1. 'SOLID' Desgin Principles

2. Our aim is to make software more stable unlike ...

3. We write code that is …

4. Rigid

5. Fragile

6. Immobile

7. Making everyone's life miserable
Source: http://www.welaf.com/13500,miserable-life-of-a-small-cat.html

8. So WHY SOLID?

9. It helps us to write code that is ...

10. Loosely coupled

11. Highly cohesive

12. Easily composable

13. Reusable

14. SOLID
 Coined by Robert C
Martin
 Not new, exsiting
principles brought
together

15. Single Responsibility Principle (SRP)

16. Single Responsibility Principle (SRP)
Open Closed Principle (OCP)

17. Single Responsibility Principle (SRP)
Open Closed Principle (OCP)
Liskov Substitution Principle (LSP)

18. Single Responsibility Principle (SRP)
Open Closed Principle (OCP)
Liskov Substitution Principle (LSP)
Interface Seggregation Principle (ISP)

19. Single Responsibility Principle (SRP)
Open Closed Principle (OCP)
Liskov Substitution Principle (LSP)
Interface Seggregation Principle (ISP)
Dependency Inversion Principle (DIP)

20. Single Responsibility Principle

21. Single Responsibility Principle
”There should be NEVER be more than
ONE reason for a class to change”

22. Single Responsibility Principle
Rectangle
Computation ----------------------- GUI
application Draw() application
Area()
GUI
What is the issue with this design?

23. Single Responsibility Principle
 Rectangle does
 Computes the area
 Draws it on the GUI
 How can we fix it?

24. Single Responsibility Principle
Rectangle
Computation -----------------------
application Area()
INHERITS
GUIRectangle GUI
----------------------- application
Draw()
GUI

25. Single Responsibility Principle
 JDK follows this (java.awt package)
 Graphics2D – drawing shapes
 Shape – for representing the geometrical shapes
 Some other examples:
 Task to download a file, parse it, and store it in a
database
 UserSetting- provide customisation feature, check
Access

26. Open Closed Principle

27. Open Closed Principle
”Modules must be OPEN for extension,
CLOSED for modification”

28. Open Closed Principle
Coined by Bertrand Meyer

29. Open Closed Principle
How can you add new features without editing the
existing module?

30. Open Closed Principle
Extend the existing modules!

31. Open Closed Principle
How is it even possible?

32. Open Closed Principle
Depend on abstractions, new features can be
added by extending the abstractions.

33. Open Closed Principle
Examples-
- Drawing shapes
- Loan approval process

34. Liskov Substitution Principle

35. Liskov Subsitution Principle
”Base classes instances must be replaceable by
the sub class instances without any change in the
application”

36. Liskov Subsitution Principle
Lets go back to an earlier example of Drawing
shape

37. Liskov Subsitution Principle
Other examples-
- Bird, Flight and Non-Flight
- Loading of settings
-Rectangle and Square

38. Interface Seggregation Principle

39. Inteface Seggregation Principle
”Clients should not depend upon the interfaces
they do not use”

40. Inteface Seggregation Principle
How can we pollute the interfaces?
OR
How do we end up creating Fat interfaces?

41. Inteface Seggregation Principle
Door, Time and TimedDoor example.
This example also violates LSP

42. Inteface Seggregation Principle
 Possible Solutions
 Use delegation- Adapter Pattern
 Use multiple inheirtance- Interfaces, Mixins

43. Dependency Inversion Principle

44. Dependency Inversion Principle
”High level modules should not depend on the low
level details modules, instead both should depend
on abstractions”

45. Dependency Inversion Principle
Copy
Read Keyboard Write Printer
What's GOOD or BAD about this design?

46. Dependency Inversion Principle
 Copy program is NOT REUSABLE
 Tightly bound to Keyboard and Printer
 Read Keyboard and Write Printer are
REUSABLE

47. Dependency Inversion Principle
 Copy program shouldn't be dependent on the
Low level Read/Write modules.
 How can we correct this?

48. Dependency Inversion Principle
Copy
Reader Writer
Keyboard Reader Print Writer
Copy program now depends on Abstractions- Reader and Writer

49. Resources
 Wikipedia – SOLID OO Design
 Robert C Martin articles.
 Jim Weirich SOLID Ruby talk at Ruby
Conference 2009.
 Rob Martin interview at Hansel Minutes.
 … and various other small articles,
presentations

50. Ideas for next learning sessions?
 GoF Design Patterns
 One or two patterns each week with code samples
 JVM Internals
 NoSQL
 Java and Concurrency
 Functional programming in Java