OOP Concepts in Java - Object, Class and Four Pillars

⏱️ 8 min read • Beginner Level • Lesson 43

Object-Oriented Programming, commonly called OOP, is a programming style where we solve problems by creating objects. Java is strongly based on OOP concepts such as class, object, encapsulation, abstraction, inheritance, and polymorphism.

Before learning OOP concepts, you should understand Java variables, Java methods, and Java data types.

What is OOP in Java?

OOP stands for Object-Oriented Programming. It is a programming approach where a program is designed using objects. An object combines data and behavior into a single unit.

Why use OOP?

Think of a real-world school system. A student has data such as name, roll number, and marks. A student also has actions such as study, attend exam, and show result. In OOP, we can represent this real-world student as an object.

Real-World Example of OOP

OOP vs Procedural Programming

Object in Java

An object is a real-world entity that has state and behavior.

For example, a laptop can be treated as an object:

Class in Java

A class is a blueprint or template used to create objects. It defines what data and methods an object will have.

For example, Student can be a class, and Ayan, Sarah, and John can be objects of that class.

Class and Object Example in Java

The following example creates a Student class and uses it to create an object.

class Student {
    String name;
    int rollNumber;

    void displayDetails() {
        System.out.println("Name: " + name);
        System.out.println("Roll Number: " + rollNumber);
    }
}

public class StudentExample {
    public static void main(String[] args) {
        Student s1 = new Student();

        s1.name = "Ayan";
        s1.rollNumber = 101;

        s1.displayDetails();
    }
}

Output:

Name: Ayan
Roll Number: 101

Four Pillars of OOP in Java

Java OOP is mainly based on four important concepts:

Encapsulation in Java

Encapsulation means binding data and methods together inside a class. It also helps protect data by restricting direct access from outside the class.

A common way to achieve encapsulation in Java is by using private variables and public getter/setter methods.

Abstraction in Java

Abstraction means hiding implementation details and showing only important information to the user.

For example, when you drive a car, you use the steering wheel, accelerator, and brake. You do not need to know the internal engine working every time you drive.

Inheritance in Java

Inheritance allows one class to acquire the properties and methods of another class. It supports code reusability.

For example, Dog can inherit common properties from an Animal class.

Polymorphism in Java

Polymorphism means many forms. In Java, it allows one action to behave differently depending on the object or situation.

OOP Concepts Summary Table

Interview Questions ⭐

🧠 Test your understanding with a quick quiz

class Student {
    String name = "Ayan";

    void display() {
        System.out.println(name);
    }
}

public class Test {
    public static void main(String[] args) {
        Student s1 = new Student();
        s1.display();
    }
}

🎉 Great job! Continue learning Java OOP step by step.