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(){

		@Override
		public WeekDay nextDay() {
			// TODO Auto-generated method stub
			return 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(){

		@Override
		public WeekDay nextDay() {
			// TODO Auto-generated method stub
			return 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;}


}

｝