Question

I want to create a Factory which returns dao-instance, depending on the Class clazz

Teammember, Scene and Equipment are my Model Classes.

My DAO’s look like this:

public class JDBCTeammemberDAO implements JdbcDAO<Teammember>

my Factory looks like this:

public class DAOFactory {

    JdbcDAO createDAO(Class clazz) {
        if(clazz.equals(Teammember.class)) {
            return new JDBCTeammemberDAO();
        }
        if(clazz.equals(Scene.class)) {
            return new JDBCSceneDAO();
        }
        if(clazz.equals(Equipment.class)) {
            return new JDBCEquipmentDAO();
        }
        return null;

    }
}

I was thinking about switch and polymorphism, but I couldn’t figure out how.

Basically I want to find the Implementation “SomeClass implements JdbcDAO”

My first approach was:

String name = clazz.getName().substring(6); // model.Teammember
Class<?> forName;
try {
    forName = Class.forName("dao.jdbc.JDBC" + name + "DAO");
    return (JdbcDAO) forName.newInstance();
} catch (ClassNotFoundException e) {
    e.printStackTrace();
} catch (InstantiationException e) {
    e.printStackTrace();
} catch (IllegalAccessException e) {
    e.printStackTrace();
}

but I don’t feel good with handling this with String method. Besides, it doesn’t work, if I have different Model and Dao names (like: JDBCMemberDAO instead of JDBCTeammemberDAO)

Answer 1

I was in a similar situation and decided to use a Dao registry to handle the issue. Using the generic dao pattern @Perception mentioned:

public interface JdbcDAO<T> {
    T find(Long id) ;
    T create(T entity);
    T update(T entity);
    void delete(T entity);
}

public class JdbcDAOImpl<T> {
    this.clazz = clazz;
    DaoRegistry.register (clazz, this);
}

Then you can have your

   public class JDBCTeammemberDAO extend JdbcDAOImpl<TeamMember> {
       public class JDBCTeammemberDAO () {
          super(TeamMember.class);
       }
   }

DaoRegistry would look something like this:

public class DaoRegistry {
        private Map<Class, JdbcDAO> daoMap;

        public synchronized void register (Class type, JdbcDao dao) {
                if (!daoMap.containsKey(type))
                     daoMap.put (type, dao);
                else
                     logger.error ("Something is really wrong because you are creating another dao for this class.", e);

            }

        public JdbcDAO get(Class type) {return daoMap.get(type);
}

This is just the jest of it, you will need to make sure it is thread-safe. Hope this helps.