In this session we covered Classes, Modules and Mixins in more depth. Ruby's inheritance mechanism was covered, as well as the uses of instance and class variables.

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