Classes and Objects

This is the big one. Everything in Java revolves around classes and objects, and now you're going to build your own. If Java were GTA Vice City, this lesson is where you stop doing missions for other people and start building your own empire

So what's a class? Think of it as a blueprint. A blueprint for a car describes what a car has (color, speed, fuel) and what it can do (drive, brake, honk). But a blueprint isn't a car itself. You build cars from the blueprint

In Java terms

A class is the blueprint
An object is a specific thing built from that blueprint (also called an instance)

class Car {
    String color;
    int speed;
}

This creates a class called Car with two fields (also called attributes or properties): a color (String) and a speed (int). No actual car exists yet, it's just the blueprint

To create an actual car, we use the new keyword

class Car {
    String color;
    int speed;
}

public class Main {
    public static void main(String[] args) {
        Car myCar = new Car();
        myCar.color = "red";
        myCar.speed = 120;

        System.out.println("Color: " + myCar.color);
        System.out.println("Speed: " + myCar.speed);
    }
}

Output

Color: red
Speed: 120

new Car() creates a new Car object. We store it in a variable called myCar. Then we use dot notation (myCar.color, myCar.speed) to set and read its fields

The power of classes is that you can create multiple objects from the same blueprint

class Character {
    String name;
    int health;
    String weapon;
}

public class Main {
    public static void main(String[] args) {
        Character tommy = new Character();
        tommy.name = "Tommy Vercetti";
        tommy.health = 100;
        tommy.weapon = "M4";

        Character lance = new Character();
        lance.name = "Lance Vance";
        lance.health = 80;
        lance.weapon = "Pistol";

        System.out.println(tommy.name + " has " + tommy.health + " HP and carries a " + tommy.weapon);
        System.out.println(lance.name + " has " + lance.health + " HP and carries a " + lance.weapon);
    }
}

Output

Tommy Vercetti has 100 HP and carries a M4
Lance Vance has 80 HP and carries a Pistol

tommy and lance are two different objects, both built from the same Character class. They each have their own name, health, and weapon values, and changing one doesn't affect the other

When you create a new object, all fields start with default values

Numbers (int, double) default to 0
Booleans default to false
Objects (String, etc.) default to null

class Player {
    String name;
    int score;
    boolean isOnline;
}

public class Main {
    public static void main(String[] args) {
        Player p = new Player();
        System.out.println(p.name);       // null
        System.out.println(p.score);      // 0
        System.out.println(p.isOnline);   // false
    }
}

A few important details about the code structure. Notice that the Car class and the Main class are separate classes in the same file. In Java, only one class per file can be public, and it must match the filename. Our platform wraps your code in the Main class automatically for simple programs, but when you define your own classes, you need to write the full structure yourself

The class you define (like Car or Character) goes outside the Main class. The Main class contains main() where your program starts running

class MyThing {
    // fields go here
}

public class Main {
    public static void main(String[] args) {
        // create objects and use them here
    }
}

Let's see why this is useful with a simple game inventory system

class Weapon {
    String name;
    int damage;
    double weight;
}

public class Main {
    public static void main(String[] args) {
        Weapon w1 = new Weapon();
        w1.name = "Katana";
        w1.damage = 50;
        w1.weight = 3.5;

        Weapon w2 = new Weapon();
        w2.name = "Rocket Launcher";
        w2.damage = 200;
        w2.weight = 15.0;

        // compare them
        if (w1.damage > w2.damage) {
            System.out.println(w1.name + " deals more damage");
        } else {
            System.out.println(w2.name + " deals more damage");
        }
    }
}

Output Rocket Launcher deals more damage

Without classes, you'd need separate variables for every single field: w1Name, w1Damage, w1Weight, w2Name, w2Damage, w2Weight... It gets messy fast. Classes let you bundle related data together into one clean package

Mission: Guard Dogs

Tommy is registering the guard dogs at his Vice City mansion. Each dog needs a name and an age on record before it goes on rotation.

Create a class that represents a guard dog, with a field for the name (String) and one for the age (int).

In main, first store the values in local variables: name1 and age1 for the first dog, name2 and age2 for the second. Then build two dog objects and set each object's fields from those variables. Finally print each dog on its own line in the format Name is X years old.

Example for a dog Rex aged 5 and one Buddy aged 3

Rex is 5 years old
Buddy is 3 years old

Example for a dog Bruno aged 7 and one Ace aged 2

Bruno is 7 years old
Ace is 2 years old