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

Slides

Available here

Warmup

In your notebook brainstorm a type of object and specific instances of that object that are at Turing. Then brainstorm 3 different attributes for those objects and 3 different behaviors of those objects.

For example:

  • Type of object: Refrigerator
  • Specific instances:
    • Staff Fridge
    • Small Fridge in Student Kitchen
    • Large Fridge in Student Kitchen
  • Attributes
    • Brand
    • Color
    • Temperature
  • Behaviors
    • Add Food
    • Remove Food
    • Change Temperature

Classes in Ruby

Overview

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 “brand”, “color”, and “temperature”. 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, “staff refrigerator” is a specific Instance of the Fridge Class. We can also say that “staff refrigerator” is a Fridge Object. Do not get confused by the terms Instance and 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.

Syntax

The syntax for defining a class is as follows:

class NameOfClass
end

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

class Dog
end

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.

Practice

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 following information into that file.

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

class Fridge
end

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 instances 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 will be a unique object.

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

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.

Initialize

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
  end
end

...

This method is run once and only once during an Object’s lifetime, when we call new. Other than that, initialize is like any other method where we can put Ruby code:

class Fridge
  def initialize
    puts "A new Fridge Object has been created"
  end
end

...

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.

Practice

Let’s 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_argument, color_argument, temperature_argument)
    @brand       = brand_argument
    @color       = color_argument
    @temperature = temperature_argument
  end
end

Because Attributes are something we want to persist throughout an object’s lifetime, we typically define them inside the initialize method because we want them to exist as soon as the object is created.

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)
puts "Number 1: #{fridge_1}"

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

require 'pry'; binding.pry

When we include the arguments to .new, Ruby will pass those arguments to the initialize method for us. 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.

What we have just done is a very common pattern. We gave our initialize method some arguments and we saved those arguments to instance variables. While this is a strong pattern, it is not a rule. For instance, you may want to set a variable in your initialize that has a default value that isn’t set using an argument:

class Fridge
  def initialize(brand, color, temperature)
    @brand       = brand
    @color       = color
    @temperature = temperature
    @contents    = []
  end
end

Independent Practice

TRY IT: With your pair, give your Person class some attributes that are set using arguments to initialize and some attributes that have default values. Make some instances of your Person class.

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:

fridge_1.brand

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.

Practice

Update your Fridge class to include the lines below.

class Fridge
  attr_reader :brand,
              :color,
              :temperature,
              :contents

  def initialize(brand, color, temperature)
    @brand       = brand
    @color       = color
    @temperature = temperature
    @contents    = []
  end
end

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.

An important thing to remember is that although there is a special syntax for creating attr_readers, they are still just methods. Remember the error we got earlier was a no method error for brand.

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.

Practice

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
  end
end

Update your runner file so that you:

  1. Create a new instance of Fridge.
  2. Print the contents of that Fridge.
  3. Add some food to the contents of the fridge using the method you just created. You can represent a food as a String.
  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.

Object Interaction

When we build more complex programs, we typically have many classes, and the instances of those classes interact in some way.

Practice

Instead of representing food as a String, let’s create a Food class to represent a food.

class Food
  attr_reader :name,
              :calories

  def initialize(name, calories)
    @name = name
    @calories = calories
  end
end

Update your runner file to add Food objects to the contents of your fridge.

Now let’s add a method for a fridge to total the number of calories in the fridge:

class Fridge
# ... attr_readers & other methods

  def total_calories
    calories = 0
    @contents.each do |food|
      calories += food.calories
    end
    calories
  end
end

Update your runner file to call this method.

In Pairs

Create a Book Class

With your partner, create a book class. Make sure that your book class has at least 3 attributes and 2 methods.

Once you’ve created your class, create a runner file that creates three separate instances of book and saves them to variables.

Create a Library Class

With your partner, create a Library class. Add attributes as you wish, but the be sure to include a @collection instance variable that starts as an empty array.

If you have time:

  • Add a add_book method that takes an instance of book and adds it to your collection.
  • Add a titles method that iterates over your collection of books and returns only their titles.
  • Add an authors method that iterates over your collection of books and returns the authors for each book. Can you make it so that it does not return any duplicate authors?
  • Pretty print: add a method that prints a table of books and authors that the library has. This will require some string manipulation to get a table to print with columns that line up.

Update your runner file to create a new library, add some books to the library, and print information about their collections.

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?