OOP Concepts in Java (4 Pillars)

In this lesson: Encapsulation, inheritance, polymorphism and abstraction in Java explained simply with real-life examples and code.

What is OOP?

Object-Oriented Programming (OOP) is a way of writing programs by organizing code around objects instead of just functions and logic. An object bundles together data (fields) and actions (methods) that work on that data — just like a real-world object such as a car has properties (color, speed) and actions (start, brake).

Java is a fully object-oriented language — almost everything in Java lives inside a class. OOP makes large programs easier to understand, reuse and maintain, which is why it is the foundation of Android, enterprise and banking software.

The Four Pillars of OOP

Every OOP concept is built on four core principles, often called the four pillars:

A Complete Example

This small program shows a class, an object, encapsulation (private field + methods) and inheritance together:

class Animal {
    private String name;           // encapsulation: private field
    Animal(String name) { this.name = name; }
    void eat() { System.out.println(name + " is eating"); }
}
class Dog extends Animal {   // inheritance
    Dog(String n) { super(n); }
    void bark() { System.out.println("Woof!"); }
}
public class Main {
    public static void main(String[] args) {
        Dog d = new Dog("Tommy");  // object creation
        d.eat();   // inherited from Animal
        d.bark();  // Dog's own method
    }
}

Tommy is eating
Woof!

Why Use OOP?

• Reusability — write code once (a class) and reuse it via objects and inheritance.
• Maintainability — changes stay contained; fixing one class doesn't break others.
• Security — encapsulation hides sensitive data behind controlled methods.
• Real-world modelling — code mirrors real objects, so it's easier to reason about.

Summary

• OOP organizes code around objects = data (fields) + actions (methods).
• Four pillars: Encapsulation, Inheritance, Polymorphism, Abstraction.
• A class is a blueprint; an object is a real instance created with new.
• OOP gives reusability, maintainability, security and real-world modelling.