To Reference the Interface Or the Class That Implements the Interface?

[JiaPei Jen]

I have some methods defined in an interface UserDAO; something like:

public interface UserDAO
{
    public String findUserPassword( String username )
        throws UserDAOSysException;
}

and I have a class MySQLUserDAO that implements UserDAO.
If I want to access the method findUserPassword( String username ) from a class called UserService, I should access the interface or the class that implements the interface? For example, which one is correct:

UserDAO ud = new UserDAO();
ud.findUserPassword( String username );

or

MySQLUserDAO msud = new MySQLUserDAO();
msud.findUserPassword( String username );

[ November 24, 2003: Message edited by: JiaPei Jen ]

[Uma Balu]

Hi,
Generally, we will create only a reference to an interface. Your code,

UserDAO ud = new UserDAO();
ud.findUserPassword( String username );

will not compile because, you are trying to instantiate an interface, which is not possible.
May be, you can use it this way,

 
UserDAO ud = new MySQLUserDAO();
ud.findUserPassword("username");

Thanks,
Uma.

[Ko Ko Naing]

Interfaces are acting as superclasses of the implementing class, because Java doesn't allow multiple inheritence... So the usage of interfaces is devoloped in Java...