Introduction to OOP (Classes and Objects)

Imagine you are an architect. You draw a Blueprint for a house. The blueprint isn't a house; you can't live in it, and it doesn't have a physical address. It’s just a set of instructions that says, "A house should have 3 bedrooms, 2 bathrooms, and be painted a certain color."

• The Class is the Blueprint. It defines what data (Variables) and what actions (Methods) a specific thing should have.

• The Object is the Physical House. Once you use the blueprint to build a house at 123 Maple St, that specific house is an "Instance" of the blueprint. You can build 100 different houses from the same blueprint—each might have a different color or owner, but they all follow the same structure.

The Two Pillars of a Class

1. Fields (Attributes): These are variables that describe the object (e.g., color, model, price).

2. Methods (Behaviors): These are functions that describe what the object can do (e.g., drive(), brake(), honk()).

The Code Example

In Java, we usually create a separate class and then use it in our Main class.

// 1. Defining the Blueprint (The Class)
class Car {
    // Fields (Attributes)
    String brand;
    int year;
    boolean isElectric;

    // Method (Behavior)
    void drive() {
        System.out.println("The " + brand + " is now zooming down the road!");
    }

    void displayInfo() {
        System.out.println("Car: " + year + " " + brand + " (Electric: " + isElectric + ")");
    }
}

// 2. Using the Blueprint (The Main Class)
public class Main {
    public static void main(String[] args) {
        // Creating the first Object (Instance)
        Car myCar = new Car(); 
        myCar.brand = "Tesla";
        myCar.year = 2024;
        myCar.isElectric = true;

        // Creating a second Object (Instance) from the same blueprint
        Car friendCar = new Car();
        friendCar.brand = "Ford Mustang";
        friendCar.year = 1969;
        friendCar.isElectric = false;

        // Using the methods
        myCar.displayInfo();
        myCar.drive();

        System.out.println("---");

        friendCar.displayInfo();
        friendCar.drive();
    }
}

Expected Output:

Car: 2024 Tesla (Electric: true)
The Tesla is now zooming down the road!
---
Car: 1969 Ford Mustang (Electric: false)
The Ford Mustang is now zooming down the road!

Before OOP, programming was just a long list of instructions. If you wanted to change how a "Car" worked, you’d have to find every mention of a car in 10,000 lines of code.

With OOP:

• Organization: Everything related to a "Car" is inside the Car class.

• Scalability: You can create thousands of "Car" objects easily.

• Realism: It allows us to model the real world. Instead of thinking about "bits and bytes," we think about "Users," "Products," and "Messages."

Note on new: The keyword new is what tells Java to go into the computer's memory and actually build the object based on the class.