Introduction to Design Patterns

Design Patterns Abhishek Chatterjee

Agenda
Introduction
Coding practices
Is there room for improvement?
Object Oriented Analysis and Design
Design Patterns – an intro
An orientation example – Strategy Pattern
Inversion of Control – Dependency Injection
Mediator Pattern
Quick look at a practical example

Introduction
This is nothing about a new technology or a framework
This is all about how and where to write code so that
The solution is scalable
The solution is flexible
Chances of errors are reduced
The solution is imagined in a more real-world way
We identify some good practices and call them names

Coding practices
Following naming conventions
Writing commented code
Wrapping code in regions
Etc. etc. etc.
What is beyond it?

Object Oriented Analysis and Design
We all know what is OOP
Figuring how good OOP will help our solution is OOAD

Design Patterns – an intro
What are design patterns?
A pattern of OOAD principle which can be applied to multiple problem scenarios is called a Design Pattern
Design Pattern is nothing but a good OOAD design and given a name

An Orientation Example – Strategy Pattern Duck quack() swim() abstract display() MallardDuck display() RedheadDuck display() Lots of other types of ducks

An Orientation Example – Strategy Pattern Duck quack() swim() abstract display() fly() MallardDuck display() RedheadDuck display() Lots of other types of ducks

An Orientation Example – Strategy Pattern Duck quack() swim() abstract display() fly() MallardDuck display() RedheadDuck display() Yay!! I can fly too!!! RubberDuck display()

Hey Abbey-Shack…. If you want to try Monster.com’s resume services, now is the time.

Who is going to rescue me now? <<Interfaces>> !!! Yes!!! That’s it! I make an IFlyable interface and RubberDuck doesn’t get to implement it… MallardDuck <<IFlyable>> display() RedheadDuck <<IFlyable>> display() RubberDuck //You can’t fly! display() They can’t see me happy 

I… I… I… I have a Question! SimCorp simulates 50 ducks… are you saying you are going to write 50 fly methods? What if there is a change in flying style and it effects 20 ducks… will all 20 ducks change? I am losing money you know…

I thought she was non-technical…
Well here is my situation
I can’t put the fly() method in the base class
If I use interface, I can’t reuse code
Alright, so this calls for a dependency split
Flying is a behavior and should be separate from the Duck object
Flying behaviors could be reused on different objects
Different ducks could fly in different ways

Strategy Pattern in Action Duck quack() swim() abstract display() MallardDuck IFlyBehavior:FlyWithWings display() RubberDuck IFlyBehavior: DontFly display() IFlyBehavior fly() FlyWithWings FlyWithRocket DontFly

Inversion of Control – Dependency Injection
Creating a dependency between two objects
Catering to that dependency without affecting scalability of the solution
How does MallardDuck know if it should use FlyWithWings behavior? Simple:
public class MallardDuck : Duck
{
IFlyBehavior flyBehavior;
public MallardDuck()
{
this.flyBehavior = new FlyWithWings();
}
}

The problem with that is…
When there is a defaulter…
For example, let’s take RubberDucks
public class RubberDuck : Duck
{
public RubberDuck()
{
this.flyBehavior = new DontFly();
}
}
So you thought I couldn’t fly? Wheeee… Look at my rocket boosters!

Constructor Injection
public class RubberDuck : Duck
{
public RubberDuck(IFlyBehavior flyBehavior)
{
this.flyBehavior = flyBehavior;
}
}
Now anyone instantiating a Duck object needs to inject the dependent class, solving the design problem of the Rubber duck with boosters

Thank You

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Introduction to Design Patterns

by neodimension on Dec 07, 2009

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Introduction to Design Patterns — Presentation Transcript