So one way is to doExampleEnum valueOfOrdinal = ExampleEnum.values()[ordinal]; which works and its easy, however, as mentioned before, ExampleEnum.values() returns a new cloned array for every call. That can be unnecessarily expensive. We can solve that by caching the array like so ExampleEnum[] values = values(). It is also "dangerous" to allow our cached array to be modified. Someone could write ExampleEnum.values[0] = ExampleEnum.type2; So I would make it private with an accessor method that does not do extra copying.

private enum ExampleEnum{
    type0, type1, type2, type3;
    private static final ExampleEnum[] values = values();
    public static ExampleEnum value(int ord) {
        return values[ord];
    }
}

您可以使用exampleenumeration .value(ordinal)来获取与ordinal关联的枚举值

这几乎肯定是个坏主意。当然，如果序号实际上是持久化的(例如，因为有人已将URL标记为书签)——这意味着将来必须始终保留枚举顺序，这对代码维护者来说可能不太明显。

为什么不使用myEnumValue.name()(和解码通过reporttypeenume . valueof (s))来编码枚举?

每个枚举都有name()，它给出一个包含枚举成员名称的字符串。

给定一个花色{红心，黑桃，梅花，方块}，Suit.Heart.name()将给出红心。

每个枚举都有一个valueOf()方法，该方法接受一个枚举类型和一个字符串，以执行相反的操作:

枚举. valueof (Suit.class， "Heart")返回Suit.Heart。

我真搞不懂为什么会有人用序数。它可能会快几纳秒，但如果枚举成员发生变化，则不安全，因为另一个开发人员可能不知道某些代码依赖于序数值(特别是在问题中引用的JSP页面中，网络和数据库开销完全占据了时间，而不是使用整数而不是字符串)。

如果我要大量使用values():

enum Suit {
   Hearts, Diamonds, Spades, Clubs;
   public static final Suit values[] = values();
}

同时wherever.java:

Suit suit = Suit.values[ordinal];

如果你想要数组是私有的，请自便:

private static final Suit values[] = values();
public static Suit get(int ordinal) { return values[ordinal]; }

...

Suit suit = Suit.get(ordinal);

注意数组的边界。

So one way is to doExampleEnum valueOfOrdinal = ExampleEnum.values()[ordinal]; which works and its easy, however, as mentioned before, ExampleEnum.values() returns a new cloned array for every call. That can be unnecessarily expensive. We can solve that by caching the array like so ExampleEnum[] values = values(). It is also "dangerous" to allow our cached array to be modified. Someone could write ExampleEnum.values[0] = ExampleEnum.type2; So I would make it private with an accessor method that does not do extra copying.

private enum ExampleEnum{
    type0, type1, type2, type3;
    private static final ExampleEnum[] values = values();
    public static ExampleEnum value(int ord) {
        return values[ord];
    }
}

您可以使用exampleenumeration .value(ordinal)来获取与ordinal关联的枚举值

这就是我用的。我并不是说它远不如上面那些简单的解决方案“有效”。当在上面的解决方案中使用无效序数值时，它所做的是提供比"ArrayIndexOutOfBounds"更清晰的异常消息。

它利用了EnumSet javadoc指定迭代器以自然顺序返回元素的事实。如果不正确，就有一个断言。

JUnit4测试演示了如何使用它。

 /**
 * convert ordinal to Enum
 * @param clzz may not be null
 * @param ordinal
 * @return e with e.ordinal( ) == ordinal
 * @throws IllegalArgumentException if ordinal out of range
 */
public static <E extends Enum<E> > E lookupEnum(Class<E> clzz, int ordinal) {
    EnumSet<E> set = EnumSet.allOf(clzz);
    if (ordinal < set.size()) {
        Iterator<E> iter = set.iterator();
        for (int i = 0; i < ordinal; i++) {
            iter.next();
        }
        E rval = iter.next();
        assert(rval.ordinal() == ordinal);
        return rval;
    }
    throw new IllegalArgumentException("Invalid value " + ordinal + " for " + clzz.getName( ) + ", must be < " + set.size());
}

@Test
public void lookupTest( ) {
    java.util.concurrent.TimeUnit tu = lookupEnum(TimeUnit.class, 3);
    System.out.println(tu);
}