Why does guava Multimap.values() return a flat collection instead of a collection of collections?

Question

I really like the Multimap class of the google guava library. It is a map type where you can add multiple values for a key, so it effectively maps from a key to a collection of some type. What I especially love is the Multimaps.index() function which takes an Iterable and a key function and returns a Multimap which groups (or indexes or maps) the elements of the Iterable by the value the function returns for each of those elements.

What I find a bit strange is that Multimap.values() returns a flat collection instead of a collection of collections? So the grouping the index function gave me is lost once Ì retrieve the values. I can circumvent that problem by calling Multimap.asMap() and then call values() on that.

Does anyone know why it may make sense that Multimap behaves that way?

Answer 1

Multimap.asMap().values() isn't a way around the problem -- it was deliberate that Multimap provides both ways of accessing it, getting a Collection<Collection<V>> via asMap().values() and getting the flattened Collection<V> with values().

More generally speaking, Multimap tries not to just be "a map to collections," but rather "a general way to associate keys with multiple values." So you get the entries() method in addition to values() and keys(). The asMap() view provides a way to treat it as a "map to collections," but that has very different semantics that aren't always what you're looking for.

In any event, the values method is just meant to fill a different niche than the one filled by asMap().values().

Answer 2

Does anyone know why it may make sense that Multimap behaves that way?

A Multimap should be viewed as ordinary map, where the keys does not need to be unique.

Key       Val
 a   ->    1
 b   ->    2
 a   ->    3

Values: {1, 2, 3}