Understand the idea

Object-oriented programming models concepts using objects with state and behavior. A class defines a type; each new object can hold its own field values. Start with a concrete example before memorizing the names of OOP principles.

Explore the details

What is Object-Oriented Programming?

OOP organises code around objects — bundles that combine data (attributes) and behaviour (methods) in one place, mirroring real-world things.

Think of a Car: it has attributes (brand, colour, speed) and methods (accelerate, brake). In OOP you define a Car class once, then create as many car objects as you need from that blueprint.

Class vs Object

• Class — the blueprint / template. Defined once. Describes what attributes and methods every object of that type will have.
• Object — a real instance created from the class using new. Each object has its own copy of the attributes.

The 4 Pillars of OOP

Why OOP?

// Define a class (blueprint)
class Car {
    String brand;
    int speed;

    void drive() {
        System.out.println(brand + " driving at " + speed + " km/h");
    }
}

// Create objects (instances)
Car car1 = new Car();
car1.brand = "Toyota"; car1.speed = 80;
car1.drive();  // Toyota driving at 80 km/h

Car car2 = new Car();
car2.brand = "BMW"; car2.speed = 120;
car2.drive();  // BMW driving at 120 km/h

• Reusability — write a class once, create hundreds of objects from it
• Organisation — related data and behaviour stay together
• DRY principle — Don't Repeat Yourself: one place to change, all objects updated
• Modularity — each class can be built, tested, and maintained independently

A complete runnable example

This complete Main.java program focuses on the lesson idea. Reference fragments above may demonstrate individual statements; this example includes the imports, entry point, and supporting types it needs.

public class Main {
    public static void main(String[] args) throws Exception {
        Bicycle bike = new Bicycle();
        bike.speed = 10;
        bike.accelerate();
        System.out.println(bike.speed);
    }
}
class Bicycle {
    int speed;
    void accelerate() { speed += 5; }
}

15

Change it. Run it. Understand it.

Load this lesson example into the editor. Change one value or statement, predict the result, then press RUN to execute Java.

Make it your own

Your learning checkpoint

• Explain the example without reading the output first.
• Make the change requested in the practice task.
• Describe why the result changed—or why compilation failed.

Ready to move on? Follow the next lesson in course order, or return to the curriculum to review a prerequisite.