How to correctly apply factory method to the DAO factory?

Question

In the documentation about DAO says:

When the underlying storage is not subject to change from one implementation to another, this strategy can be implemented using the Factory Method pattern to produce a number of DAOs needed by the application. The class diagram for this case is shown in Figure 9.3.

The figure 9.3 itself:

enter image description here

What causes misunderstanding is how we can ever apply factory method to produce the number of DAOs? As far as I understood in the case the DAO might be look like

public interface DAOFactory{

    public DAO1 getDAO1();
    public DAO2 getDAO2();

}

But it would be not a factory method. It would be an abstract factory, because we're producing a family of objects rather than a single object. Couldn't you exlain what they meant when said

this strategy can be implemented using the Factory Method pattern to produce a number of DAOs needed by the application. The class diagram for this case is shown in Figure 9.3.

The DAOFActory shoudl have 1 method like this public DAOBaseClass getDAO(BaseEntity entity) which returns DAO instance for each entity — StanislavL, Jan 15 '15 at 08:58
Well ... Your interface `DAOFactory` is an [abstract factory](http://en.wikipedia.org/wiki/Abstract_factory_pattern) that declares two [factory methods](http://en.wikipedia.org/wiki/Factory_method_pattern). — Seelenvirtuose, Jan 15 '15 at 09:09
@StanislavL So what did they mean when they talked about the Factory method? — St.Antario, Jan 15 '15 at 09:27

score 1 · Accepted Answer · edited May 23 '17 at 11:50

You are right. It is misleading. The Image depicting the use of Factory method is certainly an example of the Abstract Factory design pattern. There's just only one implementation of the factory.

There is a good explanation on the difference between a Factory Method and Abstract Factory here: Differences between Abstract Factory Pattern and Factory Method

The diagram shown in the documentation depicts the Abstract Factory pattern.

I can imagine the following use of DAOs and Factory Method

// the Service contains the business logic and uses DAOs to access data
class BaseService {

   public Object doSomething() {
       return getDao().retrieveObject("identifier");
   }

   // Factory method producing DAOs
   public IDao getDao() {
       return new BaseDao();
   }

}

// The DAO interface
interface IDao {
    Object retrieveObject(String identifier);
}


// Another Service that overwrite the type of produced DAOs 
class SmartService extends Service {

   // overwrite the factory method
   public IDao getDao() {
       return new SmartDao();
   }
}

How to correctly apply factory method to the DAO factory?

1 Answers1