Is it an example of decorator pattern?

Question

Hi, I've an example please tell me whether it is Decorator pattern or not?

public abstract class ComputerComponent  
{  
  String description ="Unknown Type";  
  public String getDescription()  
  {  
    return description;  
  }  
  public abstract double getCost();      
}  

public abstract class AccessoryDecorator  
{  
  ComputerComponent comp;  
  public abstract String getDescription();  
}  

public class PIIIConcreteComp extends ComputerComponent  
{  

  public PIIIConcreteComp()  
  {  

    description= "Pentium III";  
  }  
  public double getCost()  
  {  

    return 19950.00;    
  }  
}


  public class floppyConcreteDeco extends AccessoryDecorator  
  {  

    public floppyConcreteDeco(ComputerComponent comp)  
    { 
       this.comp=comp;  
    }  

  public String getDescription()  
  {  

    return comp.getDescription() +", floppy 1.44 mb";  
  }  

  public double getCost()  
  {  

    return 250+comp.getCost();  

  }  
}   
public class ComponentAssembly  
{  

  public static void createComponent()  
  {  

    ComputerComponent comp = new PIIConcreteComp();  

    // create a PIII computer object  

    ComputerComponent deco1= new floppyConcreteDeco(comp);  

    // decorate it with a floppy  

    //ComputerComponent deco2= newCDRomConcreteDeco(deco1);  

    ComputerComponent deco2= new floppyConcreteDeco(deco1);  

    // decorate with a CDRom or with one more floppy  

    System.out.println( deco2.getdescription() + " " + deco2.getCost());  

  }  
} 

Thank you.

Answer 1

Your point is good, but your code wouldn't even compile, mainly because ComputerComponent has to be interface which has to be implemented by AccessoryDecorator and PIIIConcreteComp (and your braces are terribly messed, by the way). Usually AccessoryDecorator would also implement "default" implementation of it's methods like getDescription() {return comp.getDescription()}.

Answer 2

This is indeed the Decorator design pattern, albeit a bit of a messy example.

Answer 3

It is a decorator pattern, but as Finbarr noted, it's a bit messy:

floppyConcreteDeco (which should be named starting with a capital letter) and/or AccessoryDecorator should extend ComputerComponent, however.

The reason is that the you'll want to use the decorator object the same way you use a "normal" ComputerComponent and you can't really do that unless you have a common base class or a common interface.