The presentation discusses software design. It discusses the characteristics of a good and bad design. Then it talks about how to achieve a good design. Then finally we discuss the SOLID Principles of Object Oriented Design. These are 5 principles compiled by Rober Cecil Martin aka Uncle Bob. The benefit of these principles is to achieve a good OO design which is high in cohesion and low in coupling thus easily adaptable to change

1. Boutique product development company
It is amazing what you can accomplish when you have a client-centric team to deliver outstanding products.

2. SOLID Principles of OO Design
Waleed Bin Dawood | Software Engineer
Boutique product development company
It is amazing what you can accomplish when you have a client-centric team to deliver outstanding products.

3. "Any fool can write code that a computer can
understand. Good programmers write code that humans
can understand"
Martin Fowler
SOLID Principles of Object Oriented Design
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Software design
What makes a design good or bad ?
Design/Code Smells
SOLID Principles
Waleed Bin Dawood | Software Engineer

4. SOLID Principles
What is software design ?
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The source code is the design
UML diagram represents part of a design
Software design process includes coding, testing,
refactoring…
The programmer is the actual software designer.
Waleed Bin Dawood | Software Engineer

5. SOLID Principles
Why do we need a good design ?
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To deliver fast
To manage change easily
To deal with complexity
Waleed Bin Dawood | Software Engineer

6. SOLID Principles
How to identify a bad design ?
“In my BillG review meeting, the whole
reporting hierarchy was there...and a
person...whose whole job during the
meeting was to keep an accurate
count of how many times Bill said the F
word. The lower the f***-count, the
better.”
-- Joel Spolsky, My First BillG Review
But may be we need some better criteria :)
Waleed Bin Dawood | Software Engineer

7. SOLID Principles
How to identify a bad design ?
Design/Code Smells
● Rigidity - The design is hard to change
● Fragility - The design is easy to break
● Immobility - The design is hard to reuse
● Viscosity - It is hard to do the right thing
Waleed Bin Dawood | Software Engineer

8. SOLID Principles
Good design
What are the characteristics of a good design ?
● High Cohesion
● Low Coupling
How to achieve a good design ?
Follow programming practices of your
language/framework
Follow OO design principles
Use design patterns
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Waleed Bin Dawood | Software Engineer

9. SOLID Principles
Let’s go SOLID
Initial
Stands for
(acronym)
Concept
S
SRP
Single responsibility principle
a class should have only a single responsibility.
O
OCP
Open/closed principle
“software entities … should be open for extension, but closed for modification”.
L
LSP
Liskov substitution principle
“objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program”.
I
ISP
Interface segregation principle
“many client-specific interfaces are better than one general-purpose interface.”
D
DIP
Dependency inversion principle
one should “Depend upon Abstractions. Do not depend upon concretions.”
Waleed Bin Dawood | Software Engineer

10. SOLID Principles
Single Responsibility Principle
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“There should never be more than one reason for a class
to change.” -- Robert Martin, SRP paper
Easier : A class should concentrate on doing one thing
and one thing only
It give you high cohesion
It’s often hard to identify different responsibilities
Waleed Bin Dawood | Software Engineer

11. SOLID Principles
Single Responsibility Principle
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Is SRP violated here ?
Two responsibilities:
o Connection management
o Data Communication
Waleed Bin Dawood | Software Engineer

12. SOLID Principles
Single Responsibility Principle
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Solution :
Two responsibilities:
o Connection management
o Data Communication
Waleed Bin Dawood | Software Engineer

13. SOLID Principles
Open/Closed Principle
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“Software entities should be open for extension, but
closed for modification.” -- Robert Martin paraphrasing
Bertrand Meyer, OCP Paper
Easier : you should be able to extend the behaviour of a
module without changing it
Abstraction is the key
Waleed Bin Dawood | Software Engineer

14. SOLID Principles
Open/Closed Principle
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Is OCP violated here ?
Waleed Bin Dawood | Software Engineer

15. SOLID Principles
Open/Closed Principle
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Solution:
Waleed Bin Dawood | Software Engineer

16. SOLID Principles
Liskov Substitution Principle
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“Functions that use pointers or references to base
classes must be able to use objects of derived classes
without knowing it.” -- Robert Martin, LSP Paper
Easier : Subclasses should behave nicely when used in
place of their parent class
Waleed Bin Dawood | Software Engineer

17. SOLID Principles
Liskov Substitution Principle
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How would you model the relationship between a square
and a rectangle ?
Should the square class extends rectangle ?
Many of you would say “yes” or “why not”. Square is a
kind of Rectangle after all. An obvious IS-A relationship
Issues in this Approach:
o Square has unnecessary attribute ‘width’
o What happens when we want to calculate area ?
o What happens if the client code is assuming height
and width to be independent of each other
Waleed Bin Dawood | Software Engineer

18. SOLID Principles
Dependency Inversion Principle
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“A. High level modules should not depend upon low level
modules. Both should depend upon abstractions.
B. Abstractions should not depend upon details. Details
should depend upon abstractions.”-- Robert Martin, DIP
paper
Easier:
o Use lot of interfaces (program to interface)
o Use abstractions
Waleed Bin Dawood | Software Engineer

19. SOLID Principles
Interface Segregation Principle
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“Clients should not be forced interfaces that they do not
use.”-- Robert Martin, ISP paper
Easier : avoid fat interfaces (cohesion)
Waleed Bin Dawood | Software Engineer

20. Human Computer Interaction
Thank you for your time
If you have any questions,
please ask now or forever
hold your peace
Waleed Bin Dawood | Software Engineer

21. Human Computer Interaction
References
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http://en.wikipedia.org/wiki/SOLID_(object-oriented_design)
http://www.slideshare.net/bbossola/geecon09-solid
http://butunclebob.com/ArticleS.UncleBob.PrinciplesOfOod
http://www.slideshare.net/intellizhang/the-oo-design-principles
http://www.slideshare.net/enbohm/solid-design-principles-9016117
http://www.oodesign.com/design-principles.html
http://objectmentor.com/resources/publishedArticles.html
Waleed Bin Dawood | Software Engineer