Java中的Enum的使用与分析

Java中的Enum的使用与分析

示例：

public enum EnumTest {
FRANK("The given name of me"),
LIU("The family name of me");
private String context;
private String getContext(){
return this.context;
}
private EnumTest(String context){
this.context = context;
}
public static void main(String[] args){
for(EnumTest name :EnumTest.values()){
System.out.println(name+" : "+name.getContext());
}
System.out.println(EnumTest.FRANK.getDeclaringClass());
}

}

Java中枚举实现的分析：

示例：

public enum Color{

RED,BLUE,BLACK,YELLOW,GREEN

}

显然，enum很像特殊的class，实际上enum声明定义的类型就是一个类。 而这些类都是类库中Enum类的子类(java.lang.Enum<E>)。它们继承了这个Enum中的许多有用的方法。我们对代码编译之后发现，编译器将enum类型单独编译成了一个字节码文件：Color.class。

Color字节码代码

final enum hr.test.Color {

// 所有的枚举值都是类静态常量

public static final enum hr.test.Color RED;

public static final enum hr.test.Color BLUE;

public static final enum hr.test.Color BLACK;

public static final enum hr.test.Color YELLOW;

public static final enum hr.test.Color GREEN;

private static final synthetic hr.test.Color[] ENUM$VALUES;

// 初始化过程，对枚举类的所有枚举值对象进行第一次初始化

static {

0 new hr.test.Color [1]

3 dup

4 ldc <String "RED"> [16] //把枚举值字符串"RED"压入操作数栈

6 iconst_0 // 把整型值0压入操作数栈

7 invokespecial hr.test.Color(java.lang.String, int) [17] //调用Color类的私有构造器创建Color对象RED

10 putstatic hr.test.Color.RED : hr.test.Color [21] //将枚举对象赋给Color的静态常量RED。

......... 枚举对象BLUE等与上同

102 return

};

// 私有构造器，外部不可能动态创建一个枚举类对象(也就是不可能动态创建一个枚举值)。

private Color(java.lang.String arg0, int arg1){

// 调用父类Enum的受保护构造器创建一个枚举对象

3 invokespecial java.lang.Enum(java.lang.String, int) [38]

};

public static hr.test.Color[] values();

// 实现Enum类的抽象方法

public static hr.test.Color valueOf(java.lang.String arg0);

}

下面我们就详细介绍enum定义的枚举类的特征及其用法。（后面均用Color举例）

1、Color枚举类就是class，而且是一个不可以被继承的final类。其枚举值(RED,BLUE...)都是Color类型的类静态常量， 我们可以通过下面的方式来得到Color枚举类的一个实例：

Color c=Color.RED;

注意：这些枚举值都是public static final的，也就是我们经常所定义的常量方式，因此枚举类中的枚举值最好全部大写。

2、即然枚举类是class，当然在枚举类型中有构造器，方法和数据域。但是，枚举类的构造器有很大的不同：

(1) 构造器只是在构造枚举值的时候被调用。

Java代码

enum Color{

RED(255,0,0),BLUE(0,0,255),BLACK(0,0,0),YELLOW(255,255,0),GREEN(0,255,0);

//构造枚举值，比如RED(255,0,0)

private Color(int rv,int gv,int bv){

this.redValue=rv;

this.greenValue=gv;

this.blueValue=bv;

}

public String toString(){ //覆盖了父类Enum的toString()

return super.toString()+"("+redValue+","+greenValue+","+blueValue+")";

}

private int redValue; //自定义数据域，private为了封装。

private int greenValue;

private int blueValue;

}

(2) 构造器只能私有private，绝对不允许有public构造器。 这样可以保证外部代码无法新构造枚举类的实例。这也是完全符合情理的，因为我们知道枚举值是public static final的常量而已。 但枚举类的方法和数据域可以允许外部访问。

Java代码

public static void main(String args[])

{

// Color colors=new Color(100,200,300); //wrong

Color color=Color.RED;

System.out.println(color); // 调用了toString()方法

}

3、所有枚举类都继承了Enum的方法，下面我们详细介绍这些方法。

(1) ordinal()方法: 返回枚举值在枚举类种的顺序。这个顺序根据枚举值声明的顺序而定。

Color.RED.ordinal(); //返回结果：0

Color.BLUE.ordinal(); //返回结果：1

(2) compareTo()方法: Enum实现了java.lang.Comparable接口，因此可以比较象与指定对象的顺序。Enum中的compareTo返回的是两个枚举值的顺序之差。当然，前提是两个枚举值必须属于同一个枚举类，否则会抛出ClassCastException()异常。(具体可见源代码)

Color.RED.compareTo(Color.BLUE); //返回结果 -1

(3) values()方法： 静态方法，返回一个包含全部枚举值的数组。

Color[] colors=Color.values();

for(Color c:colors){

System.out.print(c+",");

}//返回结果：RED,BLUE,BLACK YELLOW,GREEN,

(4) toString()方法： 返回枚举常量的名称。

Color c=Color.RED;

System.out.println(c);//返回结果: RED

(5) valueOf()方法： 这个方法和toString方法是相对应的，返回带指定名称的指定枚举类型的枚举常量。

Color.valueOf("BLUE"); //返回结果: Color.BLUE

(6) equals()方法： 比较两个枚举类对象的引用。

Java代码

//JDK源代码：

public final boolean equals(Object other) {

return this==other;

}

4、枚举类可以在switch语句中使用。

Java代码

Color color=Color.RED;

switch(color){

case RED: System.out.println("it's red");break;

case BLUE: System.out.println("it's blue");break;

case BLACK: System.out.println("it's blue");break;

}

上面内容转自: /article/5909364.html

此外:
a. 由于枚举本质上是类, 所以枚举可以实现接口
如jdk中就是用了这一功能, 源码如下:

package java.nio.file;
public interface OpenOption {
}

package java.nio.file;

/**
* Defines the standard open options.
*
* @since 1.7
*/

public enum StandardOpenOption implements OpenOption {
/**
* Open for read access.
*/
READ,

/**
* Open for write access.
*/
WRITE,

/**
* If the file is opened for {@link #WRITE} access then bytes will be written
* to the end of the file rather than the beginning.
*
* <p> If the file is opened for write access by other programs, then it
* is file system specific if writing to the end of the file is atomic.
*/
APPEND,

/**
* If the file already exists and it is opened for {@link #WRITE}
* access, then its length is truncated to 0. This option is ignored
* if the file is opened only for {@link #READ} access.
*/
TRUNCATE_EXISTING,

/**
* Create a new file if it does not exist.
* This option is ignored if the {@link #CREATE_NEW} option is also set.
* The check for the existence of the file and the creation of the file
* if it does not exist is atomic with respect to other file system
* operations.
*/
CREATE,

/**
* Create a new file, failing if the file already exists.
* The check for the existence of the file and the creation of the file
* if it does not exist is atomic with respect to other file system
* operations.
*/
CREATE_NEW,

/**
* Delete on close. When this option is present then the implementation
* makes a <em>best effort</em> attempt to delete the file when closed
* by the appropriate {@code close} method. If the {@code close} method is
* not invoked then a <em>best effort</em> attempt is made to delete the
* file when the Java virtual machine terminates (either normally, as
* defined by the Java Language Specification, or where possible, abnormally).
* This option is primarily intended for use with <em>work files</em> that
* are used solely by a single instance of the Java virtual machine. This
* option is not recommended for use when opening files that are open
* concurrently by other entities. Many of the details as to when and how
* the file is deleted are implementation specific and therefore not
* specified. In particular, an implementation may be unable to guarantee
* that it deletes the expected file when replaced by an attacker while the
* file is open. Consequently, security sensitive applications should take
* care when using this option.
*
* <p> For security reasons, this option may imply the {@link
* LinkOption#NOFOLLOW_LINKS} option. In other words, if the option is present
* when opening an existing file that is a symbolic link then it may fail
* (by throwing {@link java.io.IOException}).
*/
DELETE_ON_CLOSE,

/**
* Sparse file. When used with the {@link #CREATE_NEW} option then this
* option provides a <em>hint</em> that the new file will be sparse. The
* option is ignored when the file system does not support the creation of
* sparse files.
*/
SPARSE,

/**
* Requires that every update to the file's content or metadata be written
* synchronously to the underlying storage device.
*
* @see <a href="package-summary.html#integrity">Synchronized I/O file integrity</a>
*/
SYNC,

/**
* Requires that every update to the file's content be written
* synchronously to the underlying storage device.
*
* @see <a href="package-summary.html#integrity">Synchronized I/O file integrity</a>
*/
DSYNC;
}

b. 枚举不允许继承, 由于枚举是final类型的, 自然也不能被继承.

另参考: http://www.cnblogs.com/hemingwang0902/archive/2011/12/29/2306263.html