Classes and Objects: The Blueprint and the House

Java is an Object-Oriented Programming (OOP) language. This means instead of just writing a list of instructions, we organize our code around "things" (Objects). To create these things, we first need a Class.

1. What is a Class?

Think of a Class as a blueprint. A blueprint for a house isn't a house itself—it's just a set of instructions that describes what a house should have (walls, windows, doors).

An Object is the actual house built from that blueprint. You can build 100 different houses from the same single blueprint.

2. Anatomy of a Class

A class contains two main things:

1. Fields (Attributes): What the object knows (data).
2. Methods (Behaviors): What the object does (actions).

public class Car {
    // 1. Fields
    String model;
    String color;
    int speed;

    // 2. Methods
    void drive() {
        System.out.println("The " + model + " is moving!");
    }
}

3. Creating Objects (Instantiating)

To build an object from a class, we use the new keyword. This is called instantiation.

public class Main {
    public static void main(String[] args) {
        // Create the first object
        Car myCar = new Car();
        myCar.model = "Tesla";
        myCar.color = "Black";

        // Create a second object from the same blueprint
        Car yourCar = new Car();
        yourCar.model = "BMW";

        myCar.drive();   // Output: The Tesla is moving!
        yourCar.drive();  // Output: The BMW is moving!
    }
}

4. Why use Classes?

• Modularity: You can keep related data and functions together in one unit.
• Reusability: Once you write a User or Product class, you can use it across your entire application.
• Real-world Mapping: It's easier to code when your software structures look like real-world entities.

💻 Full Practical Example: The Player Profile

Copy these into two separate files (or one file if you remove the public keyword from the Player class):

File: Player.java

public class Player {
    String username;
    int level = 1;
    int health = 100;

    void levelUp() {
        level++;
        System.out.println(username + " reached level " + level + "!");
    }

    void showStatus() {
        System.out.printf("Player: %s | HP: %d | LVL: %d%n", username, health, level);
    }
}

File: Main.java

public class Main {
    public static void main(String[] args) {
        Player p1 = new Player();
        p1.username = "Name";
        
        p1.showStatus();
        p1.levelUp();
        p1.showStatus();
    }
}

⚠️ Important: Static vs Non-Static

You might have noticed our previous methods used static.

• Static methods belong to the Class (you don't need to create an object to use them).
• Non-static methods (like drive() or levelUp()) belong to the Object. You must create an object with new before you can call them.

💡 Challenge: The Smart Home Device

Create a class called Lamp.

1. Add a boolean field isOn.
2. Add a method pressSwitch() that flips the boolean (if it was true, make it false, and vice versa).
3. Add a method showState() that prints "Light is ON" or "Light is OFF".
4. In your main method, create two different lamps and turn one of them on!