Understanding the Fundamentals of OOPS in Java

Java is a powerful and versatile programming language widely used for building various applications. One key reason behind its popularity is its foundation in Object-Oriented Programming (OOP) principles. But what exactly is OOPs in Java, and why is it so important?

As we already know, object-oriented programming system (OOPS) is a programming paradigm centred around the concept of objects. These objects are instances of a class, which can be thought of as blueprints for creating individual pieces of data with specific properties (attributes) and behaviours (methods). It allows us to create modular, reusable, and maintainable code, which is crucial for building large and complex software systems.

In this blog, we will explore the core concepts of OOPs in Java, including classes, objects, constructors, along with its four pillars: inheritance, polymorphism, encapsulation, and abstraction, using code and diagrams, wherever necessary. These  concepts will help us understand how Java helps us in writing efficient, organised, and scalable code.

Let’s start…

Index

What is OOPS in Java?

As told above, OOPS is an essential concept of Java and a method or paradigm that is centered around the concept of objects, which can be briefly defined as an instance of a class. These objects are just like real-world entities with their unique characteristics, behaviour, methods etc, used to design and build applications. Unlike procedural programming, which relies on sequences of instructions, OOPs organize code into reusable objects, making it easier for us to manage, extend and maintain our code.

If we now try to understand the concept of OOPS, it is basically a structured way of writing code by organizing it in the form of classes and objects which makes managing the code easier.

Key Concepts of OOPS in Java

1. Objects – They are the instances of a class and represent real-world entities, which is the key strength of OOPS in Java.

2. Class – A class is a blueprint of an object or rather a blueprint that is used to create an object, as it tells us which properties and methods the object will have.

3. Method – Actions or functions that are performed by an object, like interacting with other objects, modifying data, etc, are called methods.

4. Method Overloading – A process in which multiple methods of a class have the same name and different behaviours is called method overloading.

5. Method Overriding – A process in which a method of the parent class gets overridden by the method of the child class is called method overriding.

6. Subclass – We can also call it the child class, as a subclass or a child class is defined as the class that inherits properties from other classes.

7. Super class – We can call it the parent class, as a super class or a parent class is defined as a class from which other classes inherit properties and behaviours.

8. Constructors – They are the special types of methods that do not have a return type and are initialised as soon as an object is created. They are basically used to initialise objects. They are of 3 types – default, no argument, and parameterised constructors.

9. Access Modifiers – These are also some special types of keywords that are used to restrict the visibility and accessibility of classes, constructors, etc. They are of 4 types – public, private, default and protected.

Now that we have understood the key concepts of OOPa, it’s time to understand the principles or pillars of OOPs. There are 4 pillars of OOPs – Abstraction, Encapsulation, Inheritance and Polymorphism. 

Let’s understand them briefly, as we will discuss them in detail in their respective blogs.

Pillars of OOPS in Java

As told there are four pillars – 

1. Abstraction
2. Encapsulation
3. Polymorphism
4. Inheritance

These four combined makes our code more modular, scalable and secure. Let’s understand each of them.

What is abstraction in Java?

The word abstract in English refers to something that cannot be directly seen or touched.  And in computer science, abstraction is the process of hiding complex details while exposing only the essential parts to the user. This helps simplify interaction with systems by focusing on the “what” rather than the “how.”

Let’s understand this with a real-world example: a car. When we press the accelerator pedal, the car speeds up — simple, right? But behind that simple action lies a complex system of engines, gears, and wiring. Do we open the bonnet and understand how everything works before taking our car out? Of course not — and that’s the beauty of abstraction.

All the intricate engineering is hidden from the user, and only the necessary controls (like the pedal and gear shift) are made accessible. As drivers, we don’t need to know how everything works under the hood — we just need to know what to do to make the car move.

Ways of achieving abstraction in Java

There are two ways of achieving abstraction in Java 

1. Abstract classes
2. Interface.

Through the abstract class, we cannot achieve total abstraction because it has both abstract methods (which don’t have a body) and concrete methods (which do). On the other hand, an interface allows us to achieve 100% abstraction because all its methods are abstract, meaning none of them have a body or implementation.

By using abstraction, developers can define a common interface for different classes, ensuring consistency and reducing code complexity.

Encapsulation

Encapsulation is the process of binding variables and methods together in a single unit, called a class. In simple terms, it encapsulates both data or variables and methods in a class, just like a capsule or pill that we often take, in which many different types of minerals, salts, etc, stuff have been encapsulated or mixed in magic proportions.

The purpose of encapsulation is to protect data from outside access so that it won’t get misused. To achieve encapsulation, the private access modifier is preferred as it restricts direct access from outside and only allows authorised methods to read and modify the data. This ensures data security and controlled communication between the methods as well.

Polymorphism

Polymorphism comes from the Greek words “poly”, meaning “many”, and “morph”, meaning “form” — so it means “many forms.” In programming, it refers to the ability of a single function, method, or object to take on different behaviours depending on how it’s used.

In simple terms, polymorphism lets us use the same method name to do different things. This can happen by changing the number or type of parameters (known as method overloading) or by redefining a method in a subclass (called method overriding).

This makes our code more flexible and easier to manage. Instead of rewriting similar code over and over, we can reuse method names in different ways, which helps reduce complexity, improves reusability, and makes maintenance simpler.

There are two main types of polymorphism:

1. Compile-time polymorphism (or static binding) happens through method overloading.

2. Run-time polymorphism (or dynamic binding) happens through method overriding.

Understanding and using polymorphism helps us write cleaner, more adaptable, and more powerful code.

Inheritance

As the name suggests, a process in which a class (subclass) inherits the properties and methods of another class (super class) is called inheritance. In other words, inheritance is the process of creating a new class from an already existing one.

It makes our code more flexible, maintainable, and properly structured and allows for the creation of more specialised classes from a general base class.

We can very easily achieve inheritance in Java using the extends keyword. See the image below.

In the image above, the code defines a new class Dog that inherits the properties and methods of an existing class Animal using the extends keyword.

Advantages of OOPS in Java

Code Re-usability

With the help of objects and classes, we can create code that can be used again and again, promoting the DRY principle.

Improved Structured

With the help of OOPs, we can give our code a modular design, which will not only make it easier to understand but also to debug.

Scalability

We can very easily extend our code as much as we want with minimal changes to existing code, making growth more manageable. And this is because of the DRY Principle, which makes the code easier to maintain.

Disadvantages of OOPS in Java

Lengthy Code

The size of the code written in Java with the help of OOP can be pretty long, thus making its execution slower when compared to procedural programming.

Security Checks:

Encapsulation and security features can introduce overhead, such as access checks, which can slow down the execution. 

So even though OOPs might seem slower, that’s because it’s doing some important work behind the scenes, like keeping our code organized and secure. While there is a way out of it as well, Many experienced developers write efficient OOP code by leveraging the JVM’s capabilities and employing effective design patterns. 

Not Omnipotent

Principles of OOPs cannot be used everywhere. It is applied only when it is required. It is not suitable for all types of problems.

Learning Curve

The shape of the learning curve for OOPs in Java for beginners is a sigmoid curve.  A sigmoid curve is an S-shaped curve that starts slowly, then rises steeply, and again slows down . Let’s understand it with the help of our favorite and not so favorite subject ‘Maths’. We start learning and understanding it very slowly as it has many simple and complex topics with uncountable formulas.

Learning them were hard at the beginning, but when we started solving problems, it became easier as we started to understand the logic behind each formula. And when we are done with one chapter, we can take it slow while reviewing it before our exams.

Similarly for OOPs also, we start slowly, understanding concepts like classes, objects, abstraction, etc. This can change how we think and solve problems.

As we progress with our journey of OOPS, things become challenging, and we programmers like to wrap our heads around challenges. This accelerates our progress.

And finally, after some vigorous practice and head-butting with these concepts, we have a grasp on the basics of OOPs, which flattens the curve as now these concepts become easier and more natural.

While in comparison, the curve of procedural programming is a gentle, steady upward slope, almost like a straight diagonal line (y=mx+c). This is because –

1. It is all about following instructions.

2. Straightforward concepts which can be easily grasped with regular practice.

3. And after some time, progress feels steady and consistent as we learn how to sequence commands and use functions.

Difference between OOPS and Procedural Programming

Challenges we face while making the transition from Procedural Programming to OOPS in Java

Now that we have seen the difference between procedural programming and OOPS, let’s say we want to make that shift as we think it’s worth it (which it is), there are some challenges that we will face while shifting. They are

Conceptual Shift

As told above, the learning curve of OOPS is very different from the learning curve of Procedural Programming. It is more like an S shape graph rather than a straight line. At first it is very hard to understand concepts like abstraction, polymorphism, inheritance, etc.

Before that we have to first change the way we think and approach problems. Procedural programming is all about writing codes in terms of functions and processing data step by step, but OOPS is like a different world altogether like an alien planet where we have to learn and adjust everything from scratch.

Here everything is made up of objects, we have to keep our data secure and hidden using techniques like abstraction and encapsulation.

How to write code in such a way that it can be used again and again and in different forms (next level dynamic coding) using polymorphism.  This is not an easy task and requires not only a different mindset and approach but also regular practice as told above. 

In procedural programming, we write our codes in the following way

public class Main {

    public static void main(String[] args) {
        drive(); // Call the static method
    }

    static void drive() {
        System.out.println("The car is moving");
    }

}

Output

But now that same code will look something like this

class Car {
    void drive() {
        System.out.println("The car is moving");
    }
}

public class Main {
    public static void main(String[] args) {
        Car myCar = new Car();
        myCar.drive();         
    }
}

Now this shift is huge. This is the shift that we are talking about, it’s not only conceptual but also noticeable.

Understanding Classes

In Object-Oriented Programming (OOP), classes play a foundational role. At their core, classes act as a blueprint that is used to create an object, as it tells us which properties and methods the object will have.

However, a class by itself doesn’t produce any output when the program runs—only when an object is created from it and its methods are used does the code become functional.

This concept can be confusing, especially for those transitioning from procedural programming, where writing code and immediately seeing output is the norm. In contrast, OOPs require organizing code into reusable and modular components, which involves a shift in both design and thinking.

Classes help implement the four main principles of OOP: encapsulation, inheritance, polymorphism, and abstraction. These principles make code more modular, maintainable, and scalable while hiding complex details to improve security. And finally relationships like “is-a” (inheritance) and “has-a” (composition) can be pretty confusing and challenging.

Mastering Syntax

In the above section we wrote a code that tells exactly how much difference there is between the syntax of a normal java code and code written using OOPS principles, they are leaps apart. Like in normal Java code, we don’t have to think about classes, objects and other stuff.

Rather it is only about the task that we have to perform. But here, we have to think about classes first even before creating objects as they will be created from these classes only. 

The syntax of the OOP program does not change only because of the presence of classes and objects, it changes with slight modification in our main method. The main method as we all know is the starting point of every Java program and the starting point of OOPS as well.

The main method is static in nature and is present even in simple Java programs, but what makes a program truly object-oriented is not the presence of a class, but how objects are created and used within it.

Main Method in a Simple Java Program

public class SimpleProgram {
    public static void main(String[] args) {
        System.out.println("Hello, world!");
    }
}

Main Method in an Object-Oriented Java Program

public class Main {
    public static void main(String[] args) {
        Car myCar = new Car();     // Object created
        myCar.drive();             // Object method called
    }
}

The main method is just the starting, there are concepts like the constructors and methods which syntax wise looks the same but are not, constructors do not have a return type while methods do.

And finally the access modifiers which define the visibility and accessibility of a class can confuse us as which specifier to use under which conditions or circumstances as there are four of them.

Read this blog to understand other challenges we might face while making a transition or even learning OOPS in Java

Conclusion

In this blog, we explored how Object-Oriented Programming (OOP) in Java is not just a programming paradigm but a core approach that works seamlessly with languages like python and C++ with their own set of rules. 

We saw how OOP offers significant advantages over procedural programming, bringing structure, reusability, and scalability to software development. However, like every coin has two sides, OOP also comes with its own set of challenges—such as a steeper learning curve, increased memory usage, and sometimes added complexity.

We also touched on the fundamental concepts that serve as the building blocks of both OOP and Java itself—objects, classes, constructors, methods, access modifiers, and more. And, of course, we covered the four main pillars of OOP: abstraction, encapsulation, inheritance, and polymorphism—principles that make Java robust and adaptable.

That said, we’ve only scratched the surface. Each of these concepts has depth and nuances that deserve deeper exploration. Stay tuned, as we’ll dive further into each topic in upcoming blogs!

Frequently Asked Questions