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Design Patterns.ppt
- 1. Introduction to Design Patterns “Reusable Object Oriented Software Elements”
- 2. What Is A Pattern? • "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 a way that you can use this solution a million times over, without ever doing it the same way twice." -- Christopher Alexander 1977
- 3. Design Patterns • “Each pattern is a three-part rule, which expresses a relation between a certain context, a problem and a solution.” • Hence, the common definition of a pattern: “A solution to a problem in a context.”
- 4. Why Patterns? – "Designing object-oriented software is hard and designing reusable object-oriented software is even harder." - Erich Gamma – Experienced designers reuse solutions that have worked in the past – Well-structured object-oriented systems have recurring patterns of classes and objects – Knowledge of the patterns that have worked in the past allows a designer to be more productive and the resulting designs to be more flexible and reusable
- 5. Common Uses • Common vocabulary to all developers. • Best Practices.
- 6. Properties Patterns do • Provide common vocabulary • Provide “shorthand” for effectively communicating complex principles • Help document software architecture • Capture essential parts of a design in compact form • Show more than one solution • Describe software abstractions Patterns do not • Provide exact solution • Solve all design problems • Only apply for object-oriented design
- 7. Becoming a Software Design Master • First learn the rules – E.g., the algorithms, data structures and languages of software. • Then learn the principles – E.g., structured programming, modular programming, object- oriented programming, generic programming, etc. • However, to truly master software design, one must study the designs of other masters • These designs contain patterns must be understood, memorized, and applied repeatedly • There are hundreds of these patterns
- 8. Software Patterns History – – – – – 1987 - Cunningham and Beck used Alexander’s ideas to develop a small pattern language for Smalltalk 1990 - The Gang of Four (Gamma, Helm, Johnson and Vlissides) begin work compiling a catalog of design patterns 1991 - Bruce Anderson gives first Patterns Workshop at OOPSLA 1993 - Kent Beck and Grady Booch sponsor the first meeting of what is now known as the Hillside Group 1994 - First Pattern Languages of Programs (PLoP) conference 1995 - The Gang of Four (GoF) publish the Design Patterns book
- 9. Types Of Software Patterns – Analysis – Design – Organizational – Process – Project Planning – Configuration Management
- 10. GoF Classification Of Design Patterns – Purpose - what a pattern does • Creational Patterns › Concern the process of object creation • Structural Patterns › Deal with the composition of classes and objects • Behavioral Patterns › Deal with the interaction of classes and objects – Scope - what the pattern applies to • Class Patterns › Focus on the relationships between classes and their subclasses › Involve inheritance reuse • Object Patterns › Focus on the relationships between objects › Involve composition reuse
- 11. Classification of design patterns Scope: domain over which a pattern applies Purpose: reflects what a pattern does
- 12. GoF Pattern Template – Pattern Name and Classification • A good , concise name for the pattern and the pattern's type •Short statement about what the pattern does – Also Known As • Other names for the pattern •A scenario that illustrates where the pattern would be useful – Applicability • Situations where the pattern can be used – Intent – Motivation
- 13. GoF Pattern Template (Continued) – Structure • A graphical representation of the pattern • The classes and objects participating in the pattern – Collaborations • How to do the participants interact to carry out their responsibilities • What are the pros and cons of using the pattern? – Implementation •Hints and techniques for implementing the pattern – Consequences – Participants
- 14. GoF Pattern Template (Continued) – Sample Code • Code fragments for a sample implementation • Examples of the pattern in real systems – Related Patterns • Other patterns that are closely related to the pattern – Known Uses
- 15. Benefits Of Design Patterns – Capture expertise and make it accessible to non-experts in a standard form – Facilitate communication among developers by providing a common language – Make it easier to reuse successful designs and avoid alternatives that diminish reusability – Facilitate design modifications – Improve design documentation – Improve design understandability
- 16. OBSERVER Intent: Define one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically. Also Known as : Dependents, Publish-Subscribe, Model-View
- 17. OBSERVER PROBLEM CONTEXT • An object (called as a subject) is the source of event • One or more objects (called as observers) want to know when the event occurs SOLUTION • Define a subject class that provides a method to attach observers to it • Subject maintains the set of observers attached to it • An Observer interface defines a method as a means to receive notification from subject (the event source) • All concrete observers implement this interface type. They realize the interface method to define action to be taken on event generation
- 18. … Observer • On generation of event, the subject notifies all the observers attached to it through their common interface method
- 19. …Observer CLASS DIAGRAM Subject attach(Observer O) <<interface>> Observer rec_notification() Concrete Observer1 rec_notification() Concrete Observer2 rec_notification() Implements action to be taken on receiving notification from subject
- 20. Events on JComponents are received and processed using Observer JButton addActionListener() <<interface>> ActionListener actionPerformed() Concrete Observer1(A class implementing ActionListener) actionPerformed() Concrete Observer2 actionPerformed() Implements action to be taken on receiving notification from Jbutton when it is clicked
- 21. Name in Design Pattern Actual Name Subject – JButton Observer – ActionListener ContreteObserver – class that implements ActionListener i/f type attach() – add ActionListener rec_notification() – actionPerformend Observer Design Pattern
- 22. Program to demonstrate observer pattern
- 23. Consequence: • Abstract coupling between Subject and Observer. • Support for broadcast communication