Java Enum 与 Enum 关联

Java Enum 也可以和另一个 Enum 关联起来，这种情况下可以使用 EnumMap 或者 EnumSet。

1. EnumMap

EnumMap 是一种特殊的 Map，它的 key 必须是 Enum 类型。可以用来将一个 Enum 和另一个 Enum 关联起来，例如：

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

public enum Weather {
    SUNNY, RAINY, CLOUDY, SNOWY
}

public class Example {
    private static final Map<Weekday, Weather> weatherMap = new EnumMap<>(Weekday.class);

    static {
        weatherMap.put(Weekday.MONDAY, Weather.SUNNY);
        weatherMap.put(Weekday.TUESDAY, Weather.CLOUDY);
        weatherMap.put(Weekday.WEDNESDAY, Weather.RAINY);
        weatherMap.put(Weekday.THURSDAY, Weather.SUNNY);
        weatherMap.put(Weekday.FRIDAY, Weather.SNOWY);
        weatherMap.put(Weekday.SATURDAY, Weather.CLOUDY);
        weatherMap.put(Weekday.SUNDAY, Weather.RAINY);
    }

    public static void main(String[] args) {
        System.out.println(weatherMap.get(Weekday.MONDAY)); // SUNNY
    }
}

在上面的例子中，我们使用 EnumMap 将 Weekday 枚举和 Weather 枚举关联起来。

2. EnumSet

EnumSet 是一种特殊的 Set，它的元素必须是 Enum 类型。可以用来将一个 Enum 和另一个 Enum 关联起来，例如：

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

public enum Weekend {
    SATURDAY, SUNDAY
}

public class Example {
    private static final Set<Weekday> weekdays = EnumSet.of(
            Weekday.MONDAY, Weekday.TUESDAY, Weekday.WEDNESDAY,
            Weekday.THURSDAY, Weekday.FRIDAY
    );

    public static void main(String[] args) {
        System.out.println(weekdays.contains(Weekday.MONDAY)); // true
        System.out.println(weekdays.contains(Weekday.SATURDAY)); // false
    }
}

在上面的例子中，我们使用 EnumSet 将 Weekday 枚举和 Weekend 枚举关联起来，将工作日和周末分别加入不同的 EnumSet 中。