枚舉應(yīng)該是一個有限集合組成的一個對象

一個簡單的例子：

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

枚舉最簡單的就是當(dāng)常量用，但是比常量安全，最常用的就是在switch里面。
其實(shí)枚舉可以當(dāng)個普通的類使用，可以有變量，方法等。
當(dāng)普通類使用的時候，其中有幾個注意點(diǎn)：

• enum不可以有子類，因?yàn)閑num是final的，
• enum不是繼承其它類，因?yàn)樗J(rèn)已經(jīng)繼承了Enum，但是可以實(shí)現(xiàn)Interface
• 構(gòu)造方法是private或者是默認(rèn)，不能是public的或者protected的

下面一個用作普通類的例子：

package tenum;

interface Next<T> {
    T next();
}

public enum Week implements Next<Week> {
    SUNDAY("Today is Sunday."), MONDAY("Today is Monday"), TUESDAY("Today is Tuesday"), WEDNESDAY(
            "Today is wednesday"), THURSDAY(
                    "Today is Thursday"), FRIDAY("Today is Firday"), SATURDAY("Today is Saturday");

    private String description;

    Week(String description) {
        this.description = description;
    }

    public String getDescription() {
        return description;
    }

    @Override
    public Week next() {
        switch (this) {
        case SUNDAY:
            return MONDAY;
        case MONDAY:
            return TUESDAY;
        case TUESDAY:
            return WEDNESDAY;
        case WEDNESDAY:
            return THURSDAY;
        case THURSDAY:
            return FRIDAY;
        case FRIDAY:
            return SATURDAY;
        case SATURDAY:
            return SUNDAY;
        }
        return null;
    }

    public static void main(String[] args) {
        for (Week w : values()) {
            System.out.println(w.getDescription());
        }
        System.out.println("------------------------------");
        for (Week w : Week.class.getEnumConstants()) {
            System.out.println(w.getDescription());
        }
        System.out.println("------------------------------");
        //星期一之后2天是周幾
        System.out.println(Week.MONDAY.next().next().getDescription());
    }

}

輸出結(jié)果：

Today is Sunday.
Today is Monday
Today is Tuesday
Today is wednesday
Today is Thursday
Today is Firday
Today is Saturday
------------------------------
Today is Sunday.
Today is Monday
Today is Tuesday
Today is wednesday
Today is Thursday
Today is Firday
Today is Saturday
------------------------------
Today is wednesday

上面兩種來獲取所有元素，一種是調(diào)用values()方法，一種是通過class 對象的getEnumConstants()方法。
有趣的是values()方法，enum 是繼承Eunm.java，但是Enum.java里面并沒有values()方法。怎么來的呢？其實(shí)是編譯器為我們自動生成的。如果你自己在定義這個方法，編譯器就會報錯“The enum Week already defines the method values() implicitly”。
讓我們來看看編譯器生成的class 文件。

Warning: Binary file Week contains tenum.Week
Compiled from "Week.java"
public final class tenum.Week extends java.lang.Enum<tenum.Week> implements tenum.Next<tenum.Week> {

  public static final tenum.Week SUNDAY;
  public static final tenum.Week MONDAY;
  public static final tenum.Week TUESDAY;
  public static final tenum.Week WEDNESDAY;
  public static final tenum.Week THURSDAY;
  public static final tenum.Week FRIDAY;
  public static final tenum.Week SATURDAY;
  static {};
  public java.lang.String getDescription();
  public tenum.Week next();
  public static void main(java.lang.String[]);
  public static tenum.Week[] values();
  public static tenum.Week valueOf(java.lang.String);
  public java.lang.Object next();
  static int[] $SWITCH_TABLE$tenum$Week();
}

從上面可以看到幾點(diǎn)，1）class 是final的，2）自動生成values()方法，3）繼承了Enum類

參考：
https://docs.oracle.com/javase/tutorial/java/javaOO/enum.html
https://www.ibm.com/developerworks/cn/java/j-lo-enum/index.html