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為什麼要有枚舉
問題：要定義星期幾或性别的變量，該怎麼定義？假設用1-7分别表示星期一到星期日，但有人可能會寫成int weekday = 0;或即使使用常量方式也無法阻止意外。
枚舉就是要讓某個類型的變量的取值隻能為若幹個固定值中的一個，否則，編譯器就會報錯。枚舉可以讓編譯器在編譯時就可以控制源程式中填寫的非法值，普通變量的方式在開發階段無法實作這一目标。
用普通類如何實作枚舉功能
定義一個Weekday的類來模拟枚舉功能。
私有的構造方法
每個元素分别用一個公有的靜态成員變量表示
可以有若幹公有方法或抽象方法。采用抽象方法定義nextDay就将大量的if.else語句轉移成了一個個獨立的類。
package cn.itcast.enumeration;
public class Weekday {
// 私有構造方法，不允許外界構造對象
private Weekday() {
}
// 自己構造對象
public static final Weekday SUNDAY = new Weekday();
public static final Weekday MONDAY = new Weekday();
public static final Weekday TUESDAY = new Weekday();
public static final Weekday WEDNESDAY = new Weekday();
public static final Weekday THURSDAY = new Weekday();
public static final Weekday FRIDAY = new Weekday();
public static final Weekday SATURDAY = new Weekday();
// 定義方法。注意個方法有大量的if-else判斷，為了簡化，我們可以把此方法定義為抽象的。具體看Weekday2
public Weekday nextDay() {
Weekday nextDay = null;
if (this == SUNDAY) {
nextDay = MONDAY;
} else if (this == MONDAY) {
nextDay = TUESDAY;
} else if (this == TUESDAY) {
nextDay = WEDNESDAY;
} else if (this == WEDNESDAY) {
nextDay = THURSDAY;
} else if (this == THURSDAY) {
nextDay = FRIDAY;
} else if (this == FRIDAY) {
nextDay = SATURDAY;
} else if (this == SATURDAY) {
nextDay = SUNDAY;
} else {
throw new IllegalArgumentException();
}
return nextDay;
}
// 覆寫方法，便于外界調用
@Override
public String toString() {
String toString = null;
if (this == SUNDAY) {
toString = "sunday";
} else if (this == MONDAY) {
toString = "monday";
} else if (this == TUESDAY) {
toString = "tuesday";
} else if (this == WEDNESDAY) {
toString = "wednesday";
} else if (this == THURSDAY) {
toString = "thursday";
} else if (this == FRIDAY) {
toString = "friday";
} else if (this == SATURDAY) {
toString = "saturday";
} else {
throw new IllegalArgumentException();
}
return toString;
}
}
package cn.itcast.enumeration;
public abstract class Weekday2 {
public static final Weekday2 SUNDAY = new Weekday2() {
@Override
public Weekday2 nextDay() {
return MONDAY;
}
};
public static final Weekday2 MONDAY = new Weekday2() {
@Override
public Weekday2 nextDay() {
return TUESDAY;
}
};
public static final Weekday2 TUESDAY = new Weekday2() {
@Override
public Weekday2 nextDay() {
return WEDNESDAY;
}
};
public static final Weekday2 WEDNESDAY = new Weekday2() {
@Override
public Weekday2 nextDay() {
return THURSDAY;
}
};
public static final Weekday2 THURSDAY = new Weekday2() {
@Override
public Weekday2 nextDay() {
return FRIDAY;
}
};
public static final Weekday2 FRIDAY = new Weekday2() {
@Override
public Weekday2 nextDay() {
return SATURDAY;
}
};
public static final Weekday2 SATURDAY = new Weekday2() {
@Override
public Weekday2 nextDay() {
return SUNDAY;
}
};
public abstract Weekday2 nextDay();
@Override
public String toString() {
String toString = null;
if (this == SUNDAY) {
toString = "sunday";
} else if (this == MONDAY) {
toString = "monday";
} else if (this == TUESDAY) {
toString = "tuesday";
} else if (this == WEDNESDAY) {
toString = "wednesday";
} else if (this == THURSDAY) {
toString = "thursday";
} else if (this == FRIDAY) {
toString = "friday";
} else if (this == SATURDAY) {
toString = "saturday";
} else {
throw new IllegalArgumentException();
}
return toString;
}
}
枚舉的基本應用
舉例：定義一個Weekday的枚舉。
擴充：枚舉類的values,valueOf,name,toString,ordinal等方法
總結：枚舉是一種特殊的類，其中的每個元素都是該類的一個執行個體對象，例如可以調用WeekDay.SUN.getClass().getName和WeekDay.class.getName()。
枚舉的進階應用
枚舉就相當于一個類，其中也可以定義構造方法、成員變量、普通方法和抽象方法。
枚舉元素必須位于枚舉體中的最開始部分，枚舉元素清單的後要有分号與其他成員分隔。把枚舉中的成員方法或變量等放在枚舉元素的前面，編譯器報告錯誤。
帶構造方法的枚舉
構造方法必須定義成私有的
如果有多個構造方法，該如何選擇哪個構造方法？
枚舉元素MON和MON（）的效果一樣，都是調用預設的構造方法。
帶方法的枚舉
定義枚舉TrafficLamp
實作普通的next方法
實作抽象的next方法：每個元素分别是由枚舉類的子類來生成的執行個體對象，這些子類采用類似内部類的方式進行定義。
增加上表示時間的構造方法
枚舉隻有一個成員時，就可以作為一種單例的實作方式。
package cn.itcast.enumeration;
public class EnumTest2 {
public static void main(String[] args) {
// Weekday mon = Weekday.MONDAY;
// System.out.format("%s的下一天是%s.%n", mon, mon.nextDay());
Weekday2 mon = Weekday2.MONDAY;
System.out.println(mon.name());
System.out.println(mon.ordinal());
System.out.println(Weekday2.valueOf("MONDAY"));
System.out.println(Weekday2.values().length);
}
public enum Weekday2 {
SUNDAY(0), MONDAY(1), TUESDAY(), WEDNESDAY, THURSDAY, FRIDAY, SATURDAY;
private Weekday2() {
System.out.println("無參構造方法");
}
private Weekday2(int day) {
System.out.println("有參構造方法");
}
}
public enum TrafficLamp {
RED(30) {
@Override
public TrafficLamp nextLamp() {
return GREEN;
}
},
GREEN(45) {
@Override
public TrafficLamp nextLamp() {
return YELLOW;
}
},
YELLOW(5) {
@Override
public TrafficLamp nextLamp() {
return RED;
}
};
private int time;
private TrafficLamp(int time) {
this.time = time;
}
public abstract TrafficLamp nextLamp();
public int getTime() {
return time;
}
}
}
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