1. The OO Design PrinciplesThe OO Design Principles

2. Agenda
Need of Design
Design Smells
2
Class Design Principles
Summary

3. Agenda
Need of Design
Design Smells
3
Class Design Principles
Summary

4. What's meaning
of design..?
What's the different
compared to
Design is
about how
What Design Exactly About is..?
4
compared to
analysis..?
Analysis is
about what

5. Why do we
Need
Design..?
To deliver
faster
To manage
change
Why do we need design..?
5
change
To deal with
complexity

6. Agenda
Need of Design
Design Smells
6
Class Design Principles
Summary

7. Okay... I under stand the
Importance of design
How do we know a
Design is bad..?
Design Smells
7
Are they any symptoms
Of bad design...

8. Immobility
Viscosity
Rigidity
Fragility
Design Smells(contd…)
8
ViscosityFragility

9. Rigidity
Tendency for software to be difficult to change.
The design is hard to change
9
Tendency for software to be difficult to change.
Single change causes cascade of subsequent changes in
dependent modules.
The more module must be changed the more rigid the
design.

10. Fragility
Tendency of the software to break in many places every
The design is easy to break
10
Tendency of the software to break in many places every
time it's changed .
The breakage occurs in areas with no conceptual
relationship
On every fix the software breaks in unexpected way.

11. Immobility
Inability to reuse software from other projects or modules .
Difficult to reuse
11
Inability to reuse software from other projects or modules .
The useful module have to many dependencies.
Cost of rewriting is less compared to the risk faced to
separate those part.

12. Viscosity
It's easy to do the wrong thing, but hard to do the right
Hard to do the right thing
12
It's easy to do the wrong thing, but hard to do the right
thing.
When the design preserving methods are more difficult to
use then the hacks
When dev environment is slow and inefficient developer
will be tempted to do wrong things.

13. Why design becomes rigid,
Fragile Immobile and viscousIs there any
Characteristics
For good design
Design Characteristics
13

14. High cohesion
Improper
dependencies
Between modules
Design Characteristics
14
Low coupling

15. So how can we
Achieve good
Design..?
SRP
LSP
OCP
Design Characteristics
15
ISP
DIP
So let's go SOLID..

16. Single Responsibility Principle
There should never be more then one reason
This is hard to see, as we think
Responsibility in group
16
There should never be more then one reason
to change a class.
Many responsibility means many reason to
change.

17. 1. Removes the immobility Smell from
Design.
2. Deodorizes the Rigidity Smell
Ohh.. SRP
Benefits of SRP
17

18. Open Closed Principle
Software entities should be open for extension, but closed for
Abstraction is the key
18
Software entities should be open for extension, but closed for
modification.
Keep the things that change frequently away from things that
don't change.

19. Open Closed Principle (contd…)
Open for Extension
Behavior of the module
can be extended.
Closed for Modification
The source of such a module
is invisible.
OCP is the heart of object oriented design
19
Make the module behave
in new and different ways
as the requirement of the
application change.
Resist making source change
to it.

20. Is 100%
Closer
possible
Is there any
techniques
In Reality 100% Closer
is not attainable.
Closer must be
Strategic.
Abstraction is the key
Further thinking of OCP
20
Flexibility, re usability
and maintainability is
The benefits

21. Liskov's SubstitutionPrinciple
LSP defines the inheritance principle.
Subclass should be substitutable for their base class
21
LSP defines the inheritance principle.
It makes clear that in OO design IS-A relationship is about
behavior; behavior that clients depend on.
If client uses a base class, then it should not differentiate
the base class.

22. LSP Violation Example
void g(Rectangle r)
{
r.setWidth(5);
r.setHeight(4);
if(r.getArea() != 20)
throw new
Exception("Bad area!");
Square is not
Rectangle!
22
Exception("Bad area!");
}
IS-A Relationship
Square’s behavior is
changed, so it is not
substitutable to
Rectangle

23. Further thinking of LSP
In order to be substitutable, the contract of the base class
must be honored by the derived class.
Derived class substitutable or base class, if
Design by Contract
23
Derived class substitutable or base class, if
Preconditions are no stronger then the base class
Preconditions are no weaker then the base class
Derived method should expect no more and provide no less

24. LSP Violation
The solution will likely to be put into an if else statement in the
client side.
Are you sure it works?
What happen if new
derivatives of rectangle
come..?
May be we've allocate
24
May be we've allocate
The responsibility
Wrongly.
It violates OCP

25. Interface Segregation Principle
Client should not forced to depend on methods they do not
Many client specific interface are Better then one
general purpose interface
25
Client should not forced to depend on methods they do not
use.
ISP deals with designing cohesive interfaces and avoiding
fat interfaces.
What happen when the big class changes? All depending
module must also change.

26. Service
<<Client A methods>>
+ ...
<<Client B methods>>
+ ...
<<Client C methods>>
+ ...
Client A
Client B
An violation of ISP example
26
ISP violation
Client C

27. Segregated
interface
Service
<<Interface>>
Service A
<<Client A methods>>
+ ..
<<Interface>>
Service B
Client A
An violation of ISP example: Solution
27
<<Client A methods>>
+ ...
<<Client B methods>>
+ ...
<<Client C methods>>
+ ...
Service B
<<Client B methods>>
+ ..
<<Interface>>
Service C
<<Client C methods>>
+ ..
Client B
Client C

28. Dependency Inversion Principle
High level module should not depend upon low level
Depend upon abstraction. Do not depend
Upon concretions
28
High level module should not depend upon low level
modules, both should depends upon abstraction
Abstraction should not depends upon on details, details
should depends on abstraction.

29. DIP
A DIP example
29
DIP
violation

30. DIP Summary
Inversion of dependencies is the hallmark of good object
oriented design.
If it's dependencies are inverted, it has an OO design If it's
dependencies are not inverted it has a procedural design.
Dependency injection is the core of the famous spring
30
Dependency injection is the core of the famous spring
framework
Hollywood principle: “ Don't call us, we'll call you”.

31. Summary
• A class should have only one reason to changeSRP
• A Module should be open for extension but
closed for modification.OCP
• Subclass should be substitutable for their base
LSP
31
• Subclass should be substitutable for their base
class.LSP
• Many client specific interfaces are better then
one general purpose interface .ISP
• Depends upon abstraction. Do not depends
upon concretions.DIP

32. Questions ???Questions ???