This document discusses polymorphism and object-oriented programming principles in Swift. It defines polymorphism as an object being able to take on multiple forms. Polymorphism in Swift can be achieved through class inheritance and protocol interfaces. The document also discusses the open-closed principle, which states that code should be open for extension but closed for modification. Finally, it explains the Liskov substitution principle, which requires that subclasses behave exactly like their parent classes so the subclasses can be substituted for their parents without altering behavior.

1. Object Oriented Programming
in Swift
Ch 2: Polymorphism
Sai Li @ Yowoo Tech.
2016/12/23

2. References
• Agile Software Development: Principles, Patterns,
and Practice ( : )
by Robert C. Martin
• Fundamental Object Oriented Programming
by Hsuan-Tien Lin
•
by Teddy Chen
• Essential Object-Oriented Programming
by Josh Ko

3. OOP
• Encapsulation
• Inheritance
• Polymorphism

4. Polymorphism
One Thing,
Many shapes….

6. Strawberry

7. Strawberry

9. Polymorphism in Swift
1. Class Inheritance
2. Protocol Interface
Fruit
Orange Strawberry
Playable
CD Piano

10. Polymorphism
Polymorphism means that the sender of a
stimulus does not need to know the receiving
instance’s class. The receiving instance can
belong to an arbitrary class.

11. S.O.L.I.D

12. S.O.L.I.D
• SRP: Single Responsibility Principle
• OCP: Open Closed Principle
• LSP: Liskov Substitution Principle
• ISP: Interface Segregation Principle
• DIP: Dependency Inversion Principle

13. Open Closed Principle

14. – Ivar Jacobson
“All systems change during their life cycles.
This must be born in mind when devloping
systems expected to last longer than the first
version.”

15. Open Closed Principle
Software entities (classes, modules, functions,
etc.) should be open for extension, but closed
for modification
- Bertrand Meyer (1988)

16. • Open for extension
• Closed for modification

17. Triangle?

18. • No ripple effect
• further changes of that kind are achieved by
adding new code, not by changing old code
that already works.
• Elastic

20. <Design Pattern > GoF
“Program to an interface,
not an implementation.”

22. What you can do rather than who you are.
Black Cat & White Cat

23. How ?

24. Barbara Liskov
(1939 - )
• UCB, Stanford
• 1st female CS PhD
• CLU
• von Neumann Award
• Turing Award

25. Liskov Substitution Principle
Let Φ(x) be a property provable about objects x
of type T.
Then Φ(y) should be true for objects y of type S
where S is a subtype of T

26. Liskov Substitution Principle
Subtypes must be substitutable for their base
types.

27. Liskov Substitution Principle
Subtypes must be substitutable for their base
types.
Let Φ(x) be a property provable about objects x of type T.
Then Φ(y) should be true for objects y of type S where S
is a subtype of T
( )

29. LSP Violation
p.s: Circle Ellipse Problem

30. LSP Violation

31. LSP Violation
Why?

32. Design By Contract
Class Invariant

33. Postcondition

34. Subtype func precondition can’t be stronger than base type
Subtype func postcondition can't be weaker than base type

35. Recap
• Polymorphism
• Open Closed Principle
• Liskov Substitution Principle

36. To be continued …

37. References
• https://zh.wikipedia.org/wiki/
File:Barbara_Liskov.PNG
• http://www.relatably.com/m/img/wtf-memes/
grandma-finds-the-internet-meme-generator-wtf-
b12499.jpg
• http://i.imgur.com/BTpuqPX.jpg
• wiki: https://en.wikipedia.org/wiki/Circle-
ellipse_problem