Jafe Lee

Enumerated types have no public constructor. The only instances of an enumerated type are those declared by the enum.

Enums are not Cloneable, so copies of the existing instances cannot be created.

Enums implement java.io.Serializable so they can be serialized, but the Java serialization mechanism handles them specially to ensure that no new instances are ever created.

Instances of an enumerated type are immutable: each enum value retains its identity. (We'll see later in this chapter that you can add your own fields and methods to an enumerated type, which means that you can create enumerated values that have mutable portions. This is not recommended, but does not affect the basic identity of each value.)

Instances of an enumerated type are stored in public static final fields of the type itself. Because these fields are final, they cannot be overwritten with inappropriate values: you can't assign the DownloadStatus.ERROR value to the DownloadStatus.DONE field, for example.

By convention, the values of enumerated types are written using all capital letters, just as other static final fields are.

Because there is a strictly limited set of distinct enumerated values, it is always safe to compare enum values using the = = operator instead of calling the equals() method.

Enumerated types do have a working equals( ) method, however. The method uses = =finalso that it cannot be overridden. This working equals( ) method allows enumerated values to be used as members of collections such as Set, List, and Map. internally and is

Enumerated types have a working hashCode() method consistent with their equals( )equals(), hashCode( ) is final. It allows enumerated values to be used with classes like java.util.HashMap. method. Like

Enumerated types implement java.lang.Comparable, and the compareTo() method orders enumerated values in the order in which they appear in the enum declaration.

Enumerated types include a working toString( ) method that returns the name of the enumerated value. For example, DownloadStatus.DONE.toString( ) returns the string "DONE" by default. This method is not final, and enum types can provide a custom implementation if they choose.

Enumerated types provide a static valueOf( ) method that does the opposite of the default
toString( ) method. For example, DownloadStatus.valueOf("DONE") would return DownloadStatus.DONE.

Enumerated types define a final instance method namedordinal()that returns an integer for each enumerated value. The ordinal of an enumerated value represents its position (starting at zero) in the list of value names in the enum declaration. You do not typically need to use the ordinal( ) method, but it is used by a number of enum-related facilities, as described later in the chapter.

Each enumerated type defines a static method named values( ) that returns an array of enumerated values of that type. This array contains the complete set of values, in the order they were declared, and is useful for iterating through the complete set of possible values. Because arrays are mutable, the values( ) method always returns a newly created and initialized array.

Enumerated types are subclasses of java.lang.Enum, which is new in Java 5.0. (Enum is not itself an enumerated type.) You cannot produce an enumerated type by manually extending the Enum class, and it is a compilation error to attempt this. The only way to define an enumerated type is with the enum keyword.

It is not possible to extend an enumerated type. Enumerated types are effectively final, but the final keyword is neither required nor permitted in their declarations. Because enums are effectively final, they may not be abstract.

Like classes, enumerated types may implement one or more interfaces.