {
if (e instanceof NoSuchMethodException) {
visitor.unknownConstituentHandler(methodName + '(‘ + argument.getName() + ')', this);
}
else {
throw new RuntimeException(e);
}
}
return false;
}
160/213
Prac
tice
U
se th
e in
trosp
ectio
n m
echa
nism
of J
ava
private boolean accept(
final java.lang.Class currentReceiver,
final IVisitor visitor,
final String methodName,
final boolean shouldRecurse) {
acceptClassName = currentReceiver.getName();
java.lang.Class argument = null;
try {
argument = visitor.getClass().getClassLoader().loadClass(acceptClassName);
}
catch (final ClassNotFoundException e) {
visitor.unknownConstituentHandler(methodName, this);
return false;
}
try {
final Method method = visitor.getClass().getMethod(
methodName,
new java.lang.Class[] { argument });
method.invoke(visitor, new Object[] { this });
return true;
}
catch (final Exception e) {
if (e instanceof NoSuchMethodException) {
visitor.unknownConstituentHandler(methodName + '(‘ + argument.getName() + ')', this);
}
else {
throw new RuntimeException(e);
}
}
return false;
}
catch (final Exception e) {
if (e instanceof NoSuchMethodException) {
visitor.unknownConstituentHandler(methodName + '(‘ + argument.get...;
}
else {
throw new RuntimeException(e);
}
}
161/213
Practice
Use the introspection mechanism of Java– No more duplicated accept(…) methods– Handles cases for visit(…) and open(…)+close(…)
– Plus, allows extensions to the data structure without changing all existing Visitors
162/213
Practice
Use the introspection mechanism of Java– No more duplicated accept(…) methods– Handles cases for visit(…) and open(…)+close(…)
– Plus, allows extensions to the data structure without changing all existing Visitors
163/213
Practice
Refactoring to a Visitor– Previous example of code generation from a
common AST Implementing a variant of the Visitor
– padl.kernel.impl.Constituent.accept(IVisitor)
– padl.kernel.impl.Constituent.accept(IVisitor, String)
– padl.kernel.impl.Constituent.accept(Class, IVisitor, String, boolean)
Refactoring away from the Visitor– ptidej.statement.creator.classfiles.loc.BCELLOCFinder
164/213
Prac
tice
R
efac
torin
g aw
ay fr
om th
e Vi
sito
r
final FileInputStream fis = new FileInputStream(path);
final ClassParser parser = new ClassParser(fis, path);
final JavaClass clazz = parser.parse();
clazz.accept(this.instFinder);
fis.close();
public class BCELLOCFinder implements Visitor {
private JavaClass currentClass;
public void visitCode(final Code aCode) {
}
public void visitCodeException(final CodeException aCodeException) {
}
// 18 other empty “visit” methods
public void visitJavaClass(final JavaClass aClass) {
this.currentClass = aClass;
final Method[] methods = aClass.getMethods();
for (int i = 0; i < methods.length; i++) {
this.visitMethod(methods[i]);
}
}
// 4 more empty “visit” methods
public void visitMethod(final Method aMethod) {
Integer count = null;
final String ckey = this.adaptor.adapt(this.currentClass);
final String mkey = this.adaptor.adapt(this.currentClass, aMethod);
final Map methodMap = this.methodMap(ckey);
count = this.getLOC(code);
methodMap.put(mkey, count);
}
// 6 more empty “visit” methods
}
165/213
Prac
tice
R
efac
torin
g aw
ay fr
om th
e Vi
sito
r
final FileInputStream fis = new FileInputStream(path);
final ClassParser parser = new ClassParser(fis, path);
final JavaClass clazz = parser.parse();
clazz.accept(this.instFinder);
fis.close();
public class BCELLOCFinder implements Visitor {
private JavaClass currentClass;
public void visitCode(final Code aCode) {
}
public void visitCodeException(final CodeException aCodeException) {
}
// 18 other empty “visit” methods
public void visitJavaClass(final JavaClass aClass) {
this.currentClass = aClass;
final Method[] methods = aClass.getMethods();
for (int i = 0; i < methods.length; i++) {
this.visitMethod(methods[i]);
}
}
// 4 more empty “visit” methods
public void visitMethod(final Method aMethod) {
Integer count = null;
final String ckey = this.adaptor.adapt(this.currentClass);
final String mkey = this.adaptor.adapt(this.currentClass, aMethod);
final Map methodMap = this.methodMap(ckey);
count = this.getLOC(code);
methodMap.put(mkey, count);
}
// 6 more empty “visit” methods
}
final FileInputStream fis = new FileInputStream(path);
final ClassParser parser = new ClassParser(fis, path);
final JavaClass clazz = parser.parse();
clazz.accept(this.instFinder);
fis.close();
166/213
Prac
tice
R
efac
torin
g aw
ay fr
om th
e Vi
sito
r
final FileInputStream fis = new FileInputStream(path);
final ClassParser parser = new ClassParser(fis, path);
final JavaClass clazz = parser.parse();
clazz.accept(this.instFinder);
fis.close();
public class BCELLOCFinder implements Visitor {
private JavaClass currentClass;
public void visitCode(final Code aCode) {
}
public void visitCodeException(final CodeException aCodeException) {
}
// 18 other empty “visit” methods
public void visitJavaClass(final JavaClass aClass) {
this.currentClass = aClass;
final Method[] methods = aClass.getMethods();
for (int i = 0; i < methods.length; i++) {
this.visitMethod(methods[i]);
}
}
// 4 more empty “visit” methods
public void visitMethod(final Method aMethod) {
Integer count = null;
final String ckey = this.adaptor.adapt(this.currentClass);
final String mkey = this.adaptor.adapt(this.currentClass, aMethod);
final Map methodMap = this.methodMap(ckey);
count = this.getLOC(code);
methodMap.put(mkey, count);
}
// 6 more empty “visit” methods
}
167/213
Prac
tice
R
efac
torin
g aw
ay fr
om th
e Vi
sito
r
final FileInputStream fis = new FileInputStream(path);
final ClassParser parser = new ClassParser(fis, path);
final JavaClass clazz = parser.parse();
clazz.accept(this.instFinder);
fis.close();
public class BCELLOCFinder implements Visitor {
private JavaClass currentClass;
public void visitCode(final Code aCode) {
}
public void visitCodeException(final CodeException aCodeException) {
}
// 18 other empty “visit” methods
public void visitJavaClass(final JavaClass aClass) {
this.currentClass = aClass;
final Method[] methods = aClass.getMethods();
for (int i = 0; i < methods.length; i++) {
this.visitMethod(methods[i]);
}
}
// 4 more empty “visit” methods
public void visitMethod(final Method aMethod) {
Integer count = null;
final String ckey = this.adaptor.adapt(this.currentClass);
final String mkey = this.adaptor.adapt(this.currentClass, aMethod);
final Map methodMap = this.methodMap(ckey);
count = this.getLOC(code);
methodMap.put(mkey, count);
}
// 6 more empty “visit” methods
}
public void visitJavaClass(final JavaClass aClass) {
this.currentClass = aClass;
final Method[] methods = aClass.getMethods();
for (int i = 0; i < methods.length; i++) {
this.visitMethod(methods[i]);
}
}
168/213
Practice
Refactoring away from the Visitor– 28 empty methods– Hard-coded call the visitMethod(…)
• See this.visitMethod(methods[i]);– JavaClasses do not contain other similar
objects, they are not a Composite object
– Unnecessary code, complexity
169/213
Practice
Refactoring away from the Visitor– 28 empty methods– Hard-coded call the visitMethod(…)
• See this.visitMethod(methods[i]);– JavaClasses do not contain other similar
objects, they are not a Composite object
– Unnecessary code, complexity
170/213
Practice
Unnecessary code, complexity– Trade-offs of (most of) design patterns
171/213
Practice
Unnecessary code, complexity– Trade-offs of (most of) design patterns
Flexibility
172/213
Practice
Unnecessary code, complexity– Trade-offs of (most of) design patterns
Flexibility Complexity
173/213
Practice
Trade-offs of (most of) design patterns– Flexibility
• Favouring composition over inheritance
– Complexity• More objects interacting • More messages exchanged
174/213
Practice
Trade-offs of (most of) design patterns– Flexibility
• Favouring composition over inheritance
– Complexity• More objects interacting • More messages exchanged
175/213
cu : CompilationUnit
c : Class m : Method s : Statementm : Main
generateCode( )
generateCode( )
generateCode( )
generateCode( )
m : Main cu : CompilationUnit
c : Class m : Method s : Statement v : IVisitor
accept(IVisitor)
accept(IVisitor)
accept(IVis itor)
accept(IVisitor)open(CompilationUnit)
open(Clas s)
open(Method)
visit(Statement)
close(Method)
close(Class)
close(CompilationUnit)
176/213
cu : CompilationUnit
c : Class m : Method s : Statementm : Main
generateCode( )
generateCode( )
generateCode( )
generateCode( )
m : Main cu : CompilationUnit
c : Class m : Method s : Statement v : IVisitor
accept(IVisitor)
accept(IVisitor)
accept(IVis itor)
accept(IVisitor)open(CompilationUnit)
open(Clas s)
open(Method)
visit(Statement)
close(Method)
close(Class)
close(CompilationUnit)
Plus the use of reflection!
177/213
Practice
Trade-offs of (most of) design patterns– Flexibility
• Favouring composition over inheritance
– Complexity• More objects interacting • More messages exchanged
178/213
Practice
Trade-offs of (most of) design patterns– Flexibility
• Favouring composition over inheritance
– Complexity• More objects interacting • More messages exchanged
179/213
Practice
Flexibility– Favour composition over inheritance
• Allow changing implementation• Allow safe inheritance
180/213
Practice
Flexibility– Favour composition over inheritance
• Allow changing implementation• Allow safe inheritance
Add one level of indirection
181/213
Practice
Flexibility– Favour composition over inheritance
• Allow changing implementation• Allow safe inheritance
Add one level of indirectionand (possibly)
Use double-dispatch
182/213
Practice
Add one level of indirection
“Rather than giving you a lengthy explanation, let me just point you to the Strategy pattern. It is my prototypical example for the flexibility of composition over inheritance.”
Erich Gamma, 2005
183/213
Practice
Add one level of indirection
http://itewbm.tistory.com/29
184/213
Practice
Add one level of indirection
http://itewbm.tistory.com/29
185/213
Practice
Add one level of indirection
http://itewbm.tistory.com/29
186/213
Practice
Add one level of indirection
http://programmers.stackexchange.com/questions/203210/is-context-inheritance-as-shown-by-head-first-design-patterns-duck-example-ir
187/213
188/213
Practice
Add one level of indirection
http://programmers.stackexchange.com/questions/203210/is-context-inheritance-as-shown-by-head-first-design-patterns-duck-example-ir
189/213
Practice
Add one level of indirection
http://programmers.stackexchange.com/questions/203210/is-context-inheritance-as-shown-by-head-first-design-patterns-duck-example-ir
No special code for “no behaviour”
with the NullObject design pattern
190/213
Practice
Add one level of indirection
http://programmers.stackexchange.com/questions/203210/is-context-inheritance-as-shown-by-head-first-design-patterns-duck-example-ir
Encapsulate what varies
191/213
Practice
Add one level of indirection
“You have a container, and you plug in some smaller objects. These smaller objects configure the container and customize the behaviour of the container. This is possible since the container delegates some behaviour to the smaller thing. In the end you get customization by configuration. ”
Erich Gamma, 2005(With minor adaptations)
192/213
Practice
Add one level of indirection
“You have a container, and you plug in some smaller objects. These smaller objects configure the container and customize the behaviour of the container. This is possible since the container delegates some behaviour to the smaller thing. In the end you get customization by configuration. ”
Erich Gamma, 2005(With minor adaptations)
193/213
Practice
Use double-dispatch– Object receives a message– Object sends a message back
with itself as parameter
194/213
Practice
Use double-dispatch– Object receives a message– Object sends a message back
with itself as parameter
195/213
Practice
print(CtxWriter aCTXWriter, Rect aRectangle) { ... }...print(StringWriter aStringWriter, Triangle aTriangle) { ... }...
PrinterWriter printer = new SystemWriter();IShape shape = new Square();print(printer, shape);
196/213
Practice
print(CtxWriter aCTXWriter, Rect aRectangle) { ... }...print(StringWriter aStringWriter, Triangle aTriangle) { ... }...
PrinterWriter printer = new SystemWriter();IShape shape = new Square();print(printer, shape);
197/213http://c2.com/cgi-bin/wiki?DoubleDispatchExamplehttp://www.patentstorm.us/patents/6721807/description.html
public class SystemWriter implements PrinterWriter {@Overridepublic void print(final Rect rect) {
System.out.print("Printed the rectangle: ");System.out.println(this);
}@Overridepublic void print(final Square aSquare) {
System.out.print("Printed the square: ");System.out.println(this);
}}
public class Rect implements IShape {@Overridepublic void printOn(final PrinterWriter aWriter) {
aWriter.print(this);}
}
public class Square implements IShape {@Overridepublic void printOn(final PrinterWriter aWriter) {
aWriter.print(this);}
}
public class Main {public static void main(final String[] args) {
final PrinterWriter writer = new SystemWriter();IShape shape;
shape = new Rect();shape.printOn(writer);
shape = new Square();shape.printOn(writer);
}}
public interface PrinterWriter {void print(final Rect aRect);void print(final Square aSquare);...
}
public interface IShape {void printOn(final PrinterWriter aWriter);
}
198/213http://c2.com/cgi-bin/wiki?DoubleDispatchExamplehttp://www.patentstorm.us/patents/6721807/description.html
public class SystemWriter implements PrinterWriter {@Overridepublic void print(final Rect rect) {
System.out.print("Printed the rectangle: ");System.out.println(this);
}@Overridepublic void print(final Square aSquare) {
System.out.print("Printed the square: ");System.out.println(this);
}}
public class Rect implements IShape {@Overridepublic void printOn(final PrinterWriter aWriter) {
aWriter.print(this);}
}
public class Square implements IShape {@Overridepublic void printOn(final PrinterWriter aWriter) {
aWriter.print(this);}
}
public class Main {public static void main(final String[] args) {
final PrinterWriter writer = new SystemWriter();IShape shape;
shape = new Rect();shape.printOn(writer);
shape = new Square();shape.printOn(writer);
}}
public interface PrinterWriter {void print(final Rect aRect);void print(final Square aSquare);...
}
public interface IShape {void printOn(final PrinterWriter aWriter);
}
199/213http://c2.com/cgi-bin/wiki?DoubleDispatchExamplehttp://www.patentstorm.us/patents/6721807/description.html
public class SystemWriter implements PrinterWriter {@Overridepublic void print(final Rect rect) {
System.out.print("Printed the rectangle: ");System.out.println(this);
}@Overridepublic void print(final Square aSquare) {
System.out.print("Printed the square: ");System.out.println(this);
}}
public class Rect implements IShape {@Overridepublic void printOn(final PrinterWriter aWriter) {
aWriter.print(this);}
}
public class Square implements IShape {@Overridepublic void printOn(final PrinterWriter aWriter) {
aWriter.print(this);}
}
public class Main {public static void main(final String[] args) {
final PrinterWriter writer = new SystemWriter();IShape shape;
shape = new Rect();shape.printOn(writer);
shape = new Square();shape.printOn(writer);
}}
public interface PrinterWriter {void print(final Rect aRect);void print(final Square aSquare);...
}
public interface IShape {void printOn(final PrinterWriter aWriter);
}
200/213http://c2.com/cgi-bin/wiki?DoubleDispatchExamplehttp://www.patentstorm.us/patents/6721807/description.html
public class SystemWriter implements PrinterWriter {@Overridepublic void print(final Rect rect) {
System.out.print("Printed the rectangle: ");System.out.println(this);
}@Overridepublic void print(final Square aSquare) {
System.out.print("Printed the square: ");System.out.println(this);
}}
public class Rect implements IShape {@Overridepublic void printOn(final PrinterWriter aWriter) {
aWriter.print(this);}
}
public class Square implements IShape {@Overridepublic void printOn(final PrinterWriter aWriter) {
aWriter.print(this);}
}
public class Main {public static void main(final String[] args) {
final PrinterWriter writer = new SystemWriter();IShape shape;
shape = new Rect();shape.printOn(writer);
shape = new Square();shape.printOn(writer);
}}
public interface PrinterWriter {void print(final Rect aRect);void print(final Square aSquare);...
}
public interface IShape {void printOn(final PrinterWriter aWriter);
}
201/213http://c2.com/cgi-bin/wiki?DoubleDispatchExamplehttp://www.patentstorm.us/patents/6721807/description.html
public class SystemWriter implements PrinterWriter {@Overridepublic void print(final Rect rect) {
System.out.print("Printed the rectangle: ");System.out.println(this);
}@Overridepublic void print(final Square aSquare) {
System.out.print("Printed the square: ");System.out.println(this);
}}
public class Rect implements IShape {@Overridepublic void printOn(final PrinterWriter aWriter) {
aWriter.print(this);}
}
public class Square implements IShape {@Overridepublic void printOn(final PrinterWriter aWriter) {
aWriter.print(this);}
}
public class Main {public static void main(final String[] args) {
final PrinterWriter writer = new SystemWriter();IShape shape;
shape = new Rect();shape.printOn(writer);
shape = new Square();shape.printOn(writer);
}}
public interface PrinterWriter {void print(final Rect aRect);void print(final Square aSquare);...
}
public interface IShape {void printOn(final PrinterWriter aWriter);
}
202/213http://c2.com/cgi-bin/wiki?DoubleDispatchExamplehttp://www.patentstorm.us/patents/6721807/description.html
public class SystemWriter implements PrinterWriter {@Overridepublic void print(final Rect rect) {
System.out.print("Printed the rectangle: ");System.out.println(this);
}@Overridepublic void print(final Square aSquare) {
System.out.print("Printed the square: ");System.out.println(this);
}}
public class Rect implements IShape {@Overridepublic void printOn(final PrinterWriter aWriter) {
aWriter.print(this);}
}
public class Square implements IShape {@Overridepublic void printOn(final PrinterWriter aWriter) {
aWriter.print(this);}
}
public class Main {public static void main(final String[] args) {
final PrinterWriter writer = new SystemWriter();IShape shape;
shape = new Rect();shape.printOn(writer);
shape = new Square();shape.printOn(writer);
}}
public interface PrinterWriter {void print(final Rect aRect);void print(final Square aSquare);...
}
public interface IShape {void printOn(final PrinterWriter aWriter);
}
203/213
Practice
Unnecessary code, complexity– Trade-offs of (most of) design patterns
204/213
Practice
Unnecessary code, complexity– Trade-offs of (most of) design patterns
Flexibility
205/213
Practice
Unnecessary code, complexity– Trade-offs of (most of) design patterns
Flexibility Complexity
206/213
Practice
Beware also of “bad” solutions to recurring design problems
http://www.jot.fm/issues/issue_2006_07/column4/
207/213
Practice
Anti-patterns (also antipatterns)– A common response to a
recurring problem that is usually ineffective and may be counterproductive
– Code smells are symptoms of “bad” programming
208/213
Practice
Two examples of anti-patterns– Visitor design and JavaClasses
ptidej.statement.creator.classfiles.loc.BCELLOCFinder
– Blob (aka God Class)
209/213
Outline
Definition Quality Form Example Catalogue Practice Conclusion
210/213
Conclusion
Patterns ease communication
Patterns improve regularityand quality… even without a name…
Patterns avoid surprisesi.e., reinventing the wheel differently each time
Patterns generate architectures
211/213
Conclusion
Identify a recurring design problem
Identify a design pattern that potentially solve the problem
Assess the costs and benefits of implementing the proposed solution– Quality and quality without a name
212/213
Conclusion
Identify a recurring design problem
Identify that the solution brings – Unneeded flexibility– Unnecessary complexity
Assess the costs and benefits of removing the proposed solution
213/213
Conclusion
Tools supporting design patterns– “GoF” book
• Lists, classifications, relationships [Gamma et al., 1996]
– CASE Tools• Fragments [Florijn et al., 1997]
• PatternsBox and Ptidej [Albin et al., 2001]
• Refactoring tools…– Navigation
• Tutor [Motelet, 2000]
![[MindMap]Software Patterns](https://static.fdocuments.in/doc/165x107/5495be79b47959474d8b4de6/mindmapsoftware-patterns.jpg)
