Hibernate has been around for a while and (at least as of this writing) Java 1.5 isn't yet released, so the support for enumerations in Hibernate can't take advantage of its new enum keyword. Instead, Hibernate lets you define your own typesafe enumeration classes however you like, and it provides a mechanism to help you get them into and out of a database, by translating them to and from small integer values. This is something of a regression to the world of C, but it is useful nonetheless.
C-style enumerations still appear too often in Java. Older parts of the Sun API contain many of them.
In our music database, for example, we might want to add a field to our Track class that tells us the medium from which it was imported.
The key to adding persistence support for our enumeration is to have it implement Hibernate's PersistentEnum interface. This interface has two methods, toInt() and fromInt(), that Hibernate uses to translate between the enumeration constants and values that represent them in a database.
Let's suppose we want to be able to specify whether our tracks came from cassette tapes, vinyl, VHS tapes, CDs, a broadcast, an internet download site, or a digital audio stream. (We could go really nuts and distinguish between Internet streams and satellite radio services like Sirius or XM, or radio versus television broadcast, but this is plenty to demonstrate the important ideas.)
Without any consideration of persistence, ...