[yidlug] computer/software design patterns
Post on 15-Jan-2015
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Welcome to YIDLUG
DESIGN PATTRENS
Agenda
• Do you OOPs?
• Design Patterns? Why
• Design Patterns? What
• Design Patterns? How
why care about all of this
Example of a speaker
The speakers check list
Speak the language
Know how to design a speech
The programmers check list
Know a computer language
Understand a program design
Programming without design
Or worse !
Getting startedRefresh on Object Oriented Programming (OOP)
Normal Classes vs Abstract Classes vs InterfacesNormal Class: This is a normal class
Abstract Class: These cannot be instantiated directly. They may provide partial implementations and exists solely for the purpose of inheritance.
Interfaces: Defines a contract. Concrete implementations needs to implement each property and method as per contract.
Million Dollar Question? What is Why?
Normal Class vs Abstract Class?
If a particular class is a entity which is not
required to be instantiated by the
application directly, go for Abstract Class
Employee can be abstract class
Abstract Class vs Interfaces?
If a particular class contains any default
behavior, go for Abstract Class
If every concrete class has its own
implementation, go forInterfacesWork – Management, Programming
MOVIES & ACTORS
Shahrukh KhanSalman Khan riding bike
Actors in a Movie
IS-A InheritanceShahrukh Khan is an ACTOR
HAS-A CompositionMovie has ACTORS
CAN-DO InterfacesSalman Khan can DRIVE BIKE
IS-A vs HAS-A vs CAN-DO
IS-A InheritanceShahrukh Khan is an ACTOR
HAS-A CompositionMovie has ACTORS
CAN-DO InterfacesSalman Khan can DRIVE BIKE
What is a Pattern
• An 'element of reusable software’
• A design pattern systematically names, motivates, and explains a general design that addresses a recurring design problem in object-oriented systems.
• It describes the problem, the solution, when to apply the solution, and its consequences.
• It also gives implementation hints and examples. The solution is a general arrangement of objects and classes that solve the problem. The solution is customized and implemented to solve the problem in a particular context.
History of Software Patterns
What Wikipedia says,
• A design pattern is a general repeatable solution to a commonly occurring problem in software design.
• A design pattern is not a finished design that can be transformed directly into code.
• Algorithms are not thought of as design patterns.
Let’s understand the purpose
Let’s understand design patterns by purpose
Let’s understand the purpose
+
Creational Patterns
Structural Patterns
Final Product
Behavioral Patterns
Design Patterns as per their PURPOSE
Creational Patterns
Factory Method
Factory Method Provide an interface for the creation of objects. Allow subclasses to “decide” which class to instantiate.
Factory Method
Define an interface for creating an object, but let subclass decide which class to instantiate
• Prevents creation of any additional instances, while simultaneously allowing global access
• Creation can be delayed until it is actually required
Abstract Factory Pattern
Abstract Factory Pattern
Provide an interface for creating families of related or dependent objects without specifying their concrete classes.
• Provides creation of families of related or dependent objects without specifying their concrete class
Abstract Factory Povide an interface for creating families of related or dependent objects. A factory for factories.
Singleton Ensure a class only has one instance, and provide a global point of access to it.
Prototype Make variations on copies of a basic-object.
Structural Patterns
Façade Pattern
Façade Pattern
Provide a unified interface to set of interfaces in a subsystem.
• Is generally one side of the exterior of a building, especially the front
• Origin from a FRENCH word means “face” or “front”
Façade Pattern
Its all about the service
Decorator Pattern
Decorator Pattern
Attach additional responsibilities to an object dynamically.
• Flexible alternative to subclassing for extending functionality
• Allows dynamic and transparent addition and removal of responsibilities without affecting object
simple actress(act)
in role of modern girl(go out and act)
in role of homely girl(be in home and act)
Composite Pattern
Composite Pattern
Compose objects into tree structures to represent part-whole hierarchies
• When dealing with collections of objects, there are often operations that are appropriate for both a single object and the entire collection
• Treats individual objects and composition of objects uniformly
Example: a)Every object “IS-A” shapeb)Every shape “HAS-A” draw behavior
Adapter Pattern
Adapter Pattern
Convert the interface of a class into another interface clients expect
• Lets classes work together that couldn’t work otherwise
• Works good for making compliance
ApplicationFrameworkAdapter
LegacyComponent
Decorator vs Adapter
Decorator looks like Adapter which “decorates” an existing class to make it compatible. Though, Decorator doesn’t wants you to decorate every method in the Adapted class. But who has stopped from doing it.
“Never underestimate the power of a developer”
Decorator vs Façade
Façade is a way of hiding a complex system inside a simpler interface, whereas Decorator adds function by wrapping a class.
Behavioural Patterns
Command Pattern
Command Pattern
Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests.
• If all commands can service a specific request, they can be entertained by the invoker
• Concrete command performs the action on Receiver
• Encapsulate method invocation
One Receiver
andMultiple actions
Strategy Pattern
Strategy Pattern
Define strategies for a performing a behavior and change them dynamically as per requirement
• This lets the algorithm vary independently from clients that uses it
• It is about choice, which affects outcomes
Template Method
Template Method
Define the skeleton of an algorithm in an operations, deferring some steps to subclass
• Main algorithm skeleton is defined in a base class and subclasses can then plug in their own details without affecting the algorithm itself
• Uses Inheritance – Abstract Class defines the template (algorithm) and Concrete class manages the implementation.
• Template uses the implementation on demand
Strategy Pattern vs Template Method• Strategy allows callers to change the algorithm or order of steps whereas Template method allow steps to be modified
• Strategy uses Interfaces whereas Template method uses Inheritance
CONTENT STOLEN FROM
BOOKS:
Head First Design Patterns
Design Patterns in C#
Design Patterns
WEBSITES:
Wikipedia, MSDN, doFactory and GOOGLE
Answers, anybody?
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