1. CIT 383: Administrative Scripting
Classes
CIT 383: Administrative Scripting

2. CIT 383: Administrative Scripting
Topics
Creating a Class
Initialization
Accessors and Attributes
Redefining Methods
Equality
Class Variables
CIT 383: Administrative Scripting

3. CIT 383: Administrative Scripting
What is a Class?
A class is a blueprint to create objects
Attributes: named values associated with object
Methods: actions that can be performed on object.
A Point object has
Attributes: x and y coordinates
Methods:
x to read x-coordinate
y to read y-coordinate
to_s to return a string representation
CIT 383: Administrative Scripting

4. CIT 383: Administrative Scripting
Why Create Classes?
Ruby provides many classes
Array
DateTime
File
Regexp
String
But it doesn’t provide classes for your domain
ApacheStatus
AutomateIt
PasswdCheck
Uptime
VMstats
CIT 383: Administrative Scripting

5. CIT 383: Administrative Scripting
Creating a Class
A class is defined with the class keyword
class Point
end
Instances of the class are created with new
p = Point.new
The instance has methods from Object class
p.class # Point
p.is_a? Point # true
CIT 383: Administrative Scripting

6. Initializing an Object
The initialize method is run during new
class Point
def initialize(x,y)
= x, y
end
Instance variables used with prefix
Instance variables belong to a particular object.
Only that object can access those values.
CIT 383: Administrative Scripting

7. CIT 383: Administrative Scripting
Methods
Methods are defined with the def keyword.
class Point
def x x
end
Methods are called using the . syntax.
p = Point.new(0,0)
xvalue = p.x
CIT 383: Administrative Scripting

8. CIT 383: Administrative Scripting
Assignment Methods
Use the = operator using method ending in =
class Point
def x=(value)
@x = value
end
Methods are called using the . syntax.
p = Point.new(0,0)
p.x = 10
CIT 383: Administrative Scripting

9. CIT 383: Administrative Scripting
Accessor Shortcuts
Let Ruby create accessor methods automatically.
attr_reader: create read-only methods
attr_writer: create assignment write methods
attr_accessor: create read and write methods.
Examples
class Point
attr_accessor :x, :y
end
p = Point.new(0,0)
p.x
p.y = 1
CIT 383: Administrative Scripting

10. CIT 383: Administrative Scripting
Operator Methods
Can define operators like == for class too.
class Point
def ==(o)
if o.is_a? Point
@x==o.x
else
false
end
Example
p1 = Point.new(1,1)
p2 = Point.new(1,2)
p1 == p # false
CIT 383: Administrative Scripting

11. CIT 383: Administrative Scripting
Redefining Methods
Can redefine built-in methods like to_s
class Point
def to_s
end
Example:
p = Point.new(1,2)
puts p # “(1,2)”
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12. CIT 383: Administrative Scripting
Class Variables
Class variables are shared by all objects of class.
Use differ from instance vars which belong to one object.
Example: Track number of objects of class created.
class Point
= 0
def initialize(x,y)
= x, y
+= 1
end
def count
p1 = Point.new(0,0); p2 = Point.new(1,1)
p2.count # returns 2
CIT 383: Administrative Scripting

13. CIT 383: Administrative Scripting
References
Michael Fitzgerald, Learning Ruby, O’Reilly,
David Flanagan and Yukihiro Matsumoto, The Ruby Programming Language, O’Reilly, 2008.
Hal Fulton, The Ruby Way, 2nd edition, Addison- Wesley, 2007.
Robert C. Martin, Clean Code, Prentice Hall,
Dave Thomas with Chad Fowler and Andy Hunt, Programming Ruby, 2nd edition, Pragmatic Programmers, 2005.
CIT 383: Administrative Scripting