Classes and Objects

Blueprints and real things

Java is an object-oriented language. That means programs are built from objects that hold data and can do things.

Think of it like this: A class is like the blueprint of a car: it says every car has a colour and a speed, and can drive. An object is one real car built from that blueprint – a red one, a blue one. One blueprint, many cars.

Writing a class

class Dog {
    String name;
    int age;

    void bark() {
        System.out.println(name + " says Woof!");
    }
}

public class Main {
    public static void main(String[] args) {
        Dog d1 = new Dog();
        d1.name = "Bruno";
        d1.age = 3;

        Dog d2 = new Dog();
        d2.name = "Tiger";
        d2.age = 5;

        d1.bark();
        d2.bark();
        System.out.println(d2.name + " is " + d2.age);
    }
}

Output

Bruno says Woof!
Tiger says Woof!
Tiger is 5

Each object has its own copy of the fields. Changing d1.name doesn't touch d2.

Tip: In this course everything goes in one file, so other classes are written without public. In real projects, each public class usually gets its own file, like Dog.java.