Java Enumeration (枚举类型) (1) -- 基本概念

Java 5.0新引进了一种类型：枚举类型。昨晚看了一下，语法还是比较复杂的，至少比C的枚举要复杂的很多，不过功能也强大了很多。具体语法请参见 JLS 8.9
1、定义一个功能简单的枚举类型，更定义一个简单的类很相似，例如

<!----> package  basic;
public   enum  Day
{
    MONDAY, TUESDAT, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
}

• 跟类定义一样，枚举类型可以单独放在一个文件里，当一个枚举类型用public修饰时，它对其他包可见，否则只对同一个包中的类可见，这和类定义是一样的。
• 标识符 MONDAY, TUESDAY等就称为枚举常量（enumeration constants）
• 每一个枚举常量被隐式的声明成Day的一个public、static成员，而且其类型为Day，亦就是说这些常量是self-typed的
  

2、下面的定义也是合法的：

<!---->package basic;

public enum Day
{
    MONDAY, TUESDAT, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY,
}

或

<!---->package basic;

public enum Day
{
    MONDAY, TUESDAT, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY,;
}

或

<!---->package basic;

public enum Day
{
    MONDAY, TUESDAT, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY;
}

但是当枚举类型有其他定义时，则分号;是必须的
3、声明、使用一个枚举类型：
（1）在同一个包中：

BasicMainClass.java
<!----> package  basic;

public   class  BasicMainClass
{
     public   static   void  main(String args[])
    {
        Day today  =  Day.SATURDAY;
        
        System.out.println( " Today is  "   +  today.toString().toLowerCase());
    }
}

（2）在不同包中：

OtherMainClass.java
<!----> package  other;
import  basic.Day;
public   class  OtherMainClass
{
     public   static   void  main(String [] args)
    {
        Day today  =  Day.SATURDAY;
        System.out.println( " Today is  "   +  today.toString().toLowerCase());
    }
}

4、枚举类型的性质：（摘自o'relly 出版的 Java in A Nutshell 5th)

• 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.