public class EnumTest {	public static void main(String[] args) {				WeekDay weekDay = WeekDay.MON;		System.out.println(weekDay.nextDay().toString());	}}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";	}}另一种实现public abstract class WeekDay {	private WeekDay(){}		public final static WeekDay SUN = new WeekDay(){				public WeekDay nextDay(){			return MON;		}	};	public final static WeekDay MON = new WeekDay(){		public WeekDay nextDay(){			return SUN;		}	};		public abstract WeekDay nextDay();		public String toString(){				return this ==SUN?"SUN":"MON";	}}