JDD2015: Twenty-one years of "Design Patterns" - Ralph Johnson
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The document discusses the influences and applications of the 23 design patterns introduced in the book 'Design Patterns: Elements of Reusable Object-Oriented Design' by Gamma et al. It highlights various core patterns such as mediator, observer, and value object, and emphasizes their importance in advanced object-oriented programming and design communication. Additionally, it critiques some common usage pitfalls of patterns and suggests approaches for better design integration.
JDD2015: Twenty-one years of "Design Patterns" - Ralph Johnson
- 1. Twenty-one Years of Design Patterns Ralph E. Johnson [email protected]
- 2. Design Patterns: Elements of Reusable Object-Oriented Design by Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides, 1995. Released in October 1994 at OOPSLA. Jolt Award, Dr. Dobbs Journal award, SIGPLAN Programming Languages award, SIGSOFT Software Engineering award, Aito Dahl-Nygard Prize
- 3. Other books • The Design Patterns Smalltalk Companion, Alpert, Brown and Woolf • Heads First Design Patterns, Freeman, Freeman, Bates and Sierra. • Domain Driven Design, Eric Evans • Patterns of Enterprise Application Architecture, Fowler et. al. • http://hillside.net/patterns/books
- 4. • Describes 23 object-oriented design patterns • Typical of advanced object-oriented programming • Provides a design vocabulary – Learning – Communication – Design
- 5. Core patterns • Mediator • Façade • Proxy • Command • Template Method • Adapter • Null Object • Composite • Strategy • Decorator • State • Iterator • Observer • Value Object
- 6. Composite • Idea: make abstract "component" class. • Alternative 1: every component has a (possibly empty) set of components. • Problem: many components have no components. Component Children ParagraphChapter ... *
- 7. Composite Pattern Component container childrenDo: CompositeLeaf Composite and Component have the exact same interface. • enumerating children • childrenDo: for Component does nothing • only Composite adds removes children. 1-N
- 9. Listening instead of Observing • Argument to “update” is an Event • Many kinds of EventListeners, each with their own interface • An EventListener interface can have more than one update method
- 10. Different Kinds of Listeners • ActionListener – actionPerformed(ActionEvent) • ComponentListener – componentResized(ComponentEvent) – componentMoved(ComponentEvent) – componentShown(ComponentEvent) – componentHidden(ComponentEvent) • PropertyChangeListener – propertyChange(PropertyChangeEvent)
- 11. From the book … Avoiding observer-specific update protocols: the push and pull models. Implementations of the Observer pattern often have the subject broadcast additional information about the change. The subject passes this information as an argument to Update. The amount of information may vary widely. … The pull model emphasizes the subject’s ignorance of its observers, whereas the push model assumes subjects know something about their observers’ needs. The push model might make observers less reusable, because Subject classes make assumptions about Observer classes that might not always be true.
- 12. Data Binding • Implement the model as a composite model. • Primitives are properties, lists, etc. • Model does not “assign to variable”, but “sets value of property”. • Results: Observers are reusable, don’t have to invoke update events explicitely
- 13. Value Object • Immutable object - variables set only by constructor • Equality is important, not identity • Lack of side effects - easier to make correct
- 14. Null Object if (obj == NULL) error(‘null pointer’); else obj->f(); ----------------------- obj->f(); Give an interface to “null”, make a version of null for any interface. Eliminate checking.
- 16. Dangerous Patterns • Mediator – Supposed to be used to allow colleagues to be reusable. Is often used to allow programmers to continue to be procedural. • Singleton – Supposed to be used to encapsulate global state when it cannot be eliminated. Is often used as a justification of global state.
- 17. Creational • Abstract factory (peripheral) • Factory method • Prototype • Builder • Singleton • Dependency Injection
- 18. Peripheral • Memento • Chain of responsibility • Bridge • Visitor • Type Object • Extension Object • Generation Gap
- 19. Compound • Flyweight – To save space, factor out a Value Object, and create them using a Memoizing Factory that records the flyweight pool. • Interpreter – Represent a program with a Composite, and let the class hierarchy match the grammar of the language. Define an “interpret” method in each class.
- 20. Issues: Design • Refactor to patterns • Patterns not measure of goodness • Patterns used for – Variability management – Decoupling – Dependence management • Patterns a means to an end, not the end
- 21. Ways to improve • Patterns are examples of principles • Patterns are part of overall design style – Pattern language – Evolving Frameworks – http://st- www.cs.uiuc.edu/~droberts/evolve.html
- 22. Conclusion • We could have done better. • But we did pretty well! • For more info on patterns, including conferences and publications, see http://hillside.net • [email protected]