Java 中的enum

枚举应该是一个有限集合组成的一个对象

一个简单的例子：

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

枚举最简单的就是当常量用，但是比常量安全，最常用的就是在switch里面。
其实枚举可以当个普通的类使用，可以有变量，方法等。
当普通类使用的时候，其中有几个注意点：

• enum不可以有子类，因为enum是final的，
• enum不是继承其它类，因为它默认已经继承了Enum，但是可以实现Interface
• 构造方法是private或者是默认，不能是public的或者protected的

下面一个用作普通类的例子：

package tenum;

interface Next {
    T next();
}

public enum Week implements Next {
    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天是周几
        System.out.println(Week.MONDAY.next().next().getDescription());
    }

}

输出结果：

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

上面两种来获取所有元素，一种是调用values()方法，一种是通过class 对象的getEnumConstants()方法。
有趣的是values()方法，enum 是继承Eunm.java，但是Enum.java里面并没有values()方法。怎么来的呢？其实是编译器为我们自动生成的。如果你自己在定义这个方法，编译器就会报错“The enum Week already defines the method values() implicitly”。
让我们来看看编译器生成的class 文件。

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

  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();
}

从上面可以看到几点，1）class 是final的，2）自动生成values()方法，3）继承了Enum类

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