Many enums are naturally associated with a single int value. All enums have an ordinal method, which returns the numerical position of each enum constant in its type. You may be tempted to derive an associated int value from the ordinal:

许多枚举都与一个单独的int值进行了关联。所有的枚举都有ordinal方法，这个方法返回了每个枚举常量的位置。你也许会试着从ordinal方法里获取得到一个关联的int值：

public enum Ensemble {
    SOLO, DUET, TRIO, QUARTET, QUINTET, SEXTET, SEPTET, OCTET, NONET, DECTET;
    public int numberOfMusicians() { 
        return ordinal() + 1; 
    } 
}

While this enum works, it is a maintenance nightmare. If the constants are reordered, the numberOfMusicians method will break. If you want to add a second enum constant associated with an int value that you’ve already used, you’re out of luck. For example, it might be nice to add a constant for double quartet, which, like an octet, consists of eight musicians, but there is no way to do it.

虽然这个枚举可以正常使用，但是它的维护却将是一场噩梦。如果对常量进行重排序，那么numberOfMusicians方法将被破坏。如果你想再增加一个常量，并且让这个新的常量与一个已经被使用了的int值进行绑定，那也是不行的。例如，你想为双四重奏增加一个常量，而它却像八重奏一样，有8个演奏者，那么此时你也是无法做到让numberOfMusicians返回8的。

Also, you can’t add a constant for an int value without adding constants for all intervening int values. For example, suppose you want to add a constant representing a triple quartet, which consists of twelve musicians. There is no standard term for an ensemble consisting of eleven musicians, so you are forced to add a dummy constant for the unused int value (11). At best, this is ugly. If many int values are unused, it’s impractical.

同样，若是没有为所有涉及到的int值添加常量，你也是无法为某个int值添加常量的。例如，假设你想增加一个常量用来表示一个由12个演奏者组成的三重奏。由于没有由11个演奏者组成合奏组的标准，那么此时你不得不为一个用不到的int值(11)添加一个虚拟的常量。这么做顶多就只是显得不优雅。但如果有大量的int值都是用不到的，那么这么做就不切实际了。

Luckily, there is a simple solution to these problems. Never derive a value associated with an enum from its ordinal; store it in an instance field instead:

幸运的是，有个简单的办法可以解决这些问题。那就是，永远都不要经过某个枚举的ordinal方法来获取它的绑定值，而是要将这个值通过一个实例域来进行保存：

public enum Ensemble {
    SOLO(1), DUET(2), TRIO(3), QUARTET(4), QUINTET(5), 
    SEXTET(6), SEPTET(7), OCTET(8), DOUBLE_QUARTET(8), 
    NONET(9), DECTET(10), TRIPLE_QUARTET(12);

    private final int numberOfMusicians;
    Ensemble(int size) { 
        this.numberOfMusicians = size; 
    }
    public int numberOfMusicians() { 
        return numberOfMusicians; 
    }
}

The Enum specification has this to say about ordinal: “Most programmers will have no use for this method. It is designed for use by general-purpose enum-based data structures such as EnumSet and EnumMap.” Unless you are writing code with this character, you are best off avoiding the ordinal method entirely.

Enum规范里提到ordianl方法时是这么说的：“大多数程序员将用不到这个方法。设计这个方法是来用于像EnumSet和EnumMap这种基于枚举的通用数据结构的。“ 除非你编写的代码带有这种特征，否则最好都是要避免使用ordinal方法。