【Java学习笔记】枚举类型
作者：gnuhpc
出处：http://www.cnblogs.com/gnuhpc/
java.lang.Enum > 使用enum定义。
类如：
public class EnumDemo {
enum Edge {
TOP, BOTTOM, LEFT, RIGHT//定义了一个enum类型的实例，一个枚举类型除了那些由枚举常量定义的实例外没有任何其他实例。
};
public static void main(String arg[]) {
Edge edge1 = Edge.LEFT;
Edge edge2 = Edge.LEFT;
//由于每个枚举类型的实例在一个程序中只能有一个，所以我们可以使用==来判断是否相同。
System.out.println("Can we use == to judge equals:"+ (edge1 == edge2));
int i = edge1.ordinal();
System.out.println("The value of i: " + i);
System.out.println(edge1);
String str = "";
switch(edge1) {
/* case Edge.TOP: This is wrong*/
case TOP: str = "The top edge"; break;
case BOTTOM: str = "The bottom edge"; break;
case LEFT: str = "The left edge"; break;
case RIGHT: str = "The right edge"; break;
}
System.out.println(str);
}
这就解决了switch不能对字符串进行的缺点了。注意：枚举类型也是类，里面的成员实际上是静态的final类型的枚举类型实例。
为了更好的理解枚举类型，我们用普通类模拟一下该枚举类 ：
public abstract class WeekDay {
private int workhours;
private boolean work;
private WeekDay(int workhours, boolean work) {
this.workhours = workhours;
this.work = work;
}
protected abstract WeekDay getNextDay();
public static final WeekDay MONDAY = new WeekDay(8,true){
public WeekDay getNextDay() {
return TUESDAY;
}
@Override
public String toString() {
return "MONDAY";
}
};
public static final WeekDay TUESDAY = new WeekDay(8,true){
public WeekDay getNextDay() {
return WEDNESDAY;
}
@Override
public String toString() {
return "TUESDAY";
}
};
public static final WeekDay WEDNESDAY = new WeekDay(8,true){
public WeekDay getNextDay() {
return THURSDAY;
}
@Override
public String toString() {
return "WEDNESDAY";
}
};
public static final WeekDay THURSDAY = new WeekDay(8,true){
public WeekDay getNextDay() {
return FRIDAY;
}
@Override
public String toString() {
return "THURSDAY";
}
};
public static final WeekDay FRIDAY = new WeekDay(8,true){
public WeekDay getNextDay() {
return SATURDAY;
}
@Override
public String toString() {
return "FRIDAY";
}
};
public static final WeekDay SATURDAY = new WeekDay(4,true){
public WeekDay getNextDay() {
return SUNDAY;
}
@Override
public String toString() {
return "SATURDAY";
}
};
public static final WeekDay SUNDAY = new WeekDay(0,false){
public WeekDay getNextDay() {
return MONDAY;
}
@Override
public String toString() {
return "SUNDAY";
}
};
private static void showDay(WeekDay d) {
if(d.isWeekday()) {
System.out.println(d + " is a weekday and has " +
d.getHours() + " working hours.");
} else {
System.out.println(d + " is not a weekday and has " +
d.getHours() + " working hours.");
}
}
private int getHours() {
// TODO Auto-generated method stub
return workhours;
}
private boolean isWeekday() {
return work;
}
public static void main(String[] args) {
WeekDay day = WeekDay.FRIDAY;
showDay(day);
day = WeekDay.FRIDAY.getNextDay();
showDay(day);
day = day.getNextDay();
showDay(day);
}
}
因此，有以下更复杂的应用：
public enum Day {
MONDAY (8,true){//首先使用了子类继承的匿名类方式进行了对父类构造函数的，并且在子类中完成了父类的抽象方法
public Day getNextDay() {
return TUESDAY;
}
},
TUESDAY (8,true){
public Day getNextDay() {
return WEDNESDAY;
}
},
WEDNESDAY (8,true){
public Day getNextDay() {
return THURSDAY;
}
},
THURSDAY (8,true){
public Day getNextDay() {
return FRIDAY;
}
},
FRIDAY (8,true){
public Day getNextDay() {
return SATURDAY;
}
},
SATURDAY (4,false){
public Day getNextDay() {
return SUNDAY;
}
},
SUNDAY (0,false){
public Day getNextDay() {
return MONDAY;
}
};
private int hours;
private boolean weekday;
Day(int workHours,boolean weekdayFlag) {
hours = workHours;
weekday = weekdayFlag;
}
private abstract Day getNextDay();
public int getHours() {
return(hours);
}
public boolean isWeekday() {
return(weekday);
}
private static void showDay(Day d) {
if(d.isWeekday()) {
System.out.println(d + " is a weekday and has " +
d.getHours() + " working hours.");
} else {
System.out.println(d + " is not a weekday and has " +
d.getHours() + " working hours.");
}
}
public static void main(String arg[]) {
Day day;
day = Day.TUESDAY;
showDay(day);
day = Day.SATURDAY;
showDay(day);
day = Day.THURSDAY.getNextDay();
showDay(day);
}
}
注意：
1.注意枚举类型不能定义为abstract或者final。
2.枚举类中定义了抽象方法，但是枚举类却没有实例去实现这个抽象方法，这样会报错。
3.一个枚举类型除非包含至少一个枚举常量带有class body，否则就是隐式final的。
4.嵌套 （在一个类中定义的枚举）的枚举类型是隐式static的，若显示赋予就会出错。
5.EnumSet包含了许多静态方法用来指定不同的集合建立和读取方式。
