How can I convert MultiMap<Integer, Foo> to Map<Integer, Set<Foo>> using Guava?

问题

I'm using MultiMap from Google Guava 12 like this:

Multimap<Integer, OccupancyType> pkgPOP = HashMultimap.create();

after inserting values into this multimap, I need to return:

Map<Integer, Set<OccupancyType>>

However, when I do:

return pkgPOP.asMap();

It returns me

Map<Integer, Collection<OccupancyType>>

How can I return Map<Integer, Set<OccupancyType>> instead ?

回答1:

Look at this issue and comment #2 by Kevin Bourrillion, head Guava dev:

You can double-cast the Map<K, Collection<V>> first to a raw Map and then to the Map<K, Set<V>> that you want. You'll have to suppress an unchecked warning and you should comment at that point, "Safe because SetMultimap guarantees this." I may even update the SetMultimap javadoc to mention this trick.

So do unchecked cast:

@SuppressWarnings("unchecked") // Safe because SetMultimap guarantees this.
final Map<Integer, Set<OccupancyType>> mapOfSets = 
    (Map<Integer, Set<OccupancyType>>) (Map<?, ?>) pkgPOP.asMap();

EDIT:

Since Guava 15.0 you can use helper method to do this in more elegant way:

Map<Integer, Set<OccupancyType>> mapOfSets = Multimaps.asMap(pkgPOP);

回答2:

Guava contributor here:

Do the unsafe cast. It'll be safe.

It can't return a Map<K, Set<V>> because of the way Java inheritance works. Essentially, the Multimap supertype has to return a Map<K, Collection<V>>, and because Map<K, Set<V>> isn't a subtype of Map<K, Collection<V>>, you can't override asMap() to return a Map<K, Set<V>>.

来源：https://stackoverflow.com/questions/11204143/how-can-i-convert-multimapinteger-foo-to-mapinteger-setfoo-using-guava