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DESIGN PATTERNS -- Mudabbir Warsi
AGENDA ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]
Introduction ,[object Object],“ 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, with out ever doing it the same way twice” --   Christopher Alexander: The Timeless way of Building ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]
Myths about Design Patterns ,[object Object],[object Object],[object Object],[object Object],[object Object]
Design Principles ,[object Object],[object Object],“ A module should be open for extension but closed for modification.” ,[object Object],[object Object],“ Sub-classes should be substitutable for their base class .” “ There should never be more than one reason for a class to change .” ,[object Object],“ Talk only to your immediate friends .” “ Depend in the direction of stability .”
Design Principles Contd… ,[object Object],[object Object],“ Solve the most important problem in the simplest possible way.” ,[object Object],[object Object],“ Every piece of knowledge must have a single, unambiguous and authoritative representation with in the system .” “ Many client specific interfaces are better than one general purpose interface .” ,[object Object],“ Classes that aren’t used together, should not be grouped together .” “ Depend upon Abstractions. Never depend upon Concretions .”
Basic Elements of a Design Pattern ,[object Object],[object Object],[object Object],[object Object]
Categories of Design Patterns ,[object Object],Abstract Factory Factory Method Singleton Prototype Builder Structural Adapter Decorator Flyweight Composite Bridge Facade Proxy Behavioral Mediator Iterator Visitor Observer Chain of Responsibility Strategy State Momento Command Interpreter Template Method Let you compose objects and classes into larger structures Involve object Instantiation and all provide a way to decouple a client from the object  it needs to create  Concerned with how objects and classes interact and distribute responsibility
The Pattern Life Cycle Real World  Projects Pattern Mining Author Version Preliminary Document -ation Pattern Polishing Reusable Version Pattern Reuse Incident Occurs of a pattern Discover Patterns Document Document Analyse /  Rule of Three Feedback Modification PHASE 1: MINING Author World PHASE 2: POLISHING Pattern Community World PHASE 3: REUSE Pattern User World
Pattern Hatching 4. A Miracle Occurs 7. Test the Solution 6. Instantiate the Pattern 5. Evaluate Solution 1. Familiarize yourself with Patterns 2. Characterize nature of u r problem 3. Apply Matching
Pattern Refactoring ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]
Example Design Patterns ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Singleton static getInstance( ) // Other useful Methods static uniqueInstance // Other useful data return uniqueInstance ,[object Object],[object Object]
Singleton Contd… ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]
Singleton Contd… ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]
Inside Model-View-Controller 1. The User did something 2. Change your state 3. Change your display 4. I have changed 5. I need your state information VIEW Gives you a presentation of the model. It gets the state and data to be displayed from the model CONTROLLER Takes user input and figures out what it means  to the model MODEL The model holds all the data, state and application logic. It provides an interface to manipulate and retrieve its state.
A Closer Look 1. The User did something 2. Change your state 3. Change your display 4. I have changed 5. I need your state information STRATEGY The view and controller implement the  STRATEGY  pattern: The view is an object that is configured with a strategy. The controller provides the strategy.  Using the strategy pattern keeps the view decoupled from the model. OBSERVER The model implements the  OBSERVER  to keep interested objects updated when the state change occurs.  Using the observer patterns keeps the model decoupled with the view n controller COMPOSITE The view implements the  COMPOSITE  pattern: The display consists of nested set of windows, panels, buttons, text boxes etc.  When the controller tells the view to update, it only has to tell the top view component and the  COMPOSITE  takes care of the rest.
Observer I’d like to register as an observer My State  has changed Observable Observers All these Observers will be notified whenever state changes in the Model Any object that’s interested in the state change of the Model registers with the Model as an Observer.  INTENT: Define a one – many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
Strategy The user did something We can swap in another behavior for the View by changing the Controller The Controller is the Strategy for the View – it’s the object that knows how to handle the user action. The View delegates to the Controller to handle the user actions The View only worries about the presentation, the Controller worries about translating user inputs to actions on the Model INTENT: Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithms vary independently from the client that uses it.
Composite Paint ( ) The View is a Composite of GUI components ( buttons, labels, text boxes etc). The top level component contains other components, which contain other components and so on until you get to the leaf nodes. ,[object Object],[object Object],[object Object],INTENT: Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.
MVC in Action ,[object Object],Servlet / Controller JSP / View MODEL / DB / Business Logic Bean 1. HTTP Request 5. HTTP Response 2 3 4 ,[object Object],[object Object],[object Object]
EXPERT COMMENTS Today there are more patterns than in the GOF book: Learn them as well. Go for simplicity and don’t become over-excited. If you can come up with a simpler solution without applying a pattern, then go for it. Shoot for practical extensibility. Don’t provide hypothetical generality; be extensible in ways that matter. Patterns are Tools not Rules- They need to be tweaked and adapted to your problem. John Vlissides Richard Helm Ralph Johnson Eric Gamma
Q & A

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Design patterns

  • 1. DESIGN PATTERNS -- Mudabbir Warsi
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  • 9. The Pattern Life Cycle Real World Projects Pattern Mining Author Version Preliminary Document -ation Pattern Polishing Reusable Version Pattern Reuse Incident Occurs of a pattern Discover Patterns Document Document Analyse / Rule of Three Feedback Modification PHASE 1: MINING Author World PHASE 2: POLISHING Pattern Community World PHASE 3: REUSE Pattern User World
  • 10. Pattern Hatching 4. A Miracle Occurs 7. Test the Solution 6. Instantiate the Pattern 5. Evaluate Solution 1. Familiarize yourself with Patterns 2. Characterize nature of u r problem 3. Apply Matching
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  • 15. Inside Model-View-Controller 1. The User did something 2. Change your state 3. Change your display 4. I have changed 5. I need your state information VIEW Gives you a presentation of the model. It gets the state and data to be displayed from the model CONTROLLER Takes user input and figures out what it means to the model MODEL The model holds all the data, state and application logic. It provides an interface to manipulate and retrieve its state.
  • 16. A Closer Look 1. The User did something 2. Change your state 3. Change your display 4. I have changed 5. I need your state information STRATEGY The view and controller implement the STRATEGY pattern: The view is an object that is configured with a strategy. The controller provides the strategy. Using the strategy pattern keeps the view decoupled from the model. OBSERVER The model implements the OBSERVER to keep interested objects updated when the state change occurs. Using the observer patterns keeps the model decoupled with the view n controller COMPOSITE The view implements the COMPOSITE pattern: The display consists of nested set of windows, panels, buttons, text boxes etc. When the controller tells the view to update, it only has to tell the top view component and the COMPOSITE takes care of the rest.
  • 17. Observer I’d like to register as an observer My State has changed Observable Observers All these Observers will be notified whenever state changes in the Model Any object that’s interested in the state change of the Model registers with the Model as an Observer. INTENT: Define a one – many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
  • 18. Strategy The user did something We can swap in another behavior for the View by changing the Controller The Controller is the Strategy for the View – it’s the object that knows how to handle the user action. The View delegates to the Controller to handle the user actions The View only worries about the presentation, the Controller worries about translating user inputs to actions on the Model INTENT: Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithms vary independently from the client that uses it.
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  • 21. EXPERT COMMENTS Today there are more patterns than in the GOF book: Learn them as well. Go for simplicity and don’t become over-excited. If you can come up with a simpler solution without applying a pattern, then go for it. Shoot for practical extensibility. Don’t provide hypothetical generality; be extensible in ways that matter. Patterns are Tools not Rules- They need to be tweaked and adapted to your problem. John Vlissides Richard Helm Ralph Johnson Eric Gamma
  • 22. Q & A