Java Enums are an incredibly useful feature and often under utilized because some libraries don't treat them as first class citizens. They are also often used properly but there is a recurring issue that plagues many code bases which has inspired this post. The problem is simple, how should you get an Enum by its name or value and ignore non existant values?

The Enum

The enum we will be using in our examples. Let's pick a more complex enum to also showcase looking an enum up by another field.

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 final String displayName;
    private final String symbol;
    private final 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 String getSymbol() {
        return symbol;
    }

    public CardColor getColor() {
        return color;
    }

The Problem

Using Enum.valueOf is great when you know the input is valid. However if you pass in an invalid name an exception will be thrown. In some cases this is fine. Often times we would prefer to just ignore it and return 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)

Poor Implementations

It's unfortunate how often the following two approaches appear in code bases. Please don't do this.

Enum.valueOf with Try Catch (Poor)

This bad practice is most commonly made by beginners. Exceptions shouldn't be used for control flow and could have some performance implications. Don't be lazy do it the right way.

/*
 * 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

Find By Iteration (Poor)

This approach is also quite common (see here), at least the authors know not to try / catch the exceptions. What is wrong with this approach? It's iterating over all enums until it finds the matching enum or returning null with a worst case of N where N is the number of enum values. Some may argue this is being nitpicky and its premature optimization. However, data structures and algorithms are CS fundamentals it's not that much effort to use a Map instead of iterating a collection. Will it drastically improve performance? No, but it is a good habbit. When interviewing a candidate for a job would you be happy with a linear complexity search algorithm? You shouldn't let this code review pass in that case.

/*
 * 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

Better Implementations

The following all work by using an index in the form of a Map. There are some minor differences as well as boilerplate concerns.

Static Map Index (Better)

What is the correct data structure to use for quick lookups of fixed size? A HashMap. Now with a little extra boiler plate we have a much more efficient lookup as long as we have a good hash function. A bit more verbose, it would be nice if there was a way to reduce the boilerplate.

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

Guava Enums.getIfPresent (Recommended)

This is such a common use case our friends over at Google made a very clean and boiler plate free solution for us. Under the hood it even uses WeakReferences and WeakHashMaps. Basically this code will create a global static map keyed on the Enum's class name and use it for lookups.

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

One Step Further Indexing by Field

This exact same approach can be used for additional fields of the enum. It's not uncommon to want to look up an enum by its display name or some other property.

Static Map Indexed by Field (Better)

Same approach as above but indexed on the display name instead of the enum name.

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

Static Map Indexed by Field Utility (Better)

We can't leverage Guava here since it would be difficult to create unique global keys for the static index. However that doesn't mean we can't make our own helpers!

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);
    }
}

Now we have a fairly boilerplate free generic solution.

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

Conclusion

There are several ways to solve the same problem. Some are better than others.

BONUS - Serializing an Enum as an Object with Jackson

If you happened to notice our json output is a full object not just the enum name the magic comes from the Jackson annotation @JsonFormat(shape = JsonFormat.Shape.OBJECT)