Design Patterns for 70% of genious programmers Aamir Khan Solution Architect

Design Patterns for 70% of programmers in the world Saurabh Verma Solution 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

Do you OOPs?

Design Patterns? Why

Design Patterns? What

Design Patterns? How

I hate my job

Normal Classes vs Abstract Classes vs Interfaces Normal 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 for Interfaces Work – Management, Programming

MOVIES & ACTORS Shahrukh Khan Salman Khan riding bike Actors in a Movie

IS-A vs HAS-A vs CAN-DO IS-A Inheritance Shahrukh Khan is an ACTOR HAS-A Composition Movie has ACTORS CAN-DO Interfaces Salman Khan can DRIVE BIKE IS-A Inheritance Shahrukh Khan is an ACTOR HAS-A Composition Movie has ACTORS CAN-DO Interfaces Salman 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.

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.

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 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

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

Factory Method System.Data.SqlClient.SqlClientFactory

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

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 Method System.Data.Common.DbProviderFactories

Singleton Pattern

Singleton Pattern Ensure 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

Ensure 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 Pattern System.Data.SqlClient.SqlClientFactory

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”

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 )

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

Decorator Pattern System.IO.BufferedStream:Stream

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: Every object “IS-A” shape Every shape “HAS-A” draw behavior

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:

Every object “IS-A” shape

Every shape “HAS-A” draw behavior

Composite Pattern System.Web.UI.Control (Render Mechanism)

Composite Pattern

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 Application Framework Adapter Legacy Component

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

Adapter Pattern GearsTypelib.HtmlDialogHostClass (Interop.GearsTypelib)

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 and Multiple actions

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

Command Pattern System.ComponentModel.Design.MenuCommand

Command Pattern Saurabh.Learning.DesignPatterns.Patterns.CommandPattern

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

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

Strategy Pattern System.Collections.ArrayList

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

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

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

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 Patterns Design Patterns in C# Design Patterns WEBSITES: 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 Verma Solution Architect