The document covers an educational session on Ruby programming, focusing on object-oriented concepts such as classes, attributes, inheritance, access control, and modules. It illustrates these concepts with code examples, particularly using a 'bookinstock' class and its variations. The session also includes guidance on exercises and readings for further learning in Ruby.

1. Learn Ruby 2011
Session 4

2. Our Sponsors
Flatsourcing, iSeatz, Koda, and Launchpad

3. Objects, Oh My
Objects, Attributes, Inheritance & More

4. Questions
Having any problems? Discover anything cool?

5. Classes
• Classes are the blueprints of Objects
• Classes define the way an Object keeps
state, what it’s value is
• Classes define the way an Object behaves,
what methods it provides

6. Classes & Attributes
class BookInStock
def initialize(isbn, price)
@isbn = isbn
@price = Float(price)
end
end

7. Classes & Attributes
book = BookInStock.new(“1234”, 3.00)
p book
#<BookInStock:... @isbn=”1234”, @price=3.00>

8. Classes & Attributes
class BookInStock
def initialize(isbn, price)
@isbn = isbn
@price = Float(price)
end
def to_s
“ISBN: #{@isbn}, price: #{@price}”
end
end

9. Classes & Attributes
book = BookInStock.new(“1234”, 3.00)
puts book
ISBN: 1234, price: 3.00

10. Classes & Attributes
class BookInStock
attr_reader :isbn, :price
def initialize(isbn, price)
@isbn = isbn
@price = Float(price)
end
end

11. Classes & Attributes
book = BookInStock.new(“1234”, 3.00)
puts “ISBN: #{book.isbn}, price: #{book.price}”
ISBN: 1234, price: 3.00

12. Classes & Attributes
class BookInStock
attr_reader :isbn
attr_accessor :price
def initialize(isbn, price)
@isbn = isbn
@price = Float(price)
end
end

13. Classes & Attributes
book = BookInStock.new(“1234”, 3.00)
puts “ISBN: #{book.isbn}, price: #{book.price}”
ISBN: 1234, price: 3.00
book.price = 4.00
puts “ISBN: #{book.isbn}, price: #{book.price}”
ISBN: 1234, price: 4.00

14. Classes & Attributes
class BookInStock
attr_reader :isbn
#attr_accessor :price
attr_reader :price
attr_writer :price
def initialize(isbn, price)
@isbn = isbn
@price = Float(price)
end
end

15. Classes & Attributes
class BookInStock
attr_reader :isbn, :price
def initialize(isbn, price)
@isbn = isbn
@price = Float(price)
end
def price=(new_price)
@price = Float(new_price)
end
end

16. Classes & Attributes
class BookInStock
def initialize(isbn, price)
#...
@@count += 1
end
#...
def self.count
@@count
end
end

17. Classes & Attributes
book1 = BookInStock.new(“1234”, 3.00)
puts BookInStock.count
1
book2 = BookInStock.new(“1235”, 5.19)
book3 = BookInStock.new(“1236”, 4.00)
puts BookInStock.count
3

18. Access Control
• Public methods (the default) can be called by
anyone in any context
• Protected methods can only be called by the
Class that defines them or its Subclasses
• Private methods can only be called by the
within the Class that defines them

19. Access Control
class BookInStock
#...
private
#...
protected
#...
public
#...
end

20. Access Control
class BookInStock
#...
private :method1
protected :method2, :method3
public :method4
end

21. Inheritance
• Inheritance allows a Subclass to inherit the
behavior of it’s parent Class
• Ruby is a single-inheritance language

22. Inheritance
class BookOnSale < BookInStock
end
book = BookOnSale.new(“12349”, 1.00)
puts book
puts book.class
ISBN: 12349, price: 1.00
BookOnSale

23. Modules
• Modules provide an additional means of
encapsulation
• Modules facilitate Mixins.

24. Modules
module Trig
PI = 3.141592654
def Trig.sin(x)
#...
end
def Trig.cos(x)
#...
end
end
y = Trig.sin(Trig::PI/4)

25. Mixins
• Mixins overcome the limitations of single-
inheritance
• Mixins allow behavior to be added to a class
within requiring inheritance

26. Mixins
module OnSale
def on_sale?
true
end
end
class BookOnSale < BookInStock
include OnSale
end
b = BookOnSale.new(“12340”, 1.00)
puts “Price $#{b.price}!” unless b.on_sale?
puts “ONLY $#{b.price}!” if b.on_sale?
ONLY $1.00!

27. Reviewing
• Classes & Attributes
• Access Control
• Inheritance
• Modules
• Mixins

28. For Next Week
For the New to Programming
• Read Chapters 9, 10, 11, 12 & 13 in LtP
• Complete exercises for each chapter
For Everyone
• Read Chapter 3, 4, 5 & 6 in PR1.9
• Work through Ruby Koans: about_classes,
about_open_classes, about_inheritance,
about_modules

29. Next Week
• More about Blocks and Iterators
• Handling Errors

30. Exercises
• Work on specific Ruby Koans:
• ruby about_classes.rb
• ruby about_open_classes.rb
• ruby about_inheritance.rb
• ruby about_modules.rb