Design Patternsfor 70% of genious programmers

Aamir KhanSolution Architect

Design Patternsfor 70% of programmers in the world

Saurabh VermaSolution Architect I blog at http://www.domaindrivendesign.info

Agenda• Do you OOPs?

• Design Patterns? Why

• Design Patterns? What

• Design Patterns? How

• I hate my job

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.

Shut up ! We know all this

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-DOIS-A Inheritance

Shahrukh 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 design patterns by purpose

Let’s understand the purpose

+

Creational Patterns

Structural Patterns

Final Product

Behavioral Patterns

Let’s understand the purpose

Design Patterns as per their PURPOSE

Creational Patterns

Factory Method

Factory MethodDefine 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

Factory MethodSystem.Data.SqlClient.SqlClientFactory

Abstract Factory Pattern

Abstract Factory PatternProvide 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 MethodSystem.Data.Common.DbProviderFactories

Singleton Pattern

Singleton PatternEnsure a class has only one instance, and provide a global point to it.

• Prevents creation of any additional instances, while simultaneously allowing global access

• Creation can be delayed until it is actually required

Singleton PatternSystem.Data.SqlClient.SqlClientFactory

Structural Patterns

Façade Pattern

Façade PatternProvide 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 PatternIts all about the service

Decorator Pattern

Decorator PatternAttach 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)

Decorator PatternSystem.IO.BufferedStream:Stream

Composite Pattern

Composite PatternCompose 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

Composite PatternSystem.Web.UI.Control (Render Mechanism)

Composite Pattern

Adapter Pattern

Adapter PatternConvert 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

Adapter PatternGearsTypelib.HtmlDialogHostClass

(Interop.GearsTypelib)

Decorator vs AdapterDecorator 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 FaçadeFaçade 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 PatternEncapsulate 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 invocationOne Receiver

andMultiple actions

Command PatternSystem.ComponentModel.Design.MenuCommand

Command PatternSaurabh.Learning.DesignPatterns.Patterns.CommandPa

ttern

Strategy Pattern

Strategy PatternDefine 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

Strategy PatternSystem.Collections.ArrayList

Template Method

Template MethodDefine 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

Template Method

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

I hate my job

Top 5 ADVICES NOT TO FOLLOW

Reflect when you want

Answers, anybody?

CONTENT STOLEN FROM BOOKS:Head First Design PatternsDesign Patterns in C#Design PatternsWEBSITES:Wikipedia, MSDN, doFactory and GOOGLE

Learn it all, at my blog http://www.domaindrivendesign.info

Don’t contact me at,saurabh.net [at] gmail.com

Saurabh VermaSolution Architect