Design Patterns discussion

[Ray Ye <rayye AT Catena.com>]

Hi Jacob,

 

Think this way, factory method is also called "abstract constructor", it is a method on super class, and the subclass is supposed to override this method to return a concrete implementation. e.g.,

you have a class called AbstractDocument

public class AbstractDocument implements Document {

    public abstract Document createDocument();

...

}

 

since the AbstractDocument has no idea what kind of document the subclass is going to create, so it leaves it as an abstract method, if you have a real document to realize, you can have,

public class MyDocument extends AbstractDocument {

    public Document createDocument() {

        return this;

    }

...

}

 

In this sense, it is categorized as "Class creational pattern", because the subclass has to provide the implementation. Remember, AbstractDocument is not a factory, it is a class which has a "factory method".

 

On the other hand, a factory class may be composed of a lot of factory methods to create a family of products. I will still use the above example but tweak it a bit,

public interface DocumentFactory {

    public Header createHeader();

    public Footer createFooter();

    public Content createContent();

}

 

Now you have different types of documents(headers, footers, contents) to create, and you can use abstract factory to control their types, since for the same type of document, its header, footer and content have to be compatible.

public MsDocumentFactory implements DocumentFactory {

...

}

 

public AdobeDocumentFactory implements DocumentFactory {

...

}

 

You have different factories to create different families of document. Somewhere in you application, you have to determine which factory to plug in, and there are a lot ways to do that, e.g., you have use config files, or application parameters.

 

Hope this helps,

 

Cheers,

 

Ray

-----Original Message-----
From: Jacob George [mailto:jacobg AT atinav.com]
Sent: Monday, September 22, 2003 5:00 AM
To: GOF Mailing List
Subject: [gang-of-4-patterns] Doubt on creational patterns..

Hi,

    I am new to this group and is presently trying to understand creational patterns. I got confused by the "Factory method" pattern and "Abstract Factory" pattern. Both are used for instantiating objects, right? But one is class creational while other is object creational; this point I doesn't understand clearly.

 

    Factory method pattern defers the creation of objects to its sub-classes; anyway, in a actual scenario there will be one concrete factory for creating required objects, right? This is same in the case of Abstract Factory also. So how can we call one as class creational while other as object creational? In both the cases, the type of objects created by the factory changes only by using a factory sub-class, isn't it so?

 

    I have read the respective chapters repeatedly, but was confused on reading "Factory method" & "Abstract factory". Somebody please help me clear the confusion. ThankYou.

 

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