Object-oriented (OO) applications almost always involve a domain model representing the business entities that the application deals with. The gTunes application will include a number of domain classes, including
Song. Each of these domain classes has properties associated with it. You must map those properties to a database in order to persist instances of those classes.
Developers of object-oriented applications face some difficult problems in mapping objects to a relational database. This is not because relational databases are especially difficult to work with; the trouble is that ...