幾種Java 枚舉查找且不拋異常的實現及其優劣比較

時間 2019-11-10

標籤幾種 java 枚舉查找且不異常實現及其優劣比較欄目 Java 简体版

原文原文鏈接

導讀	Java Enum是一個很是有用的功能，但不少人一般並不能充分利用，這是由於一些庫不會優先擇用該功能。一般咱們也能夠正確使用Java枚舉功能，但在許多代碼庫中每每存在着這樣一個問題，因而有了這篇文章。問題很簡單：咱們應該如何經過名稱或值獲取枚舉，並忽略不存在的值？

枚舉html

這是咱們將在示例中使用的枚舉。挑選更復雜的枚舉是爲了也能夠經過其餘字段展現查找枚舉。java

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。linux

問題git

在你知道輸入有效的時候，使用Enum.valueOf很是棒。可是，若是傳入無效的名稱，那麼將拋出異常。在某些狀況下，這很好。不過，在一般狀況下，咱們寧願忽略異常並返回null。程序員

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)

拙劣的實現github

很不幸的是，如下兩種方法在代碼庫中出現得是這麼的頻繁。反面例子，勿學。面試

Enum.valueOf With Try Catch（劣）算法

這種拙劣的作法最多見於初學者。異常不該該用於控制流，而且這樣可能會有一些性能影響。不要偷懶。你必須用正確的方式去作。spring

/*
 * 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

經過迭代查找（劣）數據結構

這種方法也很常見（見這裏），但至少程序員知道不能用try/catch來捕獲異常。那麼，這種方法有什麼問題呢？沒錯，它會迭代全部枚舉，直到找到匹配的枚舉或返回null——最壞的狀況下須要n次，其中n就是枚舉值的數量。有些人可能會認爲這微不足道，這是過早優化了而已。可是，數據結構和算法是CS基礎。使用Map而不是迭代集合要省力得多。這會大大提升性能嗎？不，但它是一個很好的習慣。在面試候選人時，你會對線性複雜度搜索算法感到滿意嗎？此時，你不該該讓這樣的代碼審查經過。

/*
 * 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

較優的實現

如下均可以經過使用Map形式的索引發做用。可是，它們之間也有一些細微的差異。

靜態Map索引（較優）

用於快速查找固定大小的正確數據結構是什麼？那就是HashMap。如今經過一些額外的樣板，只要咱們有很好的散列函數的話，咱們就能夠進行更有效的查找。稍微更冗長，但若是有辦法減小樣板的話，這將是極好的。

private static final Map<String, CardSuit> 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

Guava Enums.getIfPresent（推薦）

這是一個常見的用例，咱們在Google的朋友爲此提供了一個很是乾淨且無需樣板的解決方案。探其究竟，它甚至使用了WeakReferences和WeakHashMaps。基本說來，這段代碼將建立一個全局的靜態地圖，並以Enum的類名稱鍵入，並將其用於查找。

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

經過字段的進一步索引

這個徹底相同的方法能夠用於枚舉的其餘字段。想要經過其顯示的名稱或其餘屬性來查找枚舉並很多見。

經過字段索引的靜態Map（較優）

與上述相同的方法，可是在顯示名稱上而不是枚舉名稱上進行索引。

private static final Map<String, CardSuit> 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

經過字段索引的靜態Map（較優）

咱們不能在這裏利用Guava，由於對於靜態索引建立惟一的全局密鑰將是一件困難的事。可是，這並不意味着咱們沒有幫手！

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

如今咱們有了一個相與樣板沒多大關聯的通用解決方案。

private static final Function<String, CardSuit> 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

結論

這裏有若干方法均可用於解決同一個問題。有些拙劣，有些更優。

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

翻譯做者：碼農網 – 小峯