![Design Patterns – Eran Toch
11November 2003
MotivationMotivation
When Processing the hierarchal
collection, we need to query the
type of the object (composite or
primitive) and act differently
Diagram reference: Design
Patterns [Gof]](https://image.slidesharecdn.com/designpatterns-130620123051-phpapp01/85/Design-patterns-11-320.jpg)
![Design Patterns – Eran Toch
13November 2003
Composite: StructureComposite: Structure
• Define an abstract base class that specifies the
uniform behavior.
• Subclass the Primitive and Composite classes of the
Component class.
Diagram reference: Design
Patterns [Gof]](https://image.slidesharecdn.com/designpatterns-130620123051-phpapp01/85/Design-patterns-13-320.jpg)
![Design Patterns – Eran Toch
26November 2003
Singleton: Example Code – cont’dSingleton: Example Code – cont’d
public class TestSingleton {
public static void main(String args[]) {
// Get a reference to the single instance of Singleton.
Singleton s = Singleton.instance();
// Set the data value.
s.setData(34);
System.out.println("First reference: " + s);
System.out.println("Singleton data value is: " + s.getData());
// Get another reference to the Singleton.
// Is it the same object?
s = null;
s = Singleton.instance();
System.out.println("nSecond reference: " + s);
System.out.println("Singleton data value is: " + s.getData());
}
}
What will be the output?](https://image.slidesharecdn.com/designpatterns-130620123051-phpapp01/85/Design-patterns-26-320.jpg)
Design patterns
- 1. Design Patterns – Eran Toch Methodologies in Information System Development Design Patterns:Design Patterns: Talking in the Design LanguageTalking in the Design Language Eran Toch Methodologies in the Development of Information Systems
- 2. Design Patterns – Eran Toch 2November 2003 AgendaAgenda • What is a Design Pattern? • Structural Patterns – The Composite Pattern • Behavioral Patterns – The Strategy Pattern • Creational Patterns – The Singleton Pattern • Human Interaction Patterns – Go Back to a Safe Place • Summary
- 3. Design Patterns – Eran Toch 3November 2003 What is a Design Pattern?What is a Design Pattern? In Short, a solution for a typical problem “a description of a recurrent problem and of the core of possible solutions.“
- 4. Design Patterns – Eran Toch 4November 2003 Why do we need them?Why do we need them? • Problems are not always unique. Reusing existing experience might be proved useful. • We need a common language to describe problems and solutions. • The Design of Object-Oriented systems limits the scope of problems and solutions we can talk about
- 5. Design Patterns – Eran Toch 5November 2003 History of Design PatternsHistory of Design Patterns Christopher Alexander A Pattern Language: Towns, Buildings Construction 1970’ 1995’ 2000’ Architecture Object Oriented Software Design Other Areas: HCI, Organizational Behavior… Gang of Four (GoF) Design Patterns: Elements of Reusable Object-Oriented Software Many Authors
- 6. Design Patterns – Eran Toch 6November 2003 Structure of a design patternStructure of a design pattern • Pattern Name and Classification • Intent – a Short statement about what the pattern does • Motivation – A scenario that illustrates where the pattern would be useful • Applicability – Situations where the pattern can be used
- 7. Design Patterns – Eran Toch 7November 2003 Structure of a design pattern – cont’dStructure of a design pattern – cont’d • Structure – A graphical representation of the pattern • Participants – The classes and objects participating in the pattern • Collaborations – How to do the participants interact to carry out their responsibilities? • Consequences – What are the pros and cons of using the pattern? • Implementation – Hints and techniques for implementing the pattern
- 8. Design Patterns – Eran Toch 8November 2003 Classification of patternsClassification of patterns • Structural – Composite*, Decorator, Façade, Adapter… • Behavioral – Strategy*, Command, Observer, Chain of Responsibilities… • Creational – Singleton*, Abstract Factory, Factory Method… • Other – User interface Patterns • Go Back to a Safe Place
- 9. Design Patterns – Eran Toch 9November 2003 AgendaAgenda • What is a Design Pattern? • Structural Patterns • Behavioral Patterns • Creational Patterns • Human Interaction Patterns • Summary
- 10. Design Patterns – Eran Toch 10November 2003 The Composite PatternThe Composite Pattern • Intent: – Compose objects into tree structures to represent part-whole hierarchies. – Composite lets clients treat individual objects and compositions of objects uniformly. This is called recursive composition.
- 11. Design Patterns – Eran Toch 11November 2003 MotivationMotivation When Processing the hierarchal collection, we need to query the type of the object (composite or primitive) and act differently Diagram reference: Design Patterns [Gof]
- 12. Design Patterns – Eran Toch 12November 2003 Composite: ApplicabilityComposite: Applicability • Use the Composite pattern when: – You want to represent part-whole hierarchies of objects. – You want clients to be able to ignore the difference between compositions of objects and individual objects. Clients will treat all objects in the composite structure uniformly.
- 13. Design Patterns – Eran Toch 13November 2003 Composite: StructureComposite: Structure • Define an abstract base class that specifies the uniform behavior. • Subclass the Primitive and Composite classes of the Component class. Diagram reference: Design Patterns [Gof]
- 14. Design Patterns – Eran Toch 14November 2003 Composite: ConsequencesComposite: Consequences • Benefits – It makes it easy to add new kinds of components – It makes clients simpler, since they do not have to know if they are dealing with a leaf or a composite component • Liabilities – It makes it harder to restrict the type of components of a composite
- 15. Design Patterns – Eran Toch 15November 2003 Composite: Known UsesComposite: Known Uses 1. A GUI system has window objects which can contain various GUI components (widgets) such as, buttons and text areas. A window can also contain widget container objects which can hold other widgets. Concrete examples: Java Swing, MFC 2. Folder based file system 3. Arithmetic expressions
- 16. Design Patterns – Eran Toch 16November 2003 AgendaAgenda • What is a Design Pattern? • Structural Patterns • Behavioral Patterns • Creational Patterns • Human Interaction Patterns • Summary
- 17. Design Patterns – Eran Toch 17November 2003 The Strategy PatternThe Strategy Pattern • Intent: – Define a family of algorithms, encapsulate each one, and make them interchangeable. – Strategy lets the algorithm vary independently from clients that use it.
- 18. Design Patterns – Eran Toch 18November 2003 Strategy: MotivationStrategy: Motivation The client wants to use the algorithm without knowing which one to use in advance. We want to add new algorithms without changing the client class.
- 19. Design Patterns – Eran Toch 19November 2003 Strategy: StructureStrategy: Structure • Encapsulate the behavior of the context class in a separate algorithm class. • The context class keeps a reference to the algorithm class and passes on the messages. • The algorithm class can be sub-classed to support multiple algorithms.
- 20. Design Patterns – Eran Toch 20November 2003 Strategy: ConsequencesStrategy: Consequences • Benefits – Provides an alternative to subclassing the Context class to get a variety of algorithms or behaviors – Keeps the client clean from large conditional statements – Provides a choice of implementations for the same behavior • Liabilities – Increases the number of objects – All algorithms must use the same Strategy interface – We still need conditional statements!
- 21. Design Patterns – Eran Toch 21November 2003 Strategy: Known UsesStrategy: Known Uses • QSIA Learning System • Layout Manager in Java Swing • Sorting algorithms, memory allocation algorithms and more…
- 22. Design Patterns – Eran Toch 22November 2003 AgendaAgenda • What is a Design Pattern? • Structural Patterns • Behavioral Patterns • Creational Patterns • Human Interaction Patterns • Summary
- 23. Design Patterns – Eran Toch 23November 2003 The Singleton PatternThe Singleton Pattern • Intent. – Ensure a class only has one instance, and provide a global point of access to it. • Motivation. – Sometimes, an application needs one, and only one, instance of an object. For example, we want just one window manager. Or just one factory for a family of products. – We need to have that one instance easily accessible. – And we want to ensure that additional instances of the class can not be created.
- 24. Design Patterns – Eran Toch 24November 2003 Singleton: StructureSingleton: Structure • Declare the single instance as a private static data member. • Provide a public static member function that encapsulates all initialization code, and provides access to the instance.
- 25. Design Patterns – Eran Toch 25November 2003 Singleton: Example CodeSingleton: Example Code public class Singleton { // The private reference to the one and only instance. private static Singleton uniqueInstance = null; // An instance attribute. private int data = 0; /** * Returns a reference to the single instance. * Creates the instance if it does not yet exist. */ public static Singleton instance() { if(uniqueInstance == null) uniqueInstance = new Singleton(); return uniqueInstance; } /** * The Singleton Constructor. Note that it is private! * No client can instantiate a Singleton object! */ private Singleton() {} }
- 26. Design Patterns – Eran Toch 26November 2003 Singleton: Example Code – cont’dSingleton: Example Code – cont’d public class TestSingleton { public static void main(String args[]) { // Get a reference to the single instance of Singleton. Singleton s = Singleton.instance(); // Set the data value. s.setData(34); System.out.println("First reference: " + s); System.out.println("Singleton data value is: " + s.getData()); // Get another reference to the Singleton. // Is it the same object? s = null; s = Singleton.instance(); System.out.println("nSecond reference: " + s); System.out.println("Singleton data value is: " + s.getData()); } } What will be the output?
- 27. Design Patterns – Eran Toch 27November 2003 AgendaAgenda • What is a Design Pattern? • Structural Patterns • Behavioral Patterns • Creational Patterns • Human Interaction Patterns • Summary
- 28. Design Patterns – Eran Toch 28November 2003 Human Interaction PatternsHuman Interaction Patterns • A Pattern language to describe human interaction situations • Examples: – Go Back to a Safe Place – Form – Control Panel – WYSIWYG Editor – Hierarchical Set – Map of Navigable Spaces – More…
- 29. Design Patterns – Eran Toch 29November 2003 Go Back to a Safe PlaceGo Back to a Safe Place • Problem: – How can the artifact make navigation easy, convenient, and psychologically safe for the user? • Forces: – A user may forget where they were, if they stop using the artifact while they're in the middle of something and don't get back to it for a while. – If the user gets into a space or a state that they don't want to be in, they will want to get out of it in a safe and predictable way. – The user is more likely to explore an artifact if they are assured that they can easily get out of an undesired state or space; that assurance engenders a feeling of security.
- 30. Design Patterns – Eran Toch 30November 2003 Go Back to a Safe PlaceGo Back to a Safe Place • Solution: – Provide a way to go back to a checkpoint of the user's choice. • Examples: – The "Home" button on a Web browser – Turning back to the beginning of a chapter in a physical book or magazine – The “undo" feature on some computer applications
- 31. Design Patterns – Eran Toch 31November 2003 AgendaAgenda • What is a Design Pattern? • Structural Patterns • Behavioral Patterns • Creational Patterns • Human Interaction Patterns • Summary
- 32. Design Patterns – Eran Toch 32November 2003 SummarySummary • Patterns learned: – Composite, Strategy, Singleton, Go Back to a Safe Place. • Advantages of Design Patterns: – They capture expertise and make it accessible to non- experts. – Their names collectively form a vocabulary that helps developers communicate better. – They help people understand a system more quickly when it is documented with the patterns it uses.
- 33. Design Patterns – Eran Toch 33November 2003 ReferencesReferences • Design Patterns: Elements of Reusable Object-Oriented Software, Gamma E. et el., 1995, Addison-Wesley. • A course from Bob Tarr from UMBC University http://www.research.umbc.edu/~tarr/dp/fall00/cs491.html • The Design Patterns Java Companion, James W. Cooper (an online version of the book) http://www.patterndepot.com/put/8/JavaPatterns.htm • A Site dedicated to Design Patterns by Vince Huston http://home.earthlink.net/~huston2/dp/patterns.html • Seven Habits of Successful Pattern Writers, John Vlissides http://hillside.net/patterns/papers/7habits.html • COMMON GROUND: A Pattern Language for Human- Computer Interface Design, Jenifer Tidwell, http://www.mit.edu/~jtidwell/common_ground_onefile.html
- 34. Design Patterns – Eran Toch 34November 2003 Doesn’t it go too far?Doesn’t it go too far? • Taken from Vince Huston’s site about Design Patterns:
- 35. Design Patterns – Eran Toch Methodologies in Information System Development Thanks!Thanks!