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?