Decorator Pattern

What is Decorator Pattern?

Decorator pattern is a structural design pattern that attaches additional behaviors to an object dynamically. Decorators provide a flexible extension reason by composition rather than subclassing (inheritance).

When to use it?

Use Decorator pattern when you want to construct an object by adding small behaviors at runtime.

UML diagram

Decorator class uses composition and inheritance, it's crucial to understand their intent.
In Decorator pattern, we use the same type for both components & decorators. Decorator composites Component object to get behavior, that is, obtaining fields or methods defined in Component object. While Decorator inherits (extends) Component so that Decorator object can be declared as Component object.

Decorator pattern achieves open-closed principle, namely, open for extension and closed for modification. It's easy to add components or decorators. for instance, if you want to add another concrete decorator, you just need to create a class representing it and extends Decorator class.

Example

Imagine we're developing a system for an ice cream shop. The shop has various ice creams and toppings. The system needs to display an ice cream description (including its toppings) and cost.

• Components (ice creams) and Decorators (toppings) have common interface IceCream class, they both declare as IceCream object.
• Each concrete ice cream overrides cost method since prices are different for each.
• Topping class provides interface for concrete toppings and holds reference to an IceCream.
• If system needs another topping, say caramel source, what you need to do is just creating CaramelSource class which extends Topping class.

Implementation in Java

IceCream class:

// Component class
public abstract class IceCream {

    public String description = "Unknown ice cream";

    public String getDescription() {
        return description;
    }

    public abstract double cost();
}

ChocolateIceCream class:

// Concrete component class
public class ChocolateIceCream extends IceCream {

    public ChocolateIceCream() {
        description = "ChocolateIceCream";
    }

    @Override
    public double cost() {
        return 1.99;
    }
}

Topping class:

// Base decorator class
public abstract class Topping extends IceCream {

    public IceCream iceCream;

    // All subclasses (concrete decorator classes) need to implement getDescription method,
    // by declaring this method as abstract, we enforce all subclasses to implement this method
    public abstract String getDescription();
}

MapleNuts class:

// Concrete decorator class
public class MapleNuts extends Topping {

    public MapleNuts(IceCream iceCream) {
        this.iceCream = iceCream;
    }

    @Override
    public String getDescription() {
        return iceCream.getDescription() + ", MapleNuts";
    }

    @Override
    public double cost() {
        return iceCream.cost() + .30;
    }
}

PeanutButterShell class:

// Concrete decorator class
public class PeanutButterShell extends Topping {

    public PeanutButterShell(IceCream iceCream) {
        this.iceCream = iceCream;
    }

    @Override
    public String getDescription() {
        return iceCream.getDescription() + ", PeanutButterShell";
    }

    @Override
    public double cost() {
        return iceCream.cost() + .30;
    }
}

Client class:

public class Client {

    public static void main(String[] args) {
        IceCream iceCream = new ChocolateIceCream();
        System.out.println(iceCream.getDescription() + ", $" + iceCream.cost());

        iceCream = new MapleNuts(iceCream);
        System.out.println(iceCream.getDescription() + ", $" + iceCream.cost());

        iceCream = new PeanutButterShell(iceCream);
        System.out.println(iceCream.getDescription() + ", $" + iceCream.cost());
    }
}

Output:

ChocolateIceCream, .99
ChocolateIceCream, MapleNuts, .29
ChocolateIceCream, MapleNuts, PeanutButterShell, .59

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You can check all the design pattern implementations here.
GitHub Repository

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