除了@BradB，回答:

That is so true... It's strange that it is the only answer who mention that. When beginners discover enums, they quickly take that as a magic-trick for valid identifier checking for the compiler. And when the code is intended to be use on distributed systems, they cry... some month later. Maintain backward compatibility with enums that contains non static list of values is a real concern, and pain. This is because when you add a value to an existing enum, its type change (despite the name does not).

"Ho, wait, it may look like the same type, right? After all, they’re enums with the same name – and aren’t enums just integers under the hood?" And for these reasons, your compiler will likely not flag the use of one definition of the type itself where it was expecting the other. But in fact, they are (in most important ways) different types. Most importantly, they have different data domains – values that are acceptable given the type. By adding a value, we’ve effectively changed the type of the enum and therefore break backward compatibility.

总之:当你想使用它的时候使用它，但是，请检查所使用的数据域是一个有限的、已知的、固定的集合。

枚举是什么

enum是为枚举定义的关键字，是一种新的数据类型。应该广泛使用类型安全枚举。特别是，它们是较老api中用来表示相关项集的简单String或int常量的健壮替代。

为什么使用enum

enum是java.lang.Enum的隐式final子类 如果枚举是类的成员，则它是隐式静态的 New永远不能用于枚举，即使是在枚举类型本身中 name和valueOf简单地使用枚举常量的文本，而toString可以被覆盖以提供任何内容，如果需要的话 对于枚举常量，equals和==等于同一个东西，并且可以互换使用 枚举常量是隐式的公共静态final

Note

枚举不能扩展任何类。 enum不能是超类。 枚举常量的出现顺序被称为它们的“自然顺序”，并定义了其他项使用的顺序:compareTo，值的迭代顺序，EnumSet, EnumSet.range。 枚举可以有构造函数、静态和实例块、变量和方法，但不能有抽象方法。

Enum表示枚举，即逐个提到(一些事情)。

enum是一种包含固定常数集的数据类型。

OR

枚举就像一个类，在编译时有一组固定的实例。

例如:

public class EnumExample {
    interface SeasonInt {
        String seasonDuration();
    }

    private enum Season implements SeasonInt {
        // except the enum constants remaining code looks same as class
        // enum constants are implicitly public static final we have used all caps to specify them like Constants in Java
        WINTER(88, "DEC - FEB"), SPRING(92, "MAR - JUN"), SUMMER(91, "JUN - AUG"), FALL(90, "SEP - NOV");

        private int days;
        private String months;

        Season(int days, String months) { // note: constructor is by default private 
            this.days = days;
            this.months = months;
        }

        @Override
        public String seasonDuration() {
            return this+" -> "+this.days + "days,   " + this.months+" months";
        }

    }
    public static void main(String[] args) {
        System.out.println(Season.SPRING.seasonDuration());
        for (Season season : Season.values()){
            System.out.println(season.seasonDuration());
        }

    }
}

枚举的优点:

Enum提高了编译时检查的类型安全性，以避免在运行时出现错误。 枚举可以很容易地用于交换机 枚举可以遍历 枚举可以有字段、构造函数和方法 enum可以实现许多接口，但不能扩展任何类，因为它在内部扩展了enum类

更多的

Something none of the other answers have covered that make enums particularly powerful are the ability to have template methods. Methods can be part of the base enum and overridden by each type. And, with the behavior attached to the enum, it often eliminates the need for if-else constructs or switch statements as this blog post demonstrates - where enum.method() does what originally would be executed inside the conditional. The same example also shows the use of static imports with enums as well producing much cleaner DSL like code.

其他一些有趣的特性包括枚举提供equals()、toString()和hashCode()的实现，并实现Serializable和Comparable。

我强烈推荐Bruce Eckel的《Thinking in Java》第4版，它用了整整一章来讨论这个主题。特别具有启发性的例子是将石头剪子布(即RoShamBo)游戏作为枚举。

知道枚举就像其他具有常量字段和私有构造函数的类一样是有用的。

例如,

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

编译器按如下方式编译它;

class Weekday extends Enum
{
  public static final Weekday MONDAY  = new Weekday( "MONDAY",   0 );
  public static final Weekday TUESDAY = new Weekday( "TUESDAY ", 1 );
  public static final Weekday WEDNESDAY= new Weekday( "WEDNESDAY", 2 );
  public static final Weekday THURSDAY= new Weekday( "THURSDAY", 3 );
  public static final Weekday FRIDAY= new Weekday( "FRIDAY", 4 );
  public static final Weekday SATURDAY= new Weekday( "SATURDAY", 5 );
  public static final Weekday SUNDAY= new Weekday( "SUNDAY", 6 );

  private Weekday( String s, int i )
  {
    super( s, i );
  }

  // other methods...
}

Java允许将变量限制为只有几个预定义值之一——换句话说，枚举列表中的一个值。 使用枚举可以帮助减少代码中的bug。 下面是一个类外部枚举的例子:

enums coffeesize{BIG , HUGE , OVERWHELMING }; 
//This semicolon is optional.

这限制了咖啡的大小:大的，巨大的，或压倒性的变量。