Is there a more efficient and specialized implementation of a Map collection where Enum objects can serve as keys?
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As well as EnumMap, there is EnumSet which can also be useful.Peter Lawrey– Peter Lawrey2010-06-12 21:28:09 +00:00Commented Jun 12, 2010 at 21:28
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Exactly rightAndrey– Andrey2010-06-13 00:47:20 +00:00Commented Jun 13, 2010 at 0:47
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3 Answers
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);
5 Comments
Andrey
Yes... please check existing answers before replying. This was a self-answer post.
aioobe
So you, 1. post an easy question of which you know the answer. 2. Write up a half-baked answer yourself immediately. 3. Down-vote other answers because you feel that it was somehow "your question to answer"? I don't get it.. why?? for reputation?
Bozho
while I support your confusion, and certainly oppose the OP's downvote, I don't think there was a need for another answer about
EnumMap. (That's why I deleted mine :) )aioobe
I wrote my answer because I didn't get why the previous answer included the LibraryItem interface and a NavigableSet of LibraryItems to illustrate the use of an EnumMap. If I knew the OP wasn't interested in an answer to the question I obviously wouldn't have bothered. :)
Andrey
1) There's nothing wrong with self-answer posts, read the FAQ. I made this post because I knew EnumMap is not widely known and thought it would be beneficial to a lot of devs if they were aware of its existence. 2) My answer was not "half-baked" since it included an example as well as a link to the API. The only mistake I had was to use LibraryItem instead of LibraryItemType in one place which was in fact fixed by Donal, thank you Donal for catching that. So as far as I can see your answer was redundant... but that's why the community exists to settle disputes like these for us.
BTW, it's simpler to write:
Map<MyEnum, String> map = new EnumMap<>(MyEnum.class);
There's no need to repeat the types. This can make the code a lot easier to read.
Comments
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.class);