OO design principles & heuristics

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OO Design Principles and Heuristics
Dhaval ShahDhaval Shah

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AgendaAgenda
 What is OO Modeling?
 Why OO Modeling?
 Odors of rotting software
 Basic OO Principles
 OO Design Principles and Heuristics
 Conclusion
 Case Study

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This session will not:This session will not:
 Make you an expert in OOAD
 Turn you into a system architect
 Provide instruction on UML

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Warning!Warning!
 Most of these slides maps
directly/indirectly to various articles
available on the web, especially from
objectmentor.com
Thanks Bob!

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What is OO ModelingWhat is OO Modeling

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Why OO ModelingWhy OO Modeling
To Manage
Change
To Manage
Change
To Deliver
Faster
To Deliver
Faster
To Deal with
Complexity
To Deal with
Complexity

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Odors of rotting softwareOdors of rotting software
RigidityRigidity
FragilityFragility
ImmobilityImmobility
ViscosityViscosity
Needless
Complexity
Needless
Complexity
Needless
Repetition
Needless
Repetition
OpacityOpacity

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OO Design Principles and Heuristics
 Single Responsibility Principle
 Open Close Principle
 Liskov Substitution Principle
 Interface Segregation Principle
 Dependency Inversion Principle

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Single Responsibility PrincipleSingle Responsibility Principle
A class should have only one
reason to change

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Single Responsibility Principle [Contd.]

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Single Responsibility Principle [Contd.]

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Single Responsibility Principle [Contd.]Single Responsibility Principle [Contd.]
 What is RESPONSIBILITY?
 RESPONSIBILITY is reason for change
 How to spot multiple RESPONSIBILITIES?

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Single Responsibility Principle [Contd.]Single Responsibility Principle [Contd.]
 Example-

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Single Responsibility Principle [Contd.]Single Responsibility Principle [Contd.]
 Going from multiple responsibilities to single responsibility -

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Open Close PrincipleOpen Close Principle
Software entities should be open
for extension but closed for
modification.

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Open Close Principle [Contd.]Open Close Principle [Contd.]
 Example

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Open Close Principle [Contd.]Open Close Principle [Contd.]
 Example

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Liskov Substitution PrincipleLiskov Substitution Principle
Subtypes must be substitutable for their base
types.
 LSP is all about inheritance – One must be able to substitute subclass by
base class without things going terribly wrong

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Liskov Substitution Principle [Contd.]Liskov Substitution Principle [Contd.]
 Example

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Liskov Substitution Principle [Contd.]Liskov Substitution Principle [Contd.]
 Example
Board board = new 3DBoard();
Unit unit = board.getUnits(8,4);

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Liskov Substitution Principle [Contd.]Liskov Substitution Principle [Contd.]
 Solving 3DBoard problem

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Interface Segregation PrincipleInterface Segregation Principle
Clients should not be forced to depend upon
interfaces that they don't use.

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Interface Segregation Principle [Contd.]Interface Segregation Principle [Contd.]
 Example

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Interface Segregation Principle [Contd.]Interface Segregation Principle [Contd.]
 Separation Through Delegation

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Interface Segregation Principle [Contd.]Interface Segregation Principle [Contd.]
 Separation Through Multiple Inheritance

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Dependency Inversion PrincipleDependency Inversion Principle
a.High-level modules should not depend on low-
level modules. Both should depend on
abstractions
b.Abstractions should not depend upon details.
Details should depend upon abstractions

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Interface Segregation Principle [Contd.]Interface Segregation Principle [Contd.]
 Layering
Naive Layering Scheme

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Interface Segregation Principle [Contd.]Interface Segregation Principle [Contd.]
 Inverted Layering
Inverted Layers

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ConclusionConclusion
 Good design is needed to successfully deal with change
 The main forces driving your design should be high cohesion and low
coupling
 SOLID principles put you on the right path
WARNING : these principles cannot be applied blindly :)