What is object-oriented programming? OOP explained in depth

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Object-oriented programming (OOP) is a fundamental programming paradigm used by nearly every developer at some point in their career. OOP is the most popular programming paradigm used for software development and is taught as the standard way to code for most of a programmer’s educational career. Another popular programming paradigm is functional programming, but we won’t get into that right now.

Today we will break down the basics of what makes a program object-oriented so that you can start to utilize this paradigm in your algorithms, projects, and interviews.

Now, let’s dive into these OOP concepts and tutorials!

Here’s what will be covered:

What is Object-Oriented Programming?

Object-Oriented Programming (OOP) is a programming paradigm in computer science that relies on the concept of classes and objects. It is used to structure a software program into simple, reusable pieces of code blueprints (usually called classes), which are used to create individual instances of objects. There are many object-oriented programming languages, including JavaScript, C++, Java, and Python.

OOP languages are not necessarily restricted to the object-oriented programming paradigm. Some languages, such as JavaScript, Python, and PHP, all allow for both procedural and object-oriented programming styles.

A class is an abstract blueprint that creates more specific, concrete objects. Classes often represent broad categories, like Car or Dog that share attributes. These classes define what attributes an instance of this type will have, like color, but not the value of those attributes for a specific object.

Classes can also contain functions called methods that are available only to objects of that type. These functions are defined within the class and perform some action helpful to that specific object type.

For example, our Car class may have a repaint method that changes the color attribute of our car. This function is only helpful to objects of type Car, so we declare it within the Car class, thus making it a method.

Class templates are used as a blueprint to create individual objects. These represent specific examples of the abstract class, like myCar or goldenRetriever. Each object can have unique values to the properties defined in the class.

For example, say we created a class, Car, to contain all the properties a car must have, color, brand, and model. We then create an instance of a Car type object, myCar to represent my specific car.

We could then set the value of the properties defined in the class to describe my car without affecting other objects or the class template.

We can then reuse this class to represent any number of cars.

Benefits of OOP for software engineering

  • OOP models complex things as reproducible, simple structures
  • Reusable, OOP objects can be used across programs
  • Polymorphism allows for class-specific behavior
  • Easier to debug, classes often contain all applicable information to them
  • Securely protects sensitive information through encapsulation

How to structure OOP programs

Let’s take a real-world problem and conceptually design an OOP software program.

Imagine running a dog-sitting camp with hundreds of pets where you keep track of the names, ages, and days attended for each pet.

How would you design simple, reusable software to model the dogs?

With hundreds of dogs, it would be inefficient to write unique entries for each dog because you would be writing a lot of redundant code. Below we see what that might look like with objects rufus and fluffy.

As you can see above, there is a lot of duplicated code between both objects. The age() function appears in each object. Since we want the same information for each dog, we can use objects and classes instead.

Grouping related information together to form a class structure makes the code shorter and easier to maintain.

In the dogsitting example, here’s how a programmer could think about organizing an OOP:

  1. Create a class for all dogs as a blueprint of information and behaviors (methods) that all dogs will have, regardless of type. This is also known as the parent class.
  2. Create subclasses to represent different subcategories of dogs under the main blueprint. These are also referred to as child classes.
  3. Add unique attributes and behaviors to the child classes to represent differences
  4. Create objects from the child class that represent dogs within that subgroup

The diagram below represents how to design an OOP program by grouping the related data and behaviors together to form a simple template and then creating subgroups for specialized data and behavior.

The Dog class is a generic template containing only the structure of data and behaviors common to all dogs as attributes.

We then create two child classes of Dog, HerdingDog and TrackingDog. These have the inherited behaviors of Dog (bark()) but also behavior unique to dogs of that subtype.

Finally, we create objects of the HerdingDog type to represent the individual dogs Fluffy and Maisel.

We can also create objects like Rufus that fit under the broad class of Dog but do not fit under either HerdingDog or TrackingDog.

Building blocks of OOP

Next, we’ll take a deeper look at each of the fundamental building blocks of an OOP program used above:

  • Classes
  • Objects
  • Methods
  • Attributes

Classes

In a nutshell, classes are essentially user-defined data types. Classes are where we create a blueprint for the structure of methods and attributes. Individual objects are instantiated from this blueprint.

Classes contain fields for attributes and methods for behaviors. In our Dog class example, attributes include name & birthday, while methods include bark() and updateAttendance().

Here’s a code snippet demonstrating how to program a Dog class using the JavaScript language.

Objects

Objects are, unsurprisingly, a huge part of OOP! Objects are instances of a class created with specific data. For example, in the code snippet below, Rufus is an instance of the Dog class.

When the new class Dog is called:

  • A new object is created named rufus
  • The constructor runs name & birthday arguments, and assigns values

Programming vocabulary:

In JavaScript, objects are a type of variable. This may cause confusion because objects can be declared without a class template in JavaScript, as shown at the beginning.

Objects have states and behaviors. The state of an object is defined by data: things like names, birthdates, and other information you’d want to store about a dog. Behaviors are methods the object can undertake.

N/A What is it? Information Contained Actions Example
Classes Blueprint Attributes Behaviors defined through methods Dog Template
Objects Instance State, Data Methods Rufus, Fluffy

Attributes

Attributes are the information that is stored. Attributes are defined in the Class template. When objects are instantiated, individual objects contain data stored in the Attributes field.

The state of an object is defined by the data in the object’s attributes fields. For example, a puppy and a dog might be treated differently at a pet camp. The birthday could define the state of an object and allow the software to handle dogs of different ages differently.

Methods

Methods represent behaviors. Methods perform actions; methods might return information about an object or update an object’s data. The method’s code is defined in the class definition.

When individual objects are instantiated, these objects can call the methods defined in the class. In the code snippet below, the bark method is defined in the Dog class, and the bark() method is called on the Rufus object.

Methods often modify, update or delete data. Methods don’t have to update data though. For example, the bark() method doesn’t update any data because barking doesn’t modify any of the attributes of the Dog class: name or birthday.

The updateAttendance() method adds a day the Dog attended the pet-sitting camp. The attendance attribute is important to keep track of for billing Owners at the end of the month.

Methods are how programmers promote reusability and keep functionality encapsulated inside an object. This reusability is a great benefit when debugging. If there’s an error, there’s only one place to find it and fix it instead of many.

The underscore in _attendance denotes that the variable is protected and shouldn’t be modified directly. The updateAttendance() method changes _attendance.

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