Decorator

It allows behavior to be added to individual objects, either statically or dynamically, without affecting the behavior of other objects from the same class. It’s useful when you want to add responsibilities to objects at runtime without modifying their code.

Structure

• Component: This is the interface or abstract class that defines the behavior that can be dynamically extended.
• Concrete Component: The base class that implements the component interface. It can be decorated with additional responsibilities.
• Decorator: This abstract class or interface extends the component class and has a reference to the component object. It delegates calls to the component and adds new behavior before or after delegating.
• Concrete Decorators: These classes implement the decorator and add specific behavior to the component.

Example

// Component Interface
interface Coffee {
    String getDescription();
    double cost();
}

// ConcreteComponent
class SimpleCoffee implements Coffee {
    @Override
    public String getDescription() {
        return "Simple Coffee";
    }

    @Override
    public double cost() {
        return 2.0;  // Base cost of simple coffee
    }
}

// Decorator Class (implements the Coffee interface and holds a reference to a Coffee object)
abstract class CoffeeDecorator implements Coffee {
    protected Coffee decoratedCoffee;  // Coffee object to be decorated

    public CoffeeDecorator(Coffee coffee) {
        this.decoratedCoffee = coffee;
    }

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

    @Override
    public double cost() {
        return decoratedCoffee.cost();
    }
}

// Concrete Decorator for adding milk
class MilkDecorator extends CoffeeDecorator {
    public MilkDecorator(Coffee coffee) {
        super(coffee);
    }

    @Override
    public String getDescription() {
        return decoratedCoffee.getDescription() + ", Milk";
    }

    @Override
    public double cost() {
        return decoratedCoffee.cost() + 0.5;  // Milk adds $0.5 to the cost
    }
}

// Concrete Decorator for adding sugar
class SugarDecorator extends CoffeeDecorator {
    public SugarDecorator(Coffee coffee) {
        super(coffee);
    }

    @Override
    public String getDescription() {
        return decoratedCoffee.getDescription() + ", Sugar";
    }

    @Override
    public double cost() {
        return decoratedCoffee.cost() + 0.2;  // Sugar adds $0.2 to the cost
    }
}

// Concrete Decorator for adding whipped cream
class WhipDecorator extends CoffeeDecorator {
    public WhipDecorator(Coffee coffee) {
        super(coffee);
    }

    @Override
    public String getDescription() {
        return decoratedCoffee.getDescription() + ", Whipped Cream";
    }

    @Override
    public double cost() {
        return decoratedCoffee.cost() + 0.7;  // Whipped Cream adds $0.7 to the cost
    }
}

public class Main {
    public static void main(String[] args) {
        // Create a simple coffee
        Coffee coffee = new SimpleCoffee();
        System.out.println(coffee.getDescription() + " $" + coffee.cost());

        // Add milk to the coffee
        coffee = new MilkDecorator(coffee);
        System.out.println(coffee.getDescription() + " $" + coffee.cost());

        // Add sugar to the coffee
        coffee = new SugarDecorator(coffee);
        System.out.println(coffee.getDescription() + " $" + coffee.cost());

        // Add whipped cream to the coffee
        coffee = new WhipDecorator(coffee);
        System.out.println(coffee.getDescription() + " $" + coffee.cost());
    }
}