EnumMap or HashMap if lookup key is a String

Question

I'm trying to weigh the pros and cons of using an EnumMap over a HashMap. Since, I will always be looking up using a String, it seems that a HashMap with a String key would be the right choice. However, an EnumMap seems like better design because it conveys my intent to restrict keys to a specific enumeration. Thoughts?

Here is a make-believe example, showing how I will be using the Map:

enum AnimalType { CAT, DOG }
interface Animal {}
class Cat implements Animal {}
class Dog implements Animal {}

public class AnimalFactory {

    private static final Map<AnimalType, Animal> enumMap 
            = new EnumMap<AnimalType, Animal>(AnimalType.class);
    // versus
    private static final Map<String, Animal> stringMap 
            = new HashMap<String, Animal>();

    static {
        enumMap.put(AnimalType.CAT, new Cat());
        enumMap.put(AnimalType.DOG, new Dog());
        stringMap.put("CAT", new Cat());
        stringMap.put("DOG", new Dog());
    }
    public static Animal create(String type) {
        Animal result = enumMap.get(AnimalType.valueOf(type));
        Animal result2 = stringMap.get(type);
        return result;
    }
}

Assume that the AnimalType enum and map will ONLY be used by the AnimalFactory to create animals and nowhere else.

Which Map should I use?

Solution 1

If all valid keys can be enumerated, I would use that as it ensures you are always working with a valid value.

It can also avoid confusion as String can be used for lots of things and is easy to turn an "Animal" string into a string used for something else. As enum types are not interchangable with other types in general (unless you use a common interface), there is less chance of error in coding.

Solution 2

If the set of possible keys is finite and known in advance (as your example/question suggest), then the enum is a perfect representation of this. As other said, the use of an enum ensures that no mistake will be made in the use of the key.

Furthermore, this implementation of Map is quite optimized, as the range of the keys is known in advance (as far as I knwow, the EnumMap uses an array of length numberOfEnums internally, indexed by the enum's ordinal).

So I would also recommend EnumMap.

Two (little) things to keep in mind though :

• you won't be able to add specialized cases by inheritance (you cannot extend an enum, so no Maps of ANIMALS with specialized Maps of MAMMALS on the side for example)
• when adding a member to your enum, if you add it "in the middle" of the other members you change the ordinals. As this info can be used by the EnumMap, this can prove problematic if you reload an EnumMap constructed from the old version of the enum (for example using Serialization)