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Intro to Ruby Classes


  • Classes combine data (values) and behavior (methods) into one concept

  • Classes are the template that describes what data, and what behavior we have

  • Objects are instances of a class.

  • Classes are like cookie cutters, where objects are like the cookies

  • Instances of objects

    • We have already created many objects, just using the literal form of creating them.
    • e.g. name = "Gavin"
    • e.g. score = 42
    • e.g. instance of a string: name ="Gavin")
  • But we can make our own classes. Lets make one to keep track of information about my dog.

    class Dog
    riley =
  • and we can define methods

    class Dog
    def age
    return 1

    And then call them:

    riley =
    p riley.age
    # => 1
  • But what if there are two dogs with different ages?

  • Same behavior, different data.

  • Must give the data to the object some how

  • Done via the initialize method

class Dog
def initialize(age)
@age = age
def age
return @age
  • New type of variable, the instance variable, noted by the fact that it starts with an @
  • Every isntance of a Dog has it's own instance variables.
  • Now that we have an initialize method we must supply the age when creating each instance of a Dog.
riley =
roscoe =
  • We also added a behavior (method) to ask for the age of a Dog
  • age simply returns this Dog instances @age instance variable.
  • Thus the same behavior (knowing a Dog's age) but different data (the actual age of the dog) results in different action.
  • attr_accessor allows us to shortcut the defining of methods to return the value of the data (attribute) as well as set the value.
class Dog
attr_accessor :age, :sound, :color
def initialize(age, sound, color)
@age = age
@sound = sound
@color = color
riley =, "woof", "red")
puts "Riley is #{riley.age} years old, sounds like #{riley.sound} and is #{} in color"
# Riley gets a little older
riley.age = 2
puts "Riley is now #{riley.age} years old, sounds like #{riley.sound} and is #{} in color"


  • Classes can have a parent class from which they gain all the data and behavior from but can extend with new data and behavior.
class GoodDog < Dog
def toys
["bone", "tennis ball", "blanket"]
goodie =, "woof", "brown)
  • Inheritance can be thought of as "is a" or "is a kind of"