Object Oriented Programming


Previewing a Lesson

As you preview this lesson before the live session, keep a few things in mind:

  • This document was written with the intention of being facilitated by a human - it might be the case that you feel some parts are missing or require an explanation.
  • You are not expected to have mastery, or even fluency with these concepts when you attend the live session. You should be familiar with the vocabulary, the realm of things surrounding the concepts.
  • How can previewing this help you to get the most out of the time you have in the live session? Can you set up some notes in advance? In looking through this, do you realize you have a knowledge gap with a foundational concept that you should review? Do you already have questions? (if so, write them down so you don't forget them!).

Learning Goals

  • Explain the core concepts of Object Oriented Programming
  • Gain exposure and familiarity with the Ruby syntax for classes and object instances


  • attribute
  • class
  • instance method
  • object instance
  • Object Oriented Programming (OOP)

What is OOP?

Back End Engineering is concerned with the management of data for an application. There are different ways we can structure the data itself as well as the systems that manage it. Object Oriented Programming (OOP) is one way to design a program to manage the data in a Back End.

OOP is not exclusive to Ruby! In fact, many of the most widely used programming languages follow the patterns of OOP, including Java, C#, and Python.

Connecting OOP to Real-World Scenarios

In the introduction, we shared that Object Oriented Programming (OOP) is a paradigm for how we can organize code. Without seeing examples, that definition just feels… unhelpful. In this section, we won’t dig into code quite yet, but we’ll do some thinking and list-making to set ourselves up to write code in the next section!

Car Factory

Let’s think about some components of a Ford car factory. We may make some stuff up, but this analogy should help us build context for what OOP is.

  • The factory has a machine that is designed to make a Ford Escape
  • All Ford Escapes have four wheels and four doors
  • The color, interior fabric and engine size can vary from one Ford Escape to another

With the information above, in theory, the manager at the factory should be able to say “please make a blue Escape with a leather interior and 2.5 cylinder engine”, and that should be possible.

That example about the car factory ties directly to concepts of OOP. We will use three main technical terms today. They are below, with the connection to the car factory example:

  • class - A class is like a blueprint or template. The factory machines are designed to make a car - that is the template for ALL Ford Escapes. We could probably name it the EscapeCar class.
  • object instance - An instance is an object that is made from the class template. The blue Escape that the manager requested (and was able to physically sit in and drive) is referred to as an object or instance of the EscapeCar class.
  • attribute - An attribute is a specific characteristic about an instance that has the potential of varying from other instances. With our Escapes, color, interior and engine would be attributes, because they weren’t programmed into the template, but were extra information the manager was able to give for each specific instance of that class. The fact that the Escape has four wheels may also be an attribute; but it’s not a dynamic attribute since all Escapes have four wheels.

Brainstorm & Share

Pick an object that you see in the physical space you are in. It could be a candle, water bottle, glove, anything!

  • What type of object is it? What might the class name be?
  • Are there multiple objects/instances of it in your home, or in the world? List a few.
  • What are some of its attributes?

Draft your response to the prompts/questions in the Slack thread but don't send it until told to!

Explore Class Syntax

While this may feel uncomfortable at first, research shows that one of the most effective ways to learn is to explore. Instead of your instructor typing code and explaining every character, you are going to read some Ruby code that’s already been written. You will likely have some questions about what X or Y is doing, and you’ll also likely be able to make some deductions about what other pieces are doing. We will talk through it all after you’ve had a chance to use your brain and make some connections on your own.

Look at the code in this replit and think through the guiding questions:

  • What is the name of the class?
  • How many instances are being made?
  • What are the attributes? Which is a default attribute and which is dynamic?
  • How might you make another instance?

Let’s discuss what you probably found and maybe had questions about.

Write a New Class

In the previous section, you identified an object in your physical space and wrote a list of instances and attributes. The Navigator in this activity will direct their Driver to write a Ruby program that represents this object.


The person with the largest number of characters in their name is the Driver, the other is the Navigator. The Navigator should have this document opened, and the Driver should be screen sharing with their Terminal opened. The Navigator will direct the Driver through the steps listed below.

  • Create a new project called oop_lesson. Inside the project, create a file called first_class.rb OR, create a Ruby file in replit - your choice.
  • Inside that file, write a class. Navigator - the class should represent the object your identified in your physical space. It does not yet need attributes, just the class name.
  • Below the class declaration, create an object instance. Print it out and run the file to confirm it has been created successfully.
  • Now, add a default attribute.
  • Now, add a dynamic attribute. Remember, something about the object instance will need to change now that the class expects dynamic info.
  • Run the file again to check for errors or issues. If you don't have any, create a few more object instances. Consider adding another dynamic attribute.

Since the Navigator had to do all the talking, the Driver should practice articulating what this code does. 2-3 Drivers will be called on and expected to screen share and talk through the code when we come back together as a whole group.

Access Attributes

With the class you wrote in the previous activity try to print out just one attribute. Does it work?

Problem Solving

Use the resource provided and work together to implement the attr_reader. Work to build an understanding of its utility.

You'll need to allow read access with an attr_reader inside the class definition.

Instance Methods

Information, even when we include dynamic information, about an object is not always enough. In software engineering, we also want the ability to change information about an object. We will use instance methods to accomplish this.

Instance methods are methods defined by the developer that can be called on any existing instance object of that class. What follows is an example to illustrate the syntax:

class EscapeCar
  def initialize(color)
    @color = color
    @wheels = 4
    @gear = "park"

  def change_gear(new_gear)
    @gear = new_gear

car1 = EscapeCar.new("blue")
p car1
p car1

As a class, we will discuss what this code is doing, and add to it! Be ready to take clear notes as every word we discuss isn’t documented here in the lesson.

Connection to Apps we Use

Now that you’ve seen how this “factory” concept can be used in code, you may be wondering about how this code concept is used within apps we use every day.

We won’t get all the way there in illustrating it today, but, we can look at Instagram for a moment to talk through where we see some use of classes and instances.

Build A Class with Specifications

Many times, you’ll build a class based on specifications (commonly referred to as specs). They will usually be written in the form of tests, which we will learn about later. For now, use the specifications below to write a class and object instances from it. You should struggle a bit while completing this, but do your best and try something!


  • In a new file, write a class called Dino
  • It should have a dynamic name attribute
  • It should have a dynamic period attribute
  • It should have a color attribute, that is green by default
  • It should have a roar method that roars a sentence including its name, when called

Challenge Extensions

  • It should have a diet attribute that defaults to "vegetarian" but, if data is passed in, it can be dynamic
  • It should have a print_welcome method that prints a sentence welcoming the Dino to the world. The welcome message should vary based on the period the Dino existed (Triassic, Jurassic, and Cretaceous are examples).

FYI: Storing Many Objects

If you’d like to print out all your sentences with one line of code, one way to do that is by storing them in an array. A couple changes were made from our original code in order to write this solution.

class Message
  def initialize(recipient)
    @recipient = recipient

  def create_message
    "To #{@recipient}: Hello!"

mom = Message.new("Mom")
bff = Message.new("BFF")

messages = [

puts messages

In OOP, it is common to store instances in a list, or array, like the one above. Consider a list of Instagram posts discussed earlier. It’s likely Instagram stores all posts attached to each user profile in an Array.

Check For Understanding

Follow the directions in the README of this repository, and submit your fork in the submission form!


Use these as resources for extra practice and challenge!

Mild 🔥

You will get starter code and some step-by-step directions that walk you through what to do and why you're doing it.

This is your starter kit.

Medium 🔥🔥

You will get some starter code and some direction on what to add to it, but will be pushed to apply all of today's learning without explicit direction at times.

This is your starter kit.

Spicy 🔥🔥🔥

You will not have any starter code for this option. Choose your own topic to make a class and instances of that class. There is no starter kit; create a new file on your own!