比较不抛出异常的几种Java枚举查找实现的优缺点

这是我们将在示例中使用的枚举。 挑选更复杂的枚举是为了也可以通过其他字段展示查找枚举。

public enum CardColor {
    RED,
    BLACK,
    ;
}
// Jackson annotation to print the enum as an Object instead of the default name.
@JsonFormat(shape = JsonFormat.Shape.OBJECT)
public enum CardSuit {
    // Unicode suits - https://en.wikipedia.org/wiki/Playing_cards_in_Unicode
    SPADE("Spade", String.valueOf((char) 0x2660), CardColor.BLACK),
    HEART("Heart", String.valueOf((char) 0x2665), CardColor.RED),
    DIAMOND("Diamond", String.valueOf((char) 0x2666), CardColor.RED),
    CLUB("Club", String.valueOf((char) 0x2663), CardColor.BLACK),
    ;
    private String displayName;
    private String symbol;
    private CardColor color;
    private CardSuit(String displayName, String symbol, CardColor color) {
        this.displayName = displayName;
        this.symbol =  symbol;
        this.color = color;
    }
    public String getDisplayName() {
        return displayName;
    }
    public void setDisplayName(String displayName) {
        this.displayName = displayName;
    }
    public String getSymbol() {
        return symbol;
    }
    public void setSymbol(String symbol) {
        this.symbol = symbol;
    }
    public CardColor getColor() {
        return color;
    }
    public void setColor(CardColor color) {
        this.color = color;
    }

在 GitHub 上查看。

在你知道输入有效的时候，使用Enum.valueOf非常棒。但是，如果传入无效的名称，那么将抛出异常。在某些情况下，这很好。不过，在通常情况下，我们宁愿忽略异常并返回null。

log.debug("Running valueOf");
for (String name : names) {
    try {
        log.debug("looking up {} found {}", name, Json.serializer().toString(CardSuit.valueOf(name)));
    } catch (Exception ex) {
        log.warn("Exception Thrown", ex);
    }
}

2017-02-22 14:46:38.556 [main] DEBUG c.s.examples.common.EnumLookup - Running valueOf
2017-02-22 14:46:38.804 [main] DEBUG c.s.examples.common.EnumLookup - looking up SPADE found {"displayName":"Spade","symbol":"♠","color":"BLACK"}
2017-02-22 14:46:38.806 [main] DEBUG c.s.examples.common.EnumLookup - looking up HEART found {"displayName":"Heart","symbol":"♥","color":"RED"}
2017-02-22 14:46:38.806 [main] DEBUG c.s.examples.common.EnumLookup - looking up DIAMOND found {"displayName":"Diamond","symbol":"♦","color":"RED"}
2017-02-22 14:46:38.806 [main] DEBUG c.s.examples.common.EnumLookup - looking up CLUB found {"displayName":"Club","symbol":"♣","color":"BLACK"}
2017-02-22 14:46:38.808 [main] WARN  c.s.examples.common.EnumLookup - Exception Thrown
java.lang.IllegalArgumentException: No enum constant com.stubbornjava.examples.common.EnumLookup.CardSuit.Missing
  at java.lang.Enum.valueOf(Enum.java:238)
  at com.stubbornjava.examples.common.EnumLookup$CardSuit.valueOf(EnumLookup.java:1)
  at com.stubbornjava.examples.common.EnumLookup.main(EnumLookup.java:154)

很不幸的是，以下两种方法在代码库中出现得是这么的频繁。反面例子，勿学。

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这种拙劣的做法最常见于初学者。异常不应该用于控制流，并且这样可能会有一些性能影响。不要偷懒。你必须用正确的方式去做。

/*
 * Please don't do this! Using try / catch for
 * control flow is a bad practice.
 */
public static CardSuit trycatchValueOf(String name) {
    try {
        return CardSuit.valueOf(name);
    } catch (Exception ex) {
        log.warn("Exception Thrown", ex);
        return null;
    }
}

log.debug("Running trycatchValueOf");
for (String name : names) {
    log.debug("looking up {} found {}", name, Json.serializer().toString(CardSuit.trycatchValueOf(name)));
}

2017-02-22 14:46:38.809 [main] DEBUG c.s.examples.common.EnumLookup - Running trycatchValueOf
2017-02-22 14:46:38.809 [main] DEBUG c.s.examples.common.EnumLookup - looking up SPADE found {"displayName":"Spade","symbol":"♠","color":"BLACK"}
2017-02-22 14:46:38.809 [main] DEBUG c.s.examples.common.EnumLookup - looking up HEART found {"displayName":"Heart","symbol":"♥","color":"RED"}
2017-02-22 14:46:38.809 [main] DEBUG c.s.examples.common.EnumLookup - looking up DIAMOND found {"displayName":"Diamond","symbol":"♦","color":"RED"}
2017-02-22 14:46:38.809 [main] DEBUG c.s.examples.common.EnumLookup - looking up CLUB found {"displayName":"Club","symbol":"♣","color":"BLACK"}
2017-02-22 14:46:38.809 [main] WARN  c.s.examples.common.EnumLookup - Exception Thrown
java.lang.IllegalArgumentException: No enum constant com.stubbornjava.examples.common.EnumLookup.CardSuit.Missing
  at java.lang.Enum.valueOf(Enum.java:238)
  at com.stubbornjava.examples.common.EnumLookup$CardSuit.valueOf(EnumLookup.java:1)
  at com.stubbornjava.examples.common.EnumLookup$CardSuit.trycatchValueOf(EnumLookup.java:89)
  at com.stubbornjava.examples.common.EnumLookup.main(EnumLookup.java:171)
2017-02-22 14:46:38.809 [main] DEBUG c.s.examples.common.EnumLookup - looking up Missing found null

这种方法也很常见（见这里），但至少程序员知道不能用try/catch来捕获异常。那么，这种方法有什么问题呢？没错，它会迭代所有枚举，直到找到匹配的枚举或返回null——最坏的情况下需要n次，其中n就是枚举值的数量。有些人可能会认为这微不足道，这是过早优化了而已。但是，数据结构和算法是CS基础。使用Map而不是迭代集合要省力得多。这会大大提高性能吗？不，但它是一个很好的习惯。在面试候选人时，你会对线性复杂度搜索算法感到满意吗？此时，你不应该让这样的代码审查通过。

/*
 * Please don't do this! It is inefficient and it's
 * not very hard to use Guava or a static Map as an index.
 */
public static CardSuit iterationFindByName(String name) {
    for (CardSuit suit : CardSuit.values()) {
        if (name.equals(suit.name())) {
            return suit;
        }
    }
    return null;
}

log.debug("Running iteration");
for (String name : names) {
    log.debug("looking up {} found {}", name, Json.serializer().toString(CardSuit.iterationFindByName(name)));
}

2017-02-22 14:46:38.808 [main] DEBUG c.s.examples.common.EnumLookup - Running iteration
2017-02-22 14:46:38.809 [main] DEBUG c.s.examples.common.EnumLookup - looking up SPADE found {"displayName":"Spade","symbol":"♠","color":"BLACK"}
2017-02-22 14:46:38.809 [main] DEBUG c.s.examples.common.EnumLookup - looking up HEART found {"displayName":"Heart","symbol":"♥","color":"RED"}
2017-02-22 14:46:38.809 [main] DEBUG c.s.examples.common.EnumLookup - looking up DIAMOND found {"displayName":"Diamond","symbol":"♦","color":"RED"}
2017-02-22 14:46:38.809 [main] DEBUG c.s.examples.common.EnumLookup - looking up CLUB found {"displayName":"Club","symbol":"♣","color":"BLACK"}
2017-02-22 14:46:38.809 [main] DEBUG c.s.examples.common.EnumLookup - looking up Missing found null

以下都可以通过使用Map形式的索引起作用。但是，它们之间也有一些细微的差别。

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用于快速查找固定大小的正确数据结构是什么？那就是HashMap。现在通过一些额外的样板，只要我们有很好的散列函数的话，我们就可以进行更有效的查找。稍微更冗长，但如果有办法减少样板的话，这将是极好的。

private static final Map nameIndex =
        Maps.newHashMapWithExpectedSize(CardSuit.values().length);
static {
    for (CardSuit suit : CardSuit.values()) {
        nameIndex.put(suit.name(), suit);
    }
}
public static CardSuit lookupByName(String name) {
    return nameIndex.get(name);
}

log.debug("Running lookupByName");
for (String name : names) {
    log.debug("looking up {} found {}", name, Json.serializer().toString(CardSuit.lookupByName(name)));
}

2017-02-22 14:46:38.809 [main] DEBUG c.s.examples.common.EnumLookup - Running lookupByName
2017-02-22 14:46:38.809 [main] DEBUG c.s.examples.common.EnumLookup - looking up SPADE found {"displayName":"Spade","symbol":"♠","color":"BLACK"}
2017-02-22 14:46:38.810 [main] DEBUG c.s.examples.common.EnumLookup - looking up HEART found {"displayName":"Heart","symbol":"♥","color":"RED"}
2017-02-22 14:46:38.810 [main] DEBUG c.s.examples.common.EnumLookup - looking up DIAMOND found {"displayName":"Diamond","symbol":"♦","color":"RED"}
2017-02-22 14:46:38.813 [main] DEBUG c.s.examples.common.EnumLookup - looking up CLUB found {"displayName":"Club","symbol":"♣","color":"BLACK"}
2017-02-22 14:46:38.813 [main] DEBUG c.s.examples.common.EnumLookup - looking up Missing found null

这是一个常见的用例，我们在Google的朋友为此提供了一个非常干净且无需样板的解决方案。探其究竟，它甚至使用了WeakReferences和WeakHashMaps。基本说来，这段代码将创建一个全局的静态地图，并以Enum的类名称键入，并将其用于查找。

public static CardSuit getIfPresent(String name) {
    return Enums.getIfPresent(CardSuit.class, name).orNull();
}

log.debug("Running Guava getIfPresent");
for (String name : names) {
    log.debug("looking up {} found {}", name, Json.serializer().toString(CardSuit.getIfPresent(name)));
}

2017-02-22 14:46:38.813 [main] DEBUG c.s.examples.common.EnumLookup - Running Guava getIfPresent
2017-02-22 14:46:38.814 [main] DEBUG c.s.examples.common.EnumLookup - looking up SPADE found {"displayName":"Spade","symbol":"♠","color":"BLACK"}
2017-02-22 14:46:38.814 [main] DEBUG c.s.examples.common.EnumLookup - looking up HEART found {"displayName":"Heart","symbol":"♥","color":"RED"}
2017-02-22 14:46:38.815 [main] DEBUG c.s.examples.common.EnumLookup - looking up DIAMOND found {"displayName":"Diamond","symbol":"♦","color":"RED"}
2017-02-22 14:46:38.815 [main] DEBUG c.s.examples.common.EnumLookup - looking up CLUB found {"displayName":"Club","symbol":"♣","color":"BLACK"}
2017-02-22 14:46:38.815 [main] DEBUG c.s.examples.common.EnumLookup - looking up Missing found null

这个完全相同的方法可以用于枚举的其他字段。想要通过其显示的名称或其他属性来查找枚举并不少见。

与上述相同的方法，但是在显示名称上而不是枚举名称上进行索引。

private static final Map displayNameIndex =
        Maps.newHashMapWithExpectedSize(CardSuit.values().length);
static {
    for (CardSuit suit : CardSuit.values()) {
        displayNameIndex.put(suit.getDisplayName(), suit);
    }
}
public static CardSuit lookupByDisplayName(String name) {
    return displayNameIndex.get(name);
}

log.debug("Running lookupByDisplayName");
for (String displayName : displayNames) {
    log.debug("looking up {} found {}", displayName, Json.serializer().toString(CardSuit.lookupByDisplayName(displayName)));
}

2017-02-22 14:46:38.815 [main] DEBUG c.s.examples.common.EnumLookup - Running lookupByDisplayName
2017-02-22 14:46:38.815 [main] DEBUG c.s.examples.common.EnumLookup - looking up Spade found {"displayName":"Spade","symbol":"♠","color":"BLACK"}
2017-02-22 14:46:38.815 [main] DEBUG c.s.examples.common.EnumLookup - looking up Heart found {"displayName":"Heart","symbol":"♥","color":"RED"}
2017-02-22 14:46:38.815 [main] DEBUG c.s.examples.common.EnumLookup - looking up Diamond found {"displayName":"Diamond","symbol":"♦","color":"RED"}
2017-02-22 14:46:38.816 [main] DEBUG c.s.examples.common.EnumLookup - looking up Club found {"displayName":"Club","symbol":"♣","color":"BLACK"}
2017-02-22 14:46:38.816 [main] DEBUG c.s.examples.common.EnumLookup - looking up Missing found null

我们不能在这里利用Guava，因为对于静态索引创建唯一的全局密钥将是一件困难的事。但是，这并不意味着我们没有帮手！

public class EnumUtils {
    public static > Function lookupMap(Class clazz, Function mapper) {
        @SuppressWarnings("unchecked")
        E[] emptyArray = (E[]) Array.newInstance(clazz, 0);
        return lookupMap(EnumSet.allOf(clazz).toArray(emptyArray), mapper);
    }
    public static > Function lookupMap(E[] values, Function mapper) {
        Map index = Maps.newHashMapWithExpectedSize(values.length);
        for (E value : values) {
            index.put(mapper.apply(value), value);
        }
        return (T key) -> index.get(key);
    }
}

现在我们有了一个相与样板没多大关联的通用解决方案。

private static final Function func =
        EnumUtils.lookupMap(CardSuit.class, e -> e.getDisplayName());
public static CardSuit lookupByDisplayNameUtil(String name) {
    return func.apply(name);
}

log.debug("Running lookupByDisplayNameUtil");
for (String displayName : displayNames) {
    log.debug("looking up {} found {}", displayName, Json.serializer().toString(CardSuit.lookupByDisplayNameUtil(displayName)));
}

2017-02-22 14:46:38.816 [main] DEBUG c.s.examples.common.EnumLookup - Running lookupByDisplayNameUtil
2017-02-22 14:46:38.816 [main] DEBUG c.s.examples.common.EnumLookup - looking up Spade found {"displayName":"Spade","symbol":"♠","color":"BLACK"}
2017-02-22 14:46:38.816 [main] DEBUG c.s.examples.common.EnumLookup - looking up Heart found {"displayName":"Heart","symbol":"♥","color":"RED"}
2017-02-22 14:46:38.816 [main] DEBUG c.s.examples.common.EnumLookup - looking up Diamond found {"displayName":"Diamond","symbol":"♦","color":"RED"}
2017-02-22 14:46:38.816 [main] DEBUG c.s.examples.common.EnumLookup - looking up Club found {"displayName":"Club","symbol":"♣","color":"BLACK"}
2017-02-22 14:46:38.816 [main] DEBUG c.s.examples.common.EnumLookup - looking up Missing found null

这里有若干方法都可用于解决同一个问题。有些拙劣，有些更优。

英文原文：Java Enum Lookup by Name or Field Without Throwing Exceptions

翻译作者：码农网 – 小峰