Mainak is an experienced Technology Consultant specializing in JEE, Web Development and Open source technologies. He is currently based out of United Kingdom. He is die hard technology enthusiast trying to explore the latest in the world of technology. In past time he loves blogging on his website: http://idiotechie.com. Mainak is a DZone MVB and is not an employee of DZone and has posted 11 posts at DZone. You can read more from them at their website. View Full User Profile

Gang of Four – Decorate with Decorator Design Pattern

12.16.2012
| 13206 views |
  • submit to reddit

 Decorator pattern is one of the widely used structural patterns. This pattern dynamically changes the functionality of an object at runtime without impacting the existing functionality of the objects. In short this pattern adds additional functionalities to the object by wrapping it.

Problem statement:

Imagine a scenario where we have a pizza which is already baked with tomato and cheese. After that you just recall that you need to put some additional topping at customer’s choice. So you will need to give some additional toppings like chicken and pepper on the go.

Intent:

Add or remove additional functionalities or responsibilities from the object dynamically without impacting the original object.
At times it is required when addition of functionalities is not possible by subclassing as it might create loads of subclasses.

Solution:

So in this case we are not using inheritance to add additional functionalities to the object i.e. pizza, instead we are using composition. This pattern is useful when we don’t want to use inheritance and rather use composition.

 Structure

Decorator Design Pattern

Decorator Design Pattern Structure

Following are the participants of the Decorator Design pattern:
Component – this is the wrapper which can have additional responsibilities associated with it at runtime.
Concrete component- is the original object to which the additional functionalities are added.
Decorator-this is an abstract class which contains a reference to the component object and also implements the component interface.
Concrete decorator-they extend the decorator and builds additional functionality on top of the Component class.

Example:

Decorator Design Pattern Example

Decorator Design Pattern Example

In the above example the Pizza class acts as the Component and BasicPizza is the concrete component which needs to be decorated. The PizzaDecorator acts as a Decorator abstract class which contains a reference to the Pizza  class. The ChickenTikkaPizza is the ConcreteDecorator which builds additional functionality to the Pizza class.

Let’s summarize the steps to implement the decorator design pattern:

  1. Create an interface to the BasicPizza(Concrete Component) that we want to decorate.
  2. Create an abstract class PizzaDecorator that contains reference field of Pizza(decorated) interface.
  3. Note: The decorator(PizzaDecorator) must extend same decorated(Pizza) interface.
  4. We will need to now pass the Pizza object that you want to decorate in the constructor of decorator.
  5. Let us create Concrete Decorator(ChickenTikkaPizza) which should provide additional functionalities of additional topping.
  6. The Concrete Decorator(ChickenTikkaPizza) should extend the PizzaDecorator abstract class.
  7. Redirect methods of decorator (bakePizza()) to decorated class’s core implementation.
  8. Override methods(bakePizza()) where you need to change behavior e.g. addition of the Chicken Tikka topping.
  9. Let the client class create the Component type (Pizza) object by creating a Concrete Decorator(ChickenTikkaPizza) with help from Concrete Component(BasicPizza).
  10. To remember in short : New Component = Concrete Component + Concrete Decorator

Pizza pizza = new ChickenTikkaPizza(new BasicPizza());

Code Example:

BasicPizza.java

public String bakePizza() {
        return "Basic Pizza";
    }

Pizza.java

public interface Pizza {
    public String bakePizza();
}

PizzaDecorator.java

public abstract class PizzaDecorator implements Pizza {
    Pizza pizza;
    public PizzaDecorator(Pizza newPizza) {
        this.pizza = newPizza;
    }
    @Override
    public String bakePizza() {
        return pizza.bakePizza();
    }
}

ChickenTikkaPizza.java

public class ChickenTikkaPizza extends PizzaDecorator {
    public ChickenTikkaPizza(Pizza newPizza) {
        super(newPizza);
    }
    public String bakePizza() {
        return pizza.bakePizza() + " with Chicken topping added";
    }
}

Client.java

public static void main(String[] args) {
        Pizza pizza = new ChickenTikkaPizza(new BasicPizza());
        System.out.println(pizza.bakePizza());
 
    }

Benefits:

Decorator design pattern provide more flexibility than the standard inheritance. Inheritance also extends the parent class responsibility but in a static manner. However decorator allows doing this in dynamic fashion.

Drawback:

Code debugging might be difficult since this pattern adds functionality at runtime.

Interesting points:

  • Adapter pattern plugs different interfaces together whereas decorator pattern enhances the functionality of the object.
  • Unlike Decorator Pattern the Strategy pattern changes the original object without wrapping it.
  • While Proxy pattern controls access to the object the decorator pattern enhances the functionality of the object.
  • Both Composite and Decorator pattern uses the same tree structure but there are subtle differences between both of them. We can use composite pattern when we need to keep the group of objects having similar behavior inside another object. However decorator pattern is used when we need to modify the functionality of the object at runtime.
  • There are various live examples of decorator pattern in Java API.
    1. java.io.BufferedReader;
    1. java.io.FileReader;
    1. java.io.Reader;

If we see the constructor of the BufferedReader then we can see that the BufferedReader wraps the Reader class by adding more features e.g. readLine() which is not present in the reader class.
We can use the same format like the above example on how the client uses the decorator pattern new BufferedReader(new FileReader(new File(“File1.txt”)));
Similarly the BufferedInputStream is a decorator for the decorated object FileInputStream.
BufferedInputStream bs = new BufferedInputStream(new FileInputStream(new File(“File1.txt”)));

Anyways I hope my readers have liked this article. Please do not hesitate to provide your valuable feedback and comments.

Published at DZone with permission of Mainak Goswami, author and DZone MVB. (source)

(Note: Opinions expressed in this article and its replies are the opinions of their respective authors and not those of DZone, Inc.)

Comments

Ratnesh Singh replied on Mon, 2012/12/17 - 1:05am

Nice explanation. But I have some questions here..

Could we use multiple decorator for one class?

Is it possible without changing class definition could we implement a  new decorator?

Mainak Goswami replied on Mon, 2012/12/17 - 7:17pm

Thanks Ratnesh.

We can definitely apply more than one decorator to the Concrete Component. e.g. If you create another decorator class VeggieDelightDecorator extending from the abstract class PizzaDecorator it will add additional feature to the BasicPizza class.

With respect to the second question can you please let me know which class you are referring here? Is this the client class?

Ratnesh Singh replied on Mon, 2012/12/17 - 11:55pm

Thanks Mainak,

 Regarding second question, Say I have ChickenTikkaPizza  and want to decorate it with some Cornflex's, Could this be accomplished without changing class ChickenTikkaPizza?

As per explanation it seems we need to create another class ChickenTikkaCornPizza   which extends PizzaDecorator. Correct me if this is wrong.


Many thanks.


Mainak Goswami replied on Tue, 2012/12/18 - 7:58pm in response to: Ratnesh Singh

If I can understand your question correctly it seems you are planning to decorate the ChickenTikkaPizza class. So in this case this is the ConcreteComponent. So we will need to implement Cornflex decorator as per the decorator implementation process. Hope this helps.

Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.