Design pattern

Mhd Adel Alajami 1
DESIGN PATTERN
8/29/2014 Mhd Adel Alajami

Agenda
Mhd Adel Alajami 8/29/2014
2
 Understanding the need for design patterns
 What are design patterns
 Using design patterns
 State Design Patter
 Singleton Design Patter
 Façade Design Patter

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

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

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

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

What are design pattern ?
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 “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

Where did DP come from ?
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 23 original pattern
 Now, there are many patterns for many
different software problems

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

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

State Machine
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Classic Way – Without Pattern
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We Need Another State
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We Need Another State
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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

Think of Each State as an
object
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Think of Each State as an
object

Class Diagram
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Implement State Design Pattern
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Advantage
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Singleton Design Pattern
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Singleton Uses
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Why can’t I Just Instantiate
One?
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Prevent Instantiation
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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

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What About Singleton With
Multithreading

Before Façade Design Pattern
Need to communicate
with
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Client Classes
Subsystem classes

Façade Design Pattern
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Client Classes
Subsystem classes
Facade

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

Any Questions ?
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Design pattern

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