Decorator Pattern

Who doesn't like customizations!!. Personally I wouldn't mind extra cheese on my pizza 😉.

Just like that when you design systems made for customizations or those who let the user customize their orders you need the Decorator Design Pattern.

Imagine you’re building a vehicle configuration platform (like Tesla’s online configurator 🚘).

A customer can start with a basic car, and then add features dynamically like:

1. Sunroof 🌞
2. Sports Package 🏎️
3. Advanced Sound System 🔊 Each feature adds extra cost and description, without changing the original Car class — perfect use case for the Decorator Pattern.

Component Interface

interface Vehicle {
    String getDescription();
    double getCost();
}

Concrete Component

class BasicCar implements Vehicle {
    @Override
    public String getDescription() {
        return "Basic Car";
    }

    @Override
    public double getCost() {
        return 500000.0; // base price
    }
}

Abstract Decorator

abstract class VehicleDecorator implements Vehicle {
    protected Vehicle vehicle;

    public VehicleDecorator(Vehicle vehicle) {
        this.vehicle = vehicle;
    }

    @Override
    public String getDescription() {
        return vehicle.getDescription();
    }

    @Override
    public double getCost() {
        return vehicle.getCost();
    }
}

Concrete Decorators

Each decorator adds a unique feature and modifies cost/description.

class SunroofDecorator extends VehicleDecorator {
    public SunroofDecorator(Vehicle vehicle) {
        super(vehicle);
    }

    @Override
    public String getDescription() {
        return vehicle.getDescription() + ", Sunroof";
    }

    @Override
    public double getCost() {
        return vehicle.getCost() + 40000.0;
    }
}

class SportsPackageDecorator extends VehicleDecorator {
    public SportsPackageDecorator(Vehicle vehicle) {
        super(vehicle);
    }

    @Override
    public String getDescription() {
        return vehicle.getDescription() + ", Sports Package";
    }

    @Override
    public double getCost() {
        return vehicle.getCost() + 120000.0;
    }
}

class SoundSystemDecorator extends VehicleDecorator {
    public SoundSystemDecorator(Vehicle vehicle) {
        super(vehicle);
    }

    @Override
    public String getDescription() {
        return vehicle.getDescription() + ", Premium Sound System";
    }

    @Override
    public double getCost() {
        return vehicle.getCost() + 35000.0;
    }
}

Main Class — Building the Custom Car

public class VehicleDecoratorDemo {
    public static void main(String[] args) {
        Vehicle car = new BasicCar();
        System.out.println(car.getDescription() + " ₹" + car.getCost());

        // Add sunroof
        car = new SunroofDecorator(car);
        System.out.println(car.getDescription() + " ₹" + car.getCost());

        // Add sports package
        car = new SportsPackageDecorator(car);
        System.out.println(car.getDescription() + " ₹" + car.getCost());

        // Add sound system
        car = new SoundSystemDecorator(car);
        System.out.println(car.getDescription() + " ₹" + car.getCost());
    }
}

🧩 Real-World Analogies

Domain	Example Decorators
E-Commerce	Apply coupon, add gift wrap, add insurance
Vehicles	Add GPS, airbags, turbo engine
Gaming	Add armor, magic boost, invisibility cloak
Media Streaming	Add subtitles, change resolution, enable HDR