🧠 1. Why do we need OOP?

Imagine you're writing a program for a school to manage:

  • Students
  • Teachers
  • Courses

Without OOP, you'd write all data and logic in one place — which becomes messy, confusing, hard to update, and reuse.

✅ OOP helps us by:

Organization
Code into real-world objects
Reusability
Use code again and again
Security
Hide and protect data
Maintainability
Easy to understand and update

💡 2. What is OOP?

OOP = Object-Oriented Programming
It's a style of programming that uses objects and classes to represent real-world things.

🏠 Real-World Analogy

Imagine you're building a game where:

  • A Dog has name, color, and can bark.
  • A Car has brand, speed, and can move.

Instead of mixing everything, we create:

  • A Dog class
  • A Car class

And we create objects like:

  • Dog d1 = new Dog();
  • Car c1 = new Car();

That's the power of OOP — structure, clarity, and real-world feel.

📦 3. What is a Class?

Class Example
A class is like a blueprint or design for objects.
Example: "Car" class → tells what a car has (color, speed) and what it can do (drive)

🧱 4. What is an Object?

An object is a real thing created from the class.
Example: My red car → has speed 80, color red

✅ Java Code Example:

// Create a Car blueprint
class Car {
    String color;
    int speed;

    void drive() {
        System.out.println("Driving at speed: " + speed);
    }
}

// Main class
public class Main {
    public static void main(String[] args) {
        Car myCar = new Car();  // Create object

        myCar.color = "Red";
        myCar.speed = 100;

        System.out.println("Car color: " + myCar.color);
        myCar.drive(); // Method call
    }
}

🔐 5. What is Encapsulation?

Encapsulation Example
Encapsulation = wrapping data(variables) + methods in one unit (class), and hiding data from outside.

🔒 Real-World Example:

Bank ATM: You can use buttons (methods), but can't see the code (data is hidden)

🧰 6. What are Access Modifiers?

These control who can access your variables/methods.

Access Modifiers

🛠 7. Getter and Setter (Easy Explanation)

In Java, private variables are not directly accessible from outside the class. To access or update them safely, we use:

  • Getter – to read the value
  • Setter – to set or update the value

✅ Simple Java Example:

class Student {
    private String name; // private data (not directly visible)

    // Setter - allows us to set the name
    public void setName(String n) {
        name = n;
    }

    // Getter - allows us to get the name
    public String getName() {
        return name;
    }
}

public class Main {
    public static void main(String[] args) {
        Student s = new Student(); // Create object

        s.setName("Deepak"); // Set value using setter
        System.out.println("Student name: " + s.getName()); // Get value using getter
    }
}

💬 Output:
Student name: Deepak

🔒 Why use this?
To protect the name field from unwanted access and changes, and to give controlled access using methods.

📌 Summary of Terms

Term Meaning (Simple) Real Life Example
Class Blueprint/design of something Car design
Object Real item created from class Your red car
Encapsulation Hide data, give access using methods ATM only shows screen
Private Used inside only Secret locker key
Public Can be used by anyone TV remote
Getter/Setter To read/write private variables ATM buttons to check balance

💬 Common Interview Questions

Question What to Say
What is OOP? A style of programming using classes and objects.
What is a class? A blueprint or design for objects.
What is an object? A real instance of a class.
What is encapsulation? Wrapping variables and methods inside a class and hiding data.
Why use private and public? To control who can access data.
What are getter and setter? Methods to read/write private variables.

✅ Practice Task

Create a class BankAccount with:

  • Private balance
  • deposit(amount) method
  • getBalance() method
  • Prevent direct access to balance

Challenge: Try to implement this using the concepts you learned!