Objects, Classes, and Instances

Learning Goals

  • Describe the difference between a class and an instance of that class
  • Define a class
  • Store state in instance variables defined in initialize
  • Provide access to state using attr_readers
  • Use methods to provide behaviors to instances of a class
  • Create a new instance of a class and call methods on that instance


Available here


In your notebook brainstorm five types of objects and specific instances of that object that are at Turing.

For example:

  • Type of object: Cubby
  • Specific instances:
    • Ali’s cubby
    • Mike’s cubby
    • Sal’s cubby

Classes in Ruby


In programming, a Class is something that models:

  1. State
  2. Behavior

State is what something is. Behavior is what something does. In the previous activity, our Class was refrigerator. We modeled the state of a refrigerator by defining the attributes “color”, “size”, and “food items”. We modeled the behavior of a refrigerator by defining the methods “add food”, “remove food”, and “change temperature”.

An Instance or Object is a concrete representation of a Class. In the previous activity, “small staff refrigerator” is a specific Instance of the Fridge Class. We can also say that “small staff refrigerator” is a Fridge Object. Do not get confused by the terms Instance an Object. They mean the exact same thing (for now).

Think of a Class like a blueprint for a house and an Instance as an actual house. The blueprint is a just an idea of how the house should be built, and the house is the realization of that blueprint.


The syntax for defining a class is as follows:

class NameOfClass

So, for example, if we wanted to create a Dog class, we could do the following:

class Dog

Generally we will want to put more information in our classes to make them useful to us, but those two lines (even with no other information) will create a class.


Let’s practice together with a Fridge class. Create a directory in your classwork directory called objects_classes_and_instances. Within that directory create a fridge.rb file, and put the folling information into that file.

# ~/turing/1module/classwork/objects_classes_and_instances/fridge.rb
# Notice that `class` is lowercase while `NameOfClass` is CamelCased.

class Fridge

In the same objects_instances_and_classes directory create a runner.rb file and put the code below into that.

# ~/turing/1module/classwork/objects_classes_and_instances/runner.rb
require './fridge'
fridge_1  = Fridge.new
puts "Number 1: #{fridge_1}"

fridge_2   = Fridge.new
puts "Number 2: #{fridge_2}"

require 'pry'; binding.pry

We can run the runner.rb file from the command line if we are inside of our objects_classes_and_instances directory by typing the following: ruby runner.rb. Do that now.

Your computer should open up a pry session. Inside of that pry session type fridge_1 and hit return to see what the variable fridge_1 is holding. Then type fridge_2 to see what that variable is holding. How are those two things the same? How are they different?

Independent Practice

TRY IT: With your pair, define a Person class in your objects_classes_and_instances directory and create instances of that class in your runner file.

Attributes in Ruby Classes

Above we created a Fridge class and then also created specific intsnaces of the fridge class that we held in the variables fridge_1 and fridge_2. Generally the objects we create will come from the same template, but each instance will be different in some way.

Think about the refrigerators here in the Turing basement.

  • M1 BE refrigerator
  • M1 FE refrigerator
  • Staff refrigerator

Each one is different in important ways. For example, each one has its own:

  • brand
  • color
  • temperature
  • plugged_in
  • contents

We can model these attributes in code by using instance variables. Generally we define these instance variables in a special method called initialize that is run every time a new instance of a class is created.


When we run Fridge.new in Ruby, what actually happens? We can see from the last example that different Fridge objects (or instances) are created. Other than that, nothing happens. If we want some specific code to run when we first create a new Fridge, we need to tell Ruby what should happen when a new Fridge instance (or object) is created. We do this with the initialize method.

class Fridge
  def initialize
    #Initialize code here


This method is run once and only once during an Object’s lifetime, when we call new.

Modeling State with Attributes

The instances of the classes we’ve defined so far are basically useless. Aside from their object_id, there is nothing unique about these instances.

Remember, a class models State and Behavior. Let’s give our refrigerator some state.


In our playground file, we’ll add some attributes to the Fridge class. The @ symbol before a variable name indicates that it is an Attribute or Instance Variable. These two terms mean the exact same thing.

class Fridge
  def initialize(brand, color, temperature, plugged_in, contents)
    @brand       = brand
    @color       = color
    @temperature = temperature
    @plugged_in  = plugged_in
    @contents    = contents

we have now created a method class that will allow us to create many different instances of Fridge, each one slightly different from the last. How do we do that in practice? Update your runner file so that it includes the following:

fridge_1  = Fridge.new("Maytag", "white", 36, true, ["leftover pizza", "yogurt", "soylent"])
puts "Number 1: #{fridge_1}"

fridge_2   = Fridge.new("", "black", 40, true, [])
puts "Number 2: #{fridge_2}"

require 'pry'; binding.pry

Note that the arguments that we pass to new are order dependent. So, in the first example when we pass "Maytag" as the first argument, we are saying that the brand of the Fridge we are creating is Maytag. When we pass an empty string ("") the second time we call new we are saying that the Fridge that we created doesn’t have a name brand.

Independent Practice

TRY IT: With your pair, give your Person class some attributes and create some instances of Person.

Accessing Attributes

That’s all well and good, but what can we do with all these attributes that we’ve created? They’re no good to us if we can’t use them.

Generally, the way that we access information stored in a class is by sending it messages or calling methods on that class. We do that using . syntax.

Run your runner file again and check to see what this returns:


You should get an error that says something about the method .brand not existing (a no method error). The syntax here is correct, but we haven’t told our Fridge class how to respond when it receives the message brand.

We can do that with methods like the ones we’ve seen before, but attributes stored as instance variables are special. We can tell our class to provide access to them using attribute readers. Let’s do that now.


Update your Fridge class to include the lines below.

class Fridge
  attr_reader :brand,

  def initialize(brand, color, temperature, plugged_in, contents)
    @brand       = brand
    @color       = color
    @temperature = temperature
    @plugged_in  = plugged_in
    @contents    = contents

Run your runner file again and now see if you can call fridge_1.brand. Try to see if you can access other attributes as well.

Independent Practice

TRY IT: With your pair, create attr_readers for the attributes in your Person class.

Other Methods

We can also create other methods that will allow us to send other messages to our Fridge class. For example, let’s say we wanted to add eggs to our Fridge. We currently have a way to see what the contents of the Fridge are, but we don’t have any way to add to it. Let’s do that by creating a method called add_food that will add a food to the contents array.


Define an add_food method that allows you to put foods in your fridge. Note that we can access the @contents instance variable from anywhere within the class just by using the @ symbol.

class Fridge
# ... attr_readers & initialize method

  def add_food(food)
    @contents << food

Update your runner file so that you:

  1. Create a new instance of Fridge.
  2. Print the contents of that Fridge.
  3. Add something to the contents of the Fridge.
  4. Print the new contents of the Fridge.

Independent Practice

TRY IT: With your pair, create a have_birthday method for your Person class. This should increase the age of that person by 1.

As A Group

Updating CreditCheck

With your partner, update your CreditCheck project using the following directions.

Create a CreditCard class based on the following criteria:

  • A CreditCard is passed two arguments upon initialization
    • The first argument is a String representing the card number
    • The second argument is an Integer representing the CreditCard’s limit
  • A CreditCard can access the called card_number and limit from outside the class with attr_readers
  • A CreditCard has a method called is_valid? that takes no arguments and returns either true or false based on whether or not the card number is valid.
  • A CreditCard has a method called last_four that returns a String of the last four digits of the card number

If the previous criteria are met, you should be able to interact with the CreditCard class from a Pry session like so:

pry(main)> require './lib/credit_card'
#=> true

pry(main)> credit_card = CreditCard.new("5541808923795240", 15000)
#=> #<CreditCard:0x00007fbb1ca5f698 @card_number="5541808923795240", @limit=15000>

pry(main)> credit_card.card_number
#=> "5541808923795240"

pry(main)> credit_card.limit
#=> 15000

pry(main)> credit_card.last_four
#=> "5240"

pry(main)> credit_card.is_valid?
#=> true

Check for Understanding

With your partner, answer the questions below.

  • Classes, instances, objects
    • What is a Class?
    • What is an Instance?
    • What is an Object?
    • How are these three things alike/different?
    • What code do you have to write to create a Class? What code do you have to write to create an instance?
    • What happens when a new instance is created?
  • Attributes & Methods
    • What is an attribute? How can we recognize an attribute?
    • What is a method? How do we write methods?
    • What are parameters? How do we add parameters to methods?
    • What is a return value? How do you know what the return value of a method is? Do all methods have return values?