通过值索引获取Guava Multimap的键

Question

Suppose this is my example data which are in Multimap collection:

x -> [1,2]
y -> [1,3]
z -> [4]

then I made a list of values which might be like this:(First column is index of values , second column is value)

0 -> 1
1 -> 2
2 -> 1
3 -> 3
4 -> 4

My question is how to get paired key of a value by knowing index of that value. For example the key for index "2" , the key "y" must return.

Answer 1

If you want to do this one time, you can do simple loop over Multimap#entries() and maintain counter yourself, but:

1. order of iteration over various Mutlimap implementations could be different
2. if main use case is access keys/values by index, you shouldn't use Multimap here at all, but maintain a list (probably List<Entry<String, Integer>> ).

If you really have to have multimap and it's immutable, you could use .entries().asList() view to achieve what I described above:

//given
ImmutableListMultimap<String, Integer> multimap =
        ImmutableListMultimap.<String, Integer>builder()
                .putAll("x", 1, 2)
                .putAll("y", 1, 3)
                .putAll("z", 4)
                .build();
ImmutableList<Map.Entry<String, Integer>> entriesWithPosition =
        multimap.entries().asList();
//when
Map.Entry<String, Integer> foundEntry = entriesWithPosition.get(2);
//then
assertThat(foundEntry.getKey()).isEqualTo("y");