Java----枚举

高新技术 枚举

一、枚举

神马是枚举？

答：枚举就相当于一个类，其中也可以定义构造方法，成员变量，普通方法和抽象方法。

枚举：枚举是一个特殊的类，枚举就是要让某个类型的变量的取值只能为若干个固定值中的一个，否则编译器就会报错。枚举可以让编译器在编译时就可以控制源程序中填写的非法值。普通变量的方式无法实现这一目标。也是JDK1.5以后出现的新特性。

练习：用普通类实现枚举功能，定义一个WeekDay的类来模拟枚举的功能。

package cn.itcast.day1;public class EnumTest {public final static void main(String[] args) {// TODO Auto-generated method stubWeekDay weekday = WeekDay.MON;System.out.println(weekday.nextDay());}}

package cn.itcast.day1;public class WeekDay {private WeekDay(){}public static WeekDay SUN = new WeekDay();public static WeekDay MON = new WeekDay();public WeekDay nextDay(){if(this == SUN){return MON;}elsereturn SUN;}@Overridepublic String toString() {// TODO Auto-generated method stubreturn this==SUN?"SUM":"MON";}}

另一种方式，由于nextDay方法采用大量的if else语句进行判断，阅读性很差，所以，将nextDay方法抽象，形成一个个独立的类。

package cn.itcast.day1;public abstract class WeekDay {private WeekDay(){}public static WeekDay SUN = new WeekDay(){@Overridepublic WeekDay nextDay() {return MON;}};public static WeekDay MON = new WeekDay(){@Overridepublic WeekDay nextDay() {return SUN;}};abstract WeekDay nextDay();@Overridepublic String toString() {// TODO Auto-generated method stubreturn this==SUN?"SUM":"MON";}}

体验枚举：

package cn.itcast.day1;public class EnumTest {public final static void main(String[] args) {WeekDay weekday = WeekDay.MON;System.out.println(weekday.name());//.name()枚举名字System.out.println(weekday.toString());System.out.println(weekday.valueOf("FRI").name());//valueOf()将字符串转成枚举对象System.out.println(weekday.values().length);//values()获取枚举数组System.out.println(weekday.ordinal());//获取枚举所在数组的角标//System.out.println(weekday.nextDay());}public enum WeekDay{SUN,MON,TUE,WED,THI,FRI,SAT;}}

实现带有构造方法的枚举：

特点：构造方法必须放在枚举列表之后，枚举列表在所有成员之前。如果枚举列表之后还有其他成员，需要在结尾添加“;”区分开。如果枚举元素带有构造方法，则需要在后面加“( 参数列表 )”。

package cn.itcast.day1;public class EnumTest {public final static void main(String[] args) {WeekDay weekday = WeekDay.MON;System.out.println(weekday.name());//.name()枚举名字System.out.println(weekday.toString());System.out.println(weekday.valueOf("FRI").name());//valueOf()将字符串转成枚举对象System.out.println(weekday.values().length);//values()获取枚举数组System.out.println(weekday.ordinal());//获取枚举所在数组的角标//System.out.println(weekday.nextDay());}public enum WeekDay{SUN("SUM"),MON("MON"),TUE,WED,THI,FRI,SAT;private WeekDay() {System.out.println("first");}private WeekDay(String s) {System.out.println("socend");}}}

实现带有抽象方法的枚举：

特点：带有抽象方法的枚举必须对抽象方法实现，在枚举元素后面加上方法的实现。

如：

枚举元素{实现方法;}

例：交通灯

package cn.itcast.day1;public class TrafficLampTest {public static void main(String[] args) {TrafficLamp lamp = TrafficLamp.YELLOW;System.out.println(lamp.nextLamp().name());}}

package cn.itcast.day1;public enum TrafficLamp {RED(30) {@Overridepublic TrafficLamp nextLamp() {return GREEN;}},GREEN(45) {@Overridepublic TrafficLamp nextLamp() {return YELLOW;}},YELLOW(5) {@Overridepublic TrafficLamp nextLamp() {return RED;}};private int time;private TrafficLamp(int time) {this.time = time;}abstract TrafficLamp nextLamp();}

注：当枚举只有一个成员时，就可以作为单例模式的实现方式。