Java 5.0 Tiger: A Developer's Notebook

Chapter 3. Enumerated Types

Note

In this chapter:

• Creating an Enum

• Declaring Enums Inline

• Iterating Over Enums

• Switching on Enums

• Maps of Enums

• Sets of Enums

• Adding Methods to an Enum

• Implementing Interfaces with Enums

• Value-Specific Class Bodies

• Manually Defining an Enum

• Extending an Enum

In Java 1.4 and below, there were two basic ways to define new types: through classes and interfaces. For most object-oriented programming, this would seem to be enough. The problem is that there are still some very specific cases where neither is these is sufficient, most commonly when you need to define a finite set of allowed values for a specific data type. For instance, you might want a type called Grade that can only be assigned values of A, B, C, D, F, or Incomplete. Any other values are illegal for this type. This sort of construct is possible prior to Tiger, but it takes a lot of work, and there are still some significant problems.

Since we’re good developers and try our best to avoid a lot of work whenever possible, Sun finally helped us out with the new enumerated type (generally referred to simply as an enum). This chapter deals with enums: how to create, use, and program with them.

Creating an Enum

Creating an enumerated type involves three basic components, at a minimum:

• The enum keyword

• A name for the new type

• A list of allowed values for the type

There are several optional components that may be defined as well:

• An interface or set of interfaces that the enum implements

• Variable ...