What is Object-Oriented Programming (OOP)?

A Complete Interview-Oriented Guide with Real-World Examples (Java)

✅ Introduction

Object-Oriented Programming (OOP) is one of the most important programming paradigms used in enterprise applications, backend systems, game engines, mobile apps, financial platforms, and modern software design.

In simple words:

OOP is a programming approach where software is designed using objects.
Each object contains:

State (Data) → variables / attributes
Behavior (Actions) → methods / functions

Think of a “Car”:

State → color, model, speed
Behavior → start(), stop(), accelerate()

That’s OOP!

🔥 Why OOP?

✔ Better code reusability
✔ Cleaner and modular structure
✔ Easier to maintain and extend
✔ Closer to real-world modelling
✔ Supports scalability

No wonder Java, C++, Python, C#, Kotlin, Swift, etc., are heavily OOP-driven.

🧱 Core Concepts of OOP (4 Pillars)

Every interviewer expects clear understanding of:
1️⃣ Encapsulation
2️⃣ Inheritance
3️⃣ Polymorphism
4️⃣ Abstraction

Let’s understand each with simple language + Java code + interview clarity.

🔒 1) Encapsulation — Data Protection + Controlled Access

Definition (Interview Answer):
Encapsulation is wrapping data and methods together inside a class and restricting direct access to the data using access modifiers like private, public, etc.
Data is accessed through getters & setters.

🏦 Real-World Example

A bank does not allow directly changing balance.
You must deposit() / withdraw() through proper rules.

✅ Java Example


class BankAccount {
    private double balance;   // data hidden

    public double getBalance() {
        return balance;
    }

    public void deposit(double amount) {
        if (amount <= 0) {
            System.out.println("Invalid deposit");
            return;
        }
        balance += amount;
        System.out.println("Deposited: " + amount);
    }
}

✔ Data security
✔ Validation
✔ Controlled modification

Interview Tip:
Encapsulation supports data hiding, maintainability, and security.

🧬 2) Inheritance — Code Reusability + IS-A Relationship

Definition (Interview Answer):
Inheritance allows one class (child/subclass) to acquire properties and behaviors of another class (parent/superclass).
This represents IS-A relationship.

Example:

Car IS-A Vehicle
Dog IS-A Animal

🚗 Java Example


class Vehicle {
    void drive() {
        System.out.println("Vehicle is moving");
    }
}

class Car extends Vehicle {
    void playMusic() {
        System.out.println("Playing music in car");
    }
}

Output


Vehicle is moving
Playing music in car

✔ Code reuse
✔ Logical hierarchy
✔ Reduces duplication

🎭 3) Polymorphism — Same Action, Different Behavior

Definition (Interview Answer):
Polymorphism means one name, many forms.
In Java, it occurs in two ways:

✔ Compile-Time Polymorphism (Method Overloading)

Same method name, different parameters.


class MathUtil {
    int add(int a, int b) { return a + b; }
    double add(double a, double b) { return a + b; }
}

✔ Runtime Polymorphism (Method Overriding)

Child class provides its own implementation of a parent method.


class Payment {
    void pay(double amount) {
        System.out.println("General Payment");
    }
}

class UpiPayment extends Payment {
    void pay(double amount) {
        System.out.println("Paying using UPI");
    }
}

Key Interview Line:
Runtime polymorphism works using dynamic binding where the decision is made at runtime based on object type.

🧩 4) Abstraction — Hide Internal Complexity

Definition (Interview Answer):
Abstraction means hiding internal implementation and exposing only essential functionality.

We achieve abstraction using:
✔ Abstract Classes
✔ Interfaces

🔔 Real-World Example

When you use a Washing Machine:

You press Start()
You don't know internal motor logic
That is abstraction.

✅ Java Example — Interface Based


interface Notification {
    void send(String message);
}

class EmailNotification implements Notification {
    public void send(String message) {
        System.out.println("Email Sent: " + message);
    }
}

Caller only calls send() — internal working is hidden.

🏢 Real-World Style Example — Food Delivery Mini-System

This simple mini-example shows multiple OOP concepts together:

✔ Encapsulation → order data protected
✔ Polymorphism → multiple payment modes
✔ Abstraction → notification channel hidden
✔ Composition → Restaurant HAS-A Menu


abstract class PaymentMethod {
    abstract void pay(double amount);
}

class CardPayment extends PaymentMethod {
    void pay(double amount) {
        System.out.println("Paid using Card: " + amount);
    }
}

interface Notification {
    void send(String msg);
}

class EmailNotification implements Notification {
    public void send(String msg) {
        System.out.println("EMAIL: " + msg);
    }
}

class Order {
    private double amount;

    public Order(double amount) {
        this.amount = amount;
    }

    public double getAmount() {
        return amount;
    }
}

🎯 OOP Interview Q&A — Quick Punch Answers

❓ What is OOP?

Programming style based on objects that combine data and behavior.

❓ Why OOP?

Reusability
Maintainability
Real-world modelling
Security
Scalability

❓ What are the 4 pillars of OOP?

Encapsulation, Inheritance, Polymorphism, Abstraction

❓ Difference between Abstraction and Encapsulation?

Abstraction	Encapsulation
Hides implementation	Hides data
Focus on “what” system does	Focus on “how” data is accessed
Done using abstract classes & interfaces	Done using private variables + getters/setters

❓ What is IS-A vs HAS-A relationship?

IS-A (Inheritance)
Car IS-A Vehicle

HAS-A (Composition)
Car HAS-A Engine

❓ What is Runtime Polymorphism?

When method call is decided at runtime using overriding.

❓ Can static methods be overridden?

No. They are hidden, not overridden.

🏁 Final Summary

Object-Oriented Programming helps developers build structured, modular, and real-world like applications.

Encapsulation → protects data
Inheritance → reuses behavior
Polymorphism → flexibility
Abstraction → hides complexity

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