Java 枚举

 

为什么要用枚举？
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。

用普通类的方法模拟枚举的实现原理

public class WeekDay {private WeekDay(){}public final static WeekDay SUN=new WeekDay();public final static WeekDay MON=new WeekDay();public WeekDay nextDay(){if(this==SUN){return MON;}else{return SUN;}}public String toString(){return this==SUN?"SUN":"MON";}}public class EnumTest{public static void main(String []args)throws Exception{WeekDay weekday=WeekDay.MON;System.out.println(weekday.nextDay());}}结果打印输出：SUN

采用抽象方法---把大量的if-else语句转移成一个个独立的类

public abstract class WeekDay {private WeekDay(){}public final static WeekDay SUN=new WeekDay(){@Overridepublic WeekDay nextDay() {// TODO Auto-generated method stubreturn MON;}};//内部类没有名称是WeekDay的子类并且创建了一个对象public final static WeekDay MON=new WeekDay(){@Overridepublic WeekDay nextDay() {// TODO Auto-generated method stubreturn SUN;}};public abstract WeekDay nextDay();public String toString(){return this==SUN?"SUN":"MON";}}public class EnumTest{public static void main(String []args)throws Exception{WeekDay weekday=WeekDay.MON;System.out.println(weekday.nextDay());}}同样可以得到

以下是一个基本的枚举类

public class EnumTest{public static void main(String []args)throws Exception{//枚举定义变量的时候只能给定义的元素WeekDay week2=WeekDay.SUN;System.out.println(week2);System.out.println(week2.name());//打印自己的名字System.out.println(week2.ordinal());//排行System.out.println(Week2.valueOf("SUN"));//怎样把一个串变成一个对象（valueOf()）System.out.println(Week2.values().length)//把枚举中的每个元素都存储在数组里}public enum WeekDay{SUN,MON,TUE,WEN,THI,FRI,SAT;}}

实现带有构造方法的枚举

public class EnumTest{public static void main(String []args)throws Exception{//枚举定义变量的时候只能给定义的元素WeekDay week2=WeekDay.SUN;System.out.println(week2);System.out.println(week2.name());//打印自己的名字System.out.println(week2.ordinal());//排行System.out.println(Week2.valueOf("SUN").toString());//怎样把一个串变成一个对象（valueOf()）System.out.println(Week2.values().length)//把枚举中的每个元素都存储在数组里}public enum WeekDay{SUN(1),MON,TUE,WEN,THI,FRI,SAT;//实现构造方法的枚举----要求要是私有的并且在元素后面private WeekDay(){System.out.println("first");}//默认调用private WeekDay(int day){System.out.println("second");}//需要在元素写成SUN(1)}}

实现带有抽象方法的枚举

public class EnumTest{public static void main(String []args)throws Exception{new Date(300){};//子类的构造方法要调用父类构造方法}public enum TrafficLamp{RED(30){public TrafficLamp nextLamp(){return GREEN;}},GREEN(45){public TrafficLamp nextLamp(){return YELLOW;}},YEELOW(5){public TrafficLamp nextLamp(){return RED;}};public abstract TrafficLamp nextLamp();private int time;private TrafficLamp(int time){this.time=time;}}｝