Java 枚舉類型（Enum）是一種特殊的類，用于表示固定數量的常量值。枚舉類型可以實現一些功能，例如：

1. 定義一組有限的常量值：

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

2. 為枚舉類型添加方法和屬性：

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

    private int dayNumber;

    public Weekday(int dayNumber) {
        this.dayNumber = dayNumber;
    }

    public int getDayNumber() {
        return dayNumber;
    }

    public void setDayNumber(int dayNumber) {
        this.dayNumber = dayNumber;
    }

    public String getDayOfWeek() {
        switch (this) {
            case MONDAY:
                return "Monday";
            case TUESDAY:
                return "Tuesday";
            case WEDNESDAY:
                return "Wednesday";
            case THURSDAY:
                return "Thursday";
            case FRIDAY:
                return "Friday";
            case SATURDAY:
                return "Saturday";
            case SUNDAY:
                return "Sunday";
            default:
                throw new IllegalStateException("Unknown day: " + this);
        }
    }
}

3. 使用枚舉類型：

public class EnumExample {
    public static void main(String[] args) {
        Weekday today = Weekday.FRIDAY;
        System.out.println("Today is " + today.getDayOfWeek());
        System.out.println("Day number: " + today.getDayNumber());

        today = Weekday.WEDNESDAY;
        System.out.println("Today is " + today.getDayOfWeek());
        System.out.println("Day number: " + today.getDayNumber());
    }
}

在這個例子中，我們定義了一個名為 Weekday 的枚舉類型，它表示一周的七天。我們還為這個枚舉類型添加了一個構造函數、一個 getDayNumber() 方法和一個 getDayOfWeek() 方法。然后，在 EnumExample 類的 main 方法中，我們使用了這個枚舉類型來表示今天是星期幾以及它是星期幾的數字。