1. OO Design: S.O.L.I.D principles
Technical team
DNB 2015
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Presenters: Quang Nguyen, Lan Nguyen

2. Page 2Confidential
Outline
SOLID principles introduction
Purpose of use.
Definition, explanation, examples and benefits of
+ Single responsibility principle (SRP)
+ Open – close Principle (OCP)
+ Linkov's substitution principle (LSP)
+ Interface segregation principle (ISP)
+ Dependency inversion principle (DIP)
Conclusion
Q&A

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Design Matter: Doing Better with Less

4. Introduction
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S.O.L.I.D is an acronym for the first five object-oriented
design(OOD) principles by Robert C. Martin, popularly
known as Uncle Bob.
S – Single Responsibility principle
O – Open-closed principle
L – Liskov’s substitution principle
I – Interface segregation principle
D – Dependency Inversion principle

5. What are the issues that developers
always face to in software development
lifecycle?
Question

6. Requirements change
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Your requirements will always change (and grow) over
time
Your system need to evolve to handle the changes.
We should build a software which is smartly designed
enough to change.
.

7. What is “Smartly-designed Software” ?
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Handle the changes with minimal re-factor and effort.
Extendable
Reusable
Flexible

8. Customer and Developer satisfaction
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Customers are satisfied because the software:
+ Work well
+ Keeps working - always.
+ Can be changed, upgraded easily
+ Reuse to build other software.
+ Extendable, Flexible.
Help the developers:
+ Handle the changes easily.
+ Code better in any programming languages.

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PURPOSE OF USE
When combined together, these principles will help your
software:
 Become well and smartly-designed
 Easily handle the changes
 Make both customer and developer satisfied with their
work.
⇒Save money and time.

10. Single Responsibility principle (SRP)
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11. SRP - Definition
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“An object should have one and only one reason to
change”

12. SRP - Explanation
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Responsibility is a family of functions that serves
one particular purpose.
If you have a class having more than one reason to
change
=> you should split the class into multiple classes
base on their responsibilities.

13. SRP - Example
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14. SRP – Why splitting is important ?
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- If classes have more than one responsibility ?
- Each responsibility is an axis of change.
+ Codes become complicated
+ One change effects another
- Big classes:
+ Difficult to change
+ Harder to read
+ Bugs

15. SRP – Benefit
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Increase the reusability of source code
Make your classes smaller, lesser complicated.
Restrict effects to other modules when we change
source codes.
Easier to read and write => less to write bugs.
Higher cohesion, lower coupling.
Smaller classes and smaller methods will give you
more flexibility
More classes != more complicated

16. Open – Close Principle (OCP)
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17. OCP - Definition
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“An object should be opened for extension and
closed for modification”

18. OCP - Explanation
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- You should extend a class without modifying it.
- if you read and understand all source code to find
what they are supposed to do and update.
⇒ so hard
⇒ cost time, effort
⇒ bugs
- But it’s easy to extend the source code to meet
the new requirements.

19. OCP - Example
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20. OCP – Benefit
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- Restrict effects to others because when you
change code.
- Not need to read all old source codes that already
worked well.
- Extend the existing libraries.

21. - Template Method Design Pattern
- Strategy Design Pattern.
Reference

22. Liskov's Substitution (LSP) - Definition
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“Objects should be replaceable with their base
classes without altering the correctness of the
program”

23. LSP - Explanation
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+ Should always use a base class or interface
instead of the subclasses.
+ This works correctly with any subclasses of X.
⇒ Extends without replacing any base class's
functions

24. LSP - Example
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25. LSP – Benefit
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- This is an extension of OCP
⇒Wrong LSP => wrong OCP
⇒ Inherit all OCP's benefits.
- Preventing bugs caused by subclasses.

26. Interface segregation Principle (ISP)
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27. ISP - Definition
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“A client should not implement an interface, if it
doesn't use that”

28. ISP - Explanation
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Clients should not be forced to implement interfaces
they don't use
=> Split a fat interface which has many methods
into many small interfaces.

29. ISP - Example
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30. ISP – Benefit
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- Smaller interface => more reusable
- Extendable
- Flexible
- Allows people to use only the parts of objects that
they need.
- High cohesion, low coupling
- Comply SRP.

31. Dependence Inversion Principle (DIP)
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32. DIP - Definition
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“High level model should not depend on low level
module, both should depend on abstractions
Abstractions should not depend on details.
Details should depend on abstractions”

33. DIP - Explanation
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- Low level module: implement basic and primary
operations
- High level module: encapsulate complex logic
- The high level classes should not directly use and
heavily depend on low level classes.
=> we should create abstraction layer between high
level classes and low level classes

34. DIP - Example
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35. Demo - Explanation
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36. DIP - Explanation
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- Abstractions should not depend upon details
- Details should depend on abstractions

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- Spring frameworks' component
- Using Dependency Injection pattern
- Automatic class instantiation (from external files or
annotation)
- Entirely remove the low-level component dependencies
and achieve true DIP.
IoC Container & Dependency Injection

38. DIP – Benefit
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- Reusable
- Flexible
- More readable.
- Higher cohesion - lower coupling
- Remove dependencies between low and high level
modules.
- Violate DIP => Violate OCP or LSP.
- More classes != More complicated

39. Should we always apply DIP anywhere?
Question

40. Conclusion
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With S.O.L.I.D principles, our software:
 Easily handle the changes
 Become well and smartly-designed
 Make both customer and developer satisfied with their
work.
⇒Save money and time.

41. OOP vs OOD
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Object oriented design (OOD) = OOP + Design principles.
⇒The source code applied OOD will be:
+ Object oriented
+ Re-usable
+ Flexible: can be changed with minimal efforts.
+ Extendable: can be extended without changing existing
codes

42. Q&A
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43. References
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http://www.oodesign.com/
http://www.codeproject.com/Articles/703634/SOLID-
architecture-principles-using-simple-Csharp
https://scotch.io/bar-talk/s-o-l-i-d-the-first-five-principles-of-
object-oriented-design
Object oriented analysis and design book by Head First
Design pattern
Dependency Injection and IoC container in Spring Framework