Java in a Nutshell, 5th Edition

Enumerated Types

In previous chapters, we’ve seen the class keyword used to define class types, and the interface keyword used to define interface types. This section introduces the enum keyword, which is used to define an enumerated type (informally called an enum). Enumerated types are new in Java 5.0, and the features described here cannot be used (although they can be partially simulated) prior to that release.

We begin with the basics: how to define and use an enumerated type, including common programming idioms involving enumerated types and values. Next, we discuss the more advanced features of enums and show how to simulate enums prior to Java 5.0.

Enumerated Types Basics

An enumerated type is a reference type with a finite (usually small) set of possible values, each of which is individually listed, or enumerated. Here is a simple enumerated type defined in Java:

public enum DownloadStatus { CONNECTING, READING, DONE, ERROR }

Like class and interface, the enum keyword defines a new reference type. The single line of Java code above defines an enumerated type named DownloadStatus. The body of this type is simply a comma-separated list of the four values of the type. These values are like static final fields (which is why their names are capitalized), and you refer to them with names like DownloadStatus.CONNECTING, DownloadStatus.READING, and so on. A variable of type DownloadStatus can be assigned one of these four values or null but nothing else. The values of an enumerated ...