04/09/2023

Mhd Adel Alajami 1

DESIGN PATTERN

Mhd Adel Alajami

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Agenda

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Understanding the need for design patterns

What are design patterns

Using design patterns

State Design Patter

Singleton Design Patter

Façade Design Patter

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Software Development

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Software Development

Softwareindustry is like other industry

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Software Development

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Software Development

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the need for design patterns

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Software changes

We need to add a new features - boss

There is a change in the specification -

customer

We have found a big bug – team

Flexible code

Maintainable Code

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Use Design Pattern

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By learning theme

And then applying them in your OOP

designs

You will create more flexible and

maintainable code

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Dealing with change

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Software has lots of ways to change

There are many design pattern to deal

with changes

Design patterns address many of the

ways software changes over time

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What are design pattern ?

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Design patterns are general solutions to common problems

A pattern is a guideline for flexible and resilient code design

It does not go in detail of implementation

There is no DP for Banking system and other for games

It is a general solution for common problem in software development

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Mhd Adel Alajami

“I have a problem, when one of my

object changes , I need to let all other

objects know. Is there a good way to do

that?”

This is a common problem in software

development

There is a proven method to solve it :

The Observer Pattern

What are design pattern ?

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Where did DP come from ?

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23 original pattern

Now, there are many patterns for many

different software problems

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How To Use Design Pattern

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A design pattern is not a library, module

or package

It’s a guideline for how to solve a

problem

Higher level than a library

First understand the pattern, and then

use its design in your software

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Using Design Pattern

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Ultimate in reuse

Build from years of software

development experience

Save times, and trial and error

Not reusing code, but reusing experience

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State Machine

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Classic Way – Without Pattern

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Classic Way – Without Pattern

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Classic Way – Without Pattern

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We Need Another State

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SoftwareChanges

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We Need Another State

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We Need a Pattern Design

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Disadvantage

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Is not really OO at all

Any additions require many change to

code

Difficult to understand all the state and

transitions

Violates open closed principle

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Think of Each State as an object

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Think of Each State as an object

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Class Diagram

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Implement State Design Pattern

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Implement State Design Pattern

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Implement State Design Pattern

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Advantage

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Singleton Design Pattern

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Prevent Creation Multi Instance

Provide Global Access

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Singleton Uses

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Why can’t I Just Instantiate One?

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No Protection

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Prevent Instantiation

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Can’t Instantiation From Outside

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Prevent Instantiation

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But Every Time Will Create New Instance !!!

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Prevent Instantiation

That Ok

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Class Diagram

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What About Singleton With Multithreading

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To Avoid creation Multi Instance in Multithreading

Initialize Static Variable

Create instance even it’s not used

Disadvantage

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What About Singleton With Multithreading

synchronized

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Before Façade Design Pattern

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Client Classes

Subsystem classes

Need to communicatewith

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Façade Design Pattern

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Client Classes

Subsystem classes

Facade

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Benefits

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Shields clients from subsystem classes; reduces

the number of objects that clients deal with.

Promotes weak coupling between subsystem

and its clients.

Helps in layering the system. Helps eliminate

circular dependencies.

Makes using the subsystem easier

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Any Questions ?

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