An enumerated type is a common and useful programming abstraction allowing a value to be selected from a fixed set of named choices. These were originally well represented in Pascal, but C took such a minimal approach (essentially just letting you assign symbolic names to inter-changeable integer values) that early Java releases reserved C's enum keyword but declined to implement it. A better, object-oriented approach known as the "typesafe enum pattern" evolved and was popularized in Joshua Bloch's Effective Java Programming Language Guide (Addison-Wesley). This approach requires a fair amount of boilerplate coding, but it lets you do all kinds of interesting and powerful things. The Java 1.5 specification resuscitates the enum keyword as an easy way to get the power of typesafe enumerations without all the tedious boilerplate coding, and it provides other nifty benefits.
In this chapter:
Defining a Persistent Enumerated Type
Working with Persistent Enumerations
Regardless of how you implement an enumerated type, you're sometimes going to want to be able to persist such values to a database.