How to efficiently use Enum objects as keys in the Map data structure?

Question

Is there a more efficient and specialized implementation of a Map collection where Enum objects can serve as keys?

Answer 1

I learned this recently after I accidentally stumbled upon the answer in the Java API. If you ever have a map that uses enums as keys make sure you use EnumMap. It's very simple and much more efficient:

public interface LibraryItem{ ... }

public enum LibraryItemType{ BOOK, CD, VHS, AUDIO; ... }

Map<LibraryItemType, NavigableSet<LibraryItem>> itemsByType =
    new EnumMap<LibraryItemType, NavigableSet<LibraryItem>>(LibraryItemType.class);

Answer 2

Yes. EnumMap is precisely that; an efficient implementation of the Map interface, in which the key type must be an enum:

From the API documentation:

Class EnumMap<K extends Enum<K>,V>

A specialized Map implementation for use with enum type keys. All of the keys in an enum map must come from a single enum type that is specified, explicitly or implicitly, when the map is created. Enum maps are represented internally as arrays. This representation is extremely compact and efficient.

Example usage:

Map<MyEnum, String> map = new EnumMap<MyEnum, String>(MyEnum.class);