Enum是enumeration(列舉)的簡(jiǎn)寫形式,包含在java.lang包中.熟悉C, C++, C#, 或 Pascal人應(yīng)該對(duì)列舉有所了解,先看個(gè)例子:
public enum Season { winter, spring, summer, fall }
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一個(gè)enum是定義一組值的對(duì)象,它可以包括零個(gè)或多個(gè)值成員.它是屬于enum類型的,一個(gè)enum對(duì)象中不可有兩個(gè)或多個(gè)相同的屬性或值.在次之前的java程序員一般是 用接口的方法實(shí)現(xiàn)列舉的,如 :
public interface Season {
?? static winter = 0;
?? static spring = 1; //etc..
}
引入了enum的java的列舉的編寫方便了許多,只須定義一個(gè)enum型的對(duì)象.enum對(duì)象的值都回自動(dòng)獲得一個(gè)數(shù)字值,從0開(kāi)始,依次遞增.看一個(gè)比較簡(jiǎn)單的enum實(shí)現(xiàn)的例子:
EnumDemo.java
package net.javagarage.enums;
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/*
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 {
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????? /*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 */
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????? private enum Seasons { winter, spring,
?????? summer, fall }
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????? //list the values
????? public static void main(String[] args) {
??????????? for (Seasons s : Seasons.values()){
????????????????? System.out.println(s);
??????????? }
????? }
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}
運(yùn)行上述代碼你回得到 以下結(jié)果:
winter
spring
summer
fall
Enum的屬性調(diào)用:
下面的代碼展示了調(diào)用enum對(duì)象的方法,這也是它通常的用法:
package net.javagarage.enums;
/*
File: EnumSwitch.java
Purpose: show how to switch against the values in an enum.
*/
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public class EnumSwitch {
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????? private enum Color { red, blue, green }
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????? //list the values
????? public static void main(String[] args) {
??????????? //refer to the qualified value
??????????? doIt(Color.red);
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????? }
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????? /*note that you switch against the UNQUALIFIED name. that is, "case Color.red:" is a
?compiler error */
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????? private static void doIt(Color c){
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????? 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");
????? }
????? }
}
為enums添加屬性和方法
enums也可以象一般的類一樣添加方法和屬性,你可以為它添加靜態(tài)和非靜態(tài)的屬性或方法,這一切都象你在一般的類中做的那樣.
package net.javagarage.enums;
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/*
File: EnumDemo.java
Purpose: show how to use an enum that also defines its own fields and methods
*/
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public class EnumWithMethods {
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//declare the enum and add values to it.
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public enum Season {
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????? winter, spring, summer, fall;
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????? private final static String location = "Phoenix";
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????? public static Season getBest(){
??????????? if (location.equals("Phoenix"))
????????????????? return winter;
??????????? else
????????????????? return summer;
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????? }
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????? public static void main(String[] args) {
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????? System.out.println(Season.getBest());
????? }
}
就是這么的簡(jiǎn)單.但是有一點(diǎn)是需要注意的,那就是enums的值列表必須緊跟在enum聲明,不然編譯時(shí)將會(huì)出錯(cuò).
Enums構(gòu)造函數(shù):
和類一樣enums也可以有自己的構(gòu)造函數(shù),如下:
package net.javagarage.enums;
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public class EnumConstructor {
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????? public static void main(String[] a) {
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??????????? //call our enum using the values method
??????????? for (Temp t : Temp.values())
????????????????? System.out.println(t + " is : " + t.getValue());
????? }
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????? //make the enum
????? public enum Temp {
??????????? absoluteZero(-459), freezing(32),
??????????? boiling(212), paperBurns(451);
?
????? //constructor here
????? Temp(int value) {
??????????? this.value = value;
????? }
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????? //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;
????? }
?
????? }
}
輸出結(jié)果是:
absoluteZero is : -459
freezing is : 32
boiling is : 212
paperBurns is : 451
盡管enums有這么多的屬性,但并不是用的越多越好,如果那樣還不如直接用類來(lái)的直接.enums的優(yōu)勢(shì)在定義int最終變量?jī)H當(dāng)這些值有一定特殊含義時(shí).但是如果你需要的是一個(gè)類,就定義一個(gè)類,而不是enum.