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This presentation intends to be an introduction to Design Patterns, and elaborate their importance with a small example. Its intention is to orient the audience to start exploring more on the topic.
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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
Duckquack()swim()abstract display()
Duckquack()swim()abstract display()
MallardDuck
display()
MallardDuck
display()
RedheadDuck
display()
RedheadDuck
display()
Lots of other types of ducks
An Orientation Example – Strategy Pattern
Duckquack()swim()abstract display()fly()
Duckquack()swim()abstract display()fly()
MallardDuck
display()
MallardDuck
display()
RedheadDuck
display()
RedheadDuck
display()
Lots of other types of ducks
An Orientation Example – Strategy Pattern
Duckquack()swim()abstract display()fly()
Duckquack()swim()abstract display()fly()
MallardDuck
display()
MallardDuck
display()
RedheadDuck
display()
RedheadDuck
display()
Lots of other types of ducks
An Orientation Example – Strategy Pattern
Duckquack()swim()abstract display()fly()
Duckquack()swim()abstract display()fly()
MallardDuck
display()
MallardDuck
display()
RedheadDuck
display()
RedheadDuck
display()
Yay!! I can fly too!!!
Yay!! I can fly too!!!
RubberDuck
display()
RubberDuck
display()
Hey Abbey-Shack…. If you want to try
Monster.com’s resume services, now is the time.
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()
MallardDuck <<IFlyable>>display()
RedheadDuck <<IFlyable>>display()
RedheadDuck <<IFlyable>>display()
RubberDuck//You can’t fly!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
Duckquack()swim()abstract display()
Duckquack()swim()abstract display()
MallardDuckIFlyBehavior:FlyWithWingsdisplay()
MallardDuckIFlyBehavior:FlyWithWingsdisplay()
RubberDuckIFlyBehavior: DontFly display()
RubberDuckIFlyBehavior: DontFly display()
IFlyBehaviorfly()
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• {
– IFlyBehavior flyBehavior;– 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• {
– IFlyBehavior flyBehavior;– 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
Q & A
Thank You
