why do we need java interfaces for DAOs? I mean instead of coming through CustomerDAOInterface, if I access CustomerDAO directly from sessionbean/service-layer, what's wrong with it? If I need to add a new method in future, I can still add it to CustomerDAO, if I need to change a method in CustomerDAO I can do that. If I need to use a different persistence package I still will be working with CustomerDAO class only in both the cases.
The idea is that this way you could choose at runtime which DAO implementation to use.
A common use is to use a different DAO implementation for unittesting (for example by using DAO mocks). It would also make it possible to have, say an OracleCustomerDao, MsSqlCustomerDao, XmlCustomerDao, etc. pp.
The soul is dyed the color of its thoughts. Think only on those things that are in line with your principles and can bear the light of day. The content of your character is your choice. Day by day, what you do is who you become. Your integrity is your destiny - it is the light that guides your way. - Heraclitus