Description
Decorator pattern adds additional functionalities to a object dynamically without disturbing the other instances. The sub classes can be extended easily using Decorator pattern. Decorator pattern is a type of structural pattern and is also known as "wrapper".
Advantages
- Provides much flexibility than static class.
- Abstract class or interface is used for composition.
- Coding is simplified.
Conceptual
figure
- Component : The responsibilities for interface objects can be added dynamically.
- ConcreteComponent : Concrete Component is used to define an object to which additional responsibilities can be added.
- Decorator : Maintains a reference to a Component object and defines a interface that conforms to Component's interface.
- Concrete Decorator : Concrete Decorator is used to extend the functionality of the component by adding the state or behavior.
Examples
Defining an interface Bike
[java]
public interface Bike
{
public void assemble();
}[/java]
Creating a class BasicBike which implements the interface Bike.
[java]
class BasicBike implements Bike
{
public void assemble()//overrides the interface method
{
System.out.print("Basic Bike.");
}
}[/java]
Creating a class BikeDecorator which implements the interface Bike.
[java]
class BikeDecorator implements Bike
{
protected Bike byk;
public BikeDecorator(Bike c)
{
this.bike=c;
}
public void assemble()//overrides the interface method
{
this.bike.assemble();
}
}[/java]
Creating a class SportsBike which extends the BikeDecorator class.
[java]
class SportsBike extends BikeDecorator
{
public SportsBike(Bike c)
{
super(c);
}
public void assemble()//overrides the interface method
{
bike.assemble();
System.out.print(" Adding features of Sports Byke.");
}
}[/java]
Creating a class LuxuryBike which extends the BikeDecorator class.
[java]
class LuxuryBike extends BikeDecorator
{
public LuxuryBike(Bike c)
{
super(c);
}
public void assemble()//overrides the interface method
{
bike.assemble();
System.out.print(" Adding features of Luxury Byke.");
}
}[/java]
Creating a DecoratorPatternTest.
[java]
class DecoratorPatternTest {
public static void main(String[] args) {
Bike sportsBike = new SportsBike(new BasicByke());
sportsBike.assemble();
System.out.println("\n*****");
Bike sportsLuxuryBike = new SportsBike(new LuxuryBike(new BasicBike()));
sportsLuxuryBike.assemble();
}
}[/java]
Output
Result will be as follows.
[java]Basic Bike. Adding features of Sports Bike.
*****
Basic Bike. Adding features of Luxury Bike. Adding features of Sports Bike.[/java]
Key Points
- Additional features can be added to an object dynamically by using the Decorator Pattern.
- Additional features can be removed easily by using Decorator Pattern.
- Decorator Pattern is mostly used in java IOclasses.