java enum的用法

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public enum Scope {
Application,
Page,
Request,
Session
}
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Enum是enumeration(列举)的简写形式,包含在java.lang包中.
public enum Season { winter, spring, summer, fall }

一个enum是定义一组值的对象,它可以包括零个或多个值成员.它是属于enum类型的,一个enum对象中不可有两个或多个相同的属性或值.在次之前的java程序员一般是 用接口的方法实现列举的,
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public interface Season {

static winter = 0;

static spring = 1; //etc..

}
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引入了enum的java的列举的编写方便了许多,只须定义一个enum型的对象.enum对象的值都回自动获得一个数字值,从0开始,依次递增.看一个比较简单的enum实现的例子:

EnumDemo.java
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package net.javagarage.enums;

/*

We can loop over the values we put into the enum

using the values() method.

Note that the enum Seasons is compiled into a

separate unit, called EnumDemo$Seasons.class

*/

public class EnumDemo {

/*declare the enum and add values to it. note that, like in C#, we don't use a ; to

end this statement and we use commas to separate the values */

private enum Seasons { winter, spring,

summer, fall }

//list the values

public static void main(String[] args) {

for (Seasons s : Seasons.values()){

System.out.println(s);

}

}

}
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运行上述代码你回得到 以下结果:

winter

spring

summer

fall

Enum的属性调用:

下面的代码展示了调用enum对象的方法,这也是它通常的用法:
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package net.javagarage.enums;

/*

File: EnumSwitch.java

Purpose: show how to switch against the values in an enum.

*/

public class EnumSwitch {

private enum Color { red, blue, green }

//list the values

public static void main(String[] args) {

//refer to the qualified value

doIt(Color.red);

}

/*note that you switch against the UNQUALIFIED name. that is, "case Color.red:" is a

compiler error */

private static void doIt(Color c){

switch (c) {

case red:

System.out.println("value is " + Color.red);

break;

case green:

System.out.println("value is " + Color.green);

break;

case blue:

System.out.println("value is : " + Color.blue);

break;

default :

System.out.println("default");

}

}
}
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为enums添加属性和方法

enums也可以象一般的类一样添加方法和属性,你可以为它添加静态和非静态的属性或方法,这一切都象你在一般的类中做的那样.
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package net.javagarage.enums;

/*

File: EnumDemo.java

Purpose: show how to use an enum that also defines its own fields and methods

*/

public class EnumWithMethods {

//declare the enum and add values to it.

public enum Season {

winter, spring, summer, fall;

private final static String location = "Phoenix";

public static Season getBest(){

if (location.equals("Phoenix"))

return winter;

else

return summer;

}

public static void main(String[] args) {

System.out.println(Season.getBest());

}

}
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就是这么的简单.但是有一点是需要注意的,那就是enums的值列表必须紧跟在enum声明,不然编译时将会出错.

Enums构造函数:

和类一样enums也可以有自己的构造函数,如下:
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package net.javagarage.enums;

public class EnumConstructor {

public static void main(String[] a) {

//call our enum using the values method

for (Temp t : Temp.values())

System.out.println(t + " is : " + t.getValue());

}

//make the enum

public enum Temp {

absoluteZero(-459), freezing(32),

boiling(212), paperBurns(451);

//constructor here

Temp(int value) {

this.value = value;

}

//regular field?but make it final,

//since that is the point, to make constants

private final int value;

//regular get method

public int getValue() {

return value;

}

}

}
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输出结果是:

absoluteZero is : -459

freezing is : 32

boiling is : 212

paperBurns is : 451

尽管enums有这么多的属性,但并不是用的越多越好,如果那样还不如直接用类来的直接.enums的优势在定义int最终变量仅当这些值有一定特殊含义时.但是如果你需要的是一个类,就定义一个类,而不是enum.