Java Design Pattern: Decorator – Decorate your ...

Decorator pattern adds additional features to an existing object dynamically. In this post, I will use a simple example – decorate your girlfriend – to illustrate how decorator pattern works.

1. Decorator Pattern Story

Let’s assume you are looking for a girlfriend. There are girls from different countries such as America, China, Japan, France, etc. Different girls have different personalities and hobbies. In a dating web like eharmony.com, if each type of girl is an individual Java class, there would be thousands of classes. That is a serious problem called class explosion. Moreover, this design is not extensible. Whenever there is a new girl type, a new class needs to be created.

Let’s change the design, and let each hobby/personality becomes a decorator which can be dynamically applied to a girl.

2. Class Diagram

Girl is the abstract class at the top level, we have girls from different countries. With a GirlDecorator class, we can decorator each girl with any feature by adding a new decorator.

3. Decorator pattern Java code

Girl.java

public abstract class Girl { String description = "no particular";   public String getDescription(){ return description; } }

AmericanGirl.java

public class AmericanGirl extends Girl { public AmericanGirl(){ description = "+American"; } }

ChineseGirl.java

public class ChineseGirl extends Girl { public ChineseGirl() { description = "+Chinese"; } }

GirlDecorator.java

public abstract class GirlDecorator extends Girl { public abstract String getDescription(); }

Slim.java

public class Slim extends GirlDecorator {   private Girl girl;   public Slim(Girl g){ girl = g; }   @Override public String getDescription() { return girl.getDescription() + "+slim"; }   public void Dance(){ System.out.println("I like dancing!"); } }

We can add more method like 「Dance()」 to each decorator without any limitations.

Cook.java

public class Cook extends GirlDecorator {   private Girl girl;   public Cook(Girl g) { girl = g; }   @Override public String getDescription() { return girl.getDescription() + "+like cook"; } }

Main.java

public class Main { public static void main(String args[]){   Girl g1 = new AmericanGirl(); System.out.println(g1.getDescription());   Slim g2 = new Slim(g1); System.out.println(g2.getDescription());   Cook g3 = new Cook(g2); System.out.println(g3.getDescription());     //in addition, we can add more method for decorator class g2.Dance(); } }

Output:

+American
+American+slim
+American+slim+like cook
I like dancing!

We can also do something like this:

Girl g = new Slim(new Cook(new AmericanGirl()));

4. Decorator Pattern Used in Java Stand Library

A typical usage of Decorator pattern is Java IO classes.

Here is a simple example – BufferedReader decorates InputStreamReader.

BufferedReader input = new BufferedReader(new InputStreamReader(System.in)); //System.in is an InputStream object

InputStreamReader(InputStream in) – bridge from byte streams to character streams. InputSteamReader reads bytes and translates them into characters using the specified character encoding.

BufferedReader(Reader in) – read text from a character stream and buffer characters in order to provide efficient reading methods(e.g., readLine())