java中工厂方法模式如何使用泛型？

我的代码如下所示：

public interface BaseDAO{
// marker interface
}

public interface CustomerDAO extends BaseDAO{
public void createCustomer();
public void deleteCustomer();
public Customer getCustomer(int id);
// etc
}

public abstract class DAOFactory {
public BaseDAO getCustomerDAO();
public static DAOFactory getInstance(){
  if(system.getProperty("allowtest").equals("yes")) {
  return new TestDAOFactory();
  }
  else return new ProdDAOFactory();
}

public class TestDAOFactory extends DAOFactory{
public BaseDAO getCustomerDAO() {
  return new TestCustomerDAO(); // this is a concrete implementation
  //that extends CustomerDAO
  //and this implementation has dummy code on methods
}

public class ProdDAOFactory extends DAOFactory {
public BaseDAO getCustomerDAO() {
  return new ProdCustomerDAO(); // this implementation would have 
  // code that would connect to the database and do some stuff..
}
}

现在，我确实知道这段代码有味道......原因有很多。然而，这段代码也在这里：http://java.sun.com/blueprints/corej2eepatterns/Patterns/DataAccessObject.html http://java.sun.com/blueprints/corej2eepatterns/Patterns/DataAccessObject.html，参见9.8

我打算做的是这样的： 1）根据环境（系统属性）在运行时切换我的 DAO 实现。 2）利用java泛型，这样我就可以避免类型转换...... 例如做这样的事情：

CustomerDAO dao = factory.getCustomerDAO();
dao.getCustomer();

相对于：

CustomerDAO dao = (CustomerDAO) factory.getCustomerDAO();
dao.getCustomer();

请提出您的想法和建议。

你应该这样定义工厂：

public abstract class DAOFactory<DAO extends BaseDAO> {
public DAO getCustomerDAO();
public static <DAO extends BaseDAO> DAOFactory<DAO> getInstance(Class<DAO> typeToken){
  // instantiate the the proper factory by using the typeToken.
  if(system.getProperty("allowtest").equals("yes")) {
  return new TestDAOFactory();
  }
  else return new ProdDAOFactory();
}

getInstance 应该返回一个正确类型的 DAOFactory。

工厂变量的类型为：

DAOFactory<CustomerDAO> factory = DAOFactory<CustomerDAO>.getInstance(CustomerDAO.class);

并且用法将被正确键入：

CustomerDAO dao = factory.getCustomerDAO();
dao.getCustomer();

唯一的问题可能是 getInstance 方法内部需要进行强制转换。