Java Enum

一、枚举的以及使用

Java 枚举是一个特殊的类，一般表示一组常量，比如一年的 4 个季节，一年的 12 个月份，一个星期的 7 天，方向有东南西北等。

Java 枚举类使用 enum 关键字来定义，各个常量使用逗号 , 来分割。

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// 定义
enum Day{
    MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
}

// 使用
Day day= Day.MONDAY;// 此时day就是MONDAY这个常量，可以用if等去判断，也可以直接输出
//也可以通过类调用
Day day= Day.MONDAY;
System.out.println(Day.MONDAY);
if(day==Day.MONDAY){
	System.out.println("yes");
}

枚举类中加入其他常量与方法

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//定义
public enum Day {
    MONDAY(1), TUESDAY(2), WEDNESDAY(3), THURSDAY(4),
    FRIDAY(5), SATURDAY(6), SUNDAY(7);

    Integer integer;

    Day(Integer integer) {
        this.integer = integer;
    }
}
//使用
Day day = Day.MONDAY;
System.out.println(day.integer);
System.out.println(Day.MONDAY.integer);

常用方法

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// values()方法，获取枚举的所有值
Day [] days = Day.values();
System.out.println(Arrays.toString(days));
// [MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY]

// ordinal()获取索引值，类似于数组
Day day = Day.MONDAY;
System.out.println(day.ordinal());
// 0

// valueOf()返回对应的字符串的枚举类型的值
Day day = Day.valueOf("MONDAY");

二、深入理解枚举

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public enum Day {
    MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
}

//反编译Day.class
final class Day extends Enum //实际上继承了Enum枚举类，同时是final，不可被继承
{
    //编译器为我们添加的静态的values()方法
    public static Day[] values()
    {
        return (Day[])$VALUES.clone();
    }
    //编译器为我们添加的静态的valueOf()方法，注意间接调用了Enum也类的valueOf方法
    public static Day valueOf(String s)
    {
        return (Day)Enum.valueOf(Day, s);
    }
    //私有构造函数
    private Day(String s, int i /*,Integer integer*/)
    {
        super(s, i);
        // 如果有其他参数，此处也会同时构造
        // this.integer = integer
    }
     //前面定义的7种枚举实例
    public static final Day MONDAY;
    public static final Day TUESDAY;
    public static final Day WEDNESDAY;
    public static final Day THURSDAY;
    public static final Day FRIDAY;
    public static final Day SATURDAY;
    public static final Day SUNDAY;
    private static final Day $VALUES[];

    static 
    {    
        //实例化枚举实例
        MONDAY = new Day("MONDAY", 0);
        TUESDAY = new Day("TUESDAY", 1);
        WEDNESDAY = new Day("WEDNESDAY", 2);
        THURSDAY = new Day("THURSDAY", 3);
        FRIDAY = new Day("FRIDAY", 4);
        SATURDAY = new Day("SATURDAY", 5);
        SUNDAY = new Day("SUNDAY", 6);
        $VALUES = (new Day[] {
            MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
        });
    }
}