The document discusses software design principles known as SOLID principles. It introduces each principle - Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation and Dependency Injection. For each principle, it provides a brief definition and example to illustrate how the principle can be applied to create software that is flexible, modular and able to handle changes over time. It concludes by recommending additional resources to learn more about applying these principles.

1. Software Design Principles

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3. • S.O.L.I.D. principles
• Where to learn more
• Let’s fix some code!
Overview

4. “The intention is that these principles, when
applied together, will make it more likely that
a programmer will create a system that is
easy to maintain and extend over time.”

5. SRP
OCP
LSP
ISP
DIP
Single
Responsibility
Principle

6. Single Responsibility Principle
“A class should have only one
reason to change.”

11. SRP
OCP
LSP
ISP
DIP
Open/Closed
Principle

12. Open/Closed Principle
“Open for extension, closed for
modification.”

15. SRP
OCP
LSP
ISP
DIP
Liskov
Substitution
Principle

16. Liskov Substitution Principle
“if S is a subtype of T, then
objects of type T may
be replaced with objects of type
S"

17. Liskov Substitution Principle
“Let q(x) be a property provable
about objects x of type T.
Then q(y) should be provable for
objects y of type S where S is a
subtype of T.”

24. Liskov Substitution Principle
“if S is a subtype of T, then
objects of type T may
be replaced with objects of type
S"

25. SRP
OCP
LSP
ISP
DIP
Interface
Substitution
Principle

26. SRP
OCP
LSP
ISP
DIP
Dependency
Injection
Principle

27. Dependency Injection Principle
• High-level modules should not
depend on low-level modules. Both
should depend on abstractions.
• Abstractions should not depend
upon details. Details should depend
upon abstractions.

30. SOLID
The only certainty is that your
app will CHANGE. Program
using the S.O.L.I.D. principles to
account for that change.
Flexible, Modular, Decoupled

31. Solid code is a
pleasure to write and
work with.

32. Additional Resources
• Sandi Metz – SOLID Object
Oriented Design
• Ruby Science by Thoughtbot
• Uncle Bob – Principles of OOD
• Design Patterns in Ruby