Question

I have a class which does operations on the database, but I want to keep it as general and abstract as possible, thus by Object return types.

Here’s an example method of that class:

public static List<Object> getAll(String table) throws HibernateException{
        Session sessie = HibernateUtil.getSessionFactory().openSession();
        Transaction trans = sessie.beginTransaction();

        List<Object> objects = sessie.createSQLQuery("SELECT * FROM " + table + ";").list();
        trans.commit();
        sessie.close();
        return objects;
 }

Now I have another class User which should have a a method getAll with a List<User> return type and the implementation of the method is the same.

Is this even possible? I know it isn’t possible to overload methods with different return types. Is this doable with some kind of a design pattern, interface setup?

So basicly I don’t want to do the implementation again, it is already specified in the getAll method which returns an object. I just want to perform that method but with dynamic return types.

The reason I am asking this is because I don’t want to do the following somewhere else in my code:

User user = new User(...);
// this should be user.getAll() and the type of that method should be an User, not an object
(User)user.getAll() 

The reason I am doing it this way is because I can have other tables in the future whom would be able to call the getAll method but get a cast object back as return type and not the type object.

I am looking at the cleanest and best OO solution, even if it means that I need to restructure my code.

Answer 1

If you really need it you can make your class generic with returned type as generic type parameter, so you can have:

public static List<T> getAll(String table)

and then do

user = new YourGenericClass<User>();
List<User> list = user.getAll("user");

It’s ugly though as you pass the name of the table as a parameter. You should probably rethink your data access layer design .

edit:

Ok to make it clearer:

//Template generic class
public class GenericDAOAndModelClass<T> {
    public static List<T> getAll(String table) { your code};
    public static T getOne(String table) { your other code };

}

public class User extends GenericDAOAndModelClass<User> {
    user methods and fields

}

and then you can call it like

List<User> userList = User.getAll("user");

User u = User.getOne("user");

You could put a static field in in every class extending GenericDAOAndModelClass to represent table name, so that you don’t have to pass the parameter to getAll and getOne.