1

OO Design Principles and Heuristics

Dhaval Dhaval ShahShah

2

AgendaAgenda What is OO Modeling?

Why OO Modeling?

Odors of rotting software

Basic OO Principles

OO Design Principles and Heuristics

Conclusion

Case Study

3

This session will not:This session will not: Make you an expert in OOAD

Turn you into a system architect

Provide instruction on UML

4

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

Thanks Bob!

5

What is OO ModelingWhat is OO Modeling

6

Why OO ModelingWhy OO Modeling

To Manage Change

To Deliver Faster

To Deal with Complexity

7

Odors of rotting softwareOdors of rotting software

Rigidity

FragilityImmobility

Viscosity Needless Complexity

Needless Repetition

Opacity

8

OO Design Principles and Heuristics Single Responsibility Principle

Open Close Principle

Liskov Substitution Principle

Interface Segregation Principle

Dependency Inversion Principle

9

Single Responsibility PrincipleSingle Responsibility Principle

A class should have only one reason to change

10

Single Responsibility Principle [Contd.]

11

Single Responsibility Principle [Contd.]

12

Single Responsibility Principle Single Responsibility Principle [Contd.][Contd.] What is RESPONSIBILITY?

RESPONSIBILITY is reason for change

How to spot multiple RESPONSIBILITIES?

13

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

14

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

15

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

16

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

17

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

18

Liskov Substitution PrincipleLiskov Substitution PrincipleSubtypes 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

19

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

20

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

Board board = new 3DBoard();

Unit unit = board.getUnits(8,4);

21

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

22

Interface Segregation PrincipleInterface Segregation Principle

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

23

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

24

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

25

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

26

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

27

Interface Segregation Principle Interface Segregation Principle [Contd.][Contd.] Layering

Naive Layering Scheme

28

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

Inverted Layers

29

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 :)