Guava: construct a Multimap by inverting a Map

Question

why does Guava doesn't have the following factory call to create a MultiMap from a normal Map?

public static <K,V> MultiMap<K,V> invertMap(Map<V,K> map);

I have program-names mapped to an integer of how often they were called. I'd like to invert this, so that i can ultimately construct a TreeMap, sorted by call-count, which then are the keys leading to one or multiple program-names.

Duplicate of http://stackoverflow.com/questions/3678601/how-to-do-map-inversion-with-guava-with-non-unique-values — mtpettyp, Nov 16 '11 at 19:12

Mark Peters · Accepted Answer · 2011-11-16T21:32:05.227

11

How about:

public static <K,V> Multimap<K,V> invertMap(Map<V,K> map) {
    return Multimaps.invertFrom(Multimaps.forMap(map), ArrayListMultimap.create());
}

Doesn't seem like this requires a dedicated function. You can even get back to a TreeMap pretty easily:

Map<String, Integer> programCounts;
TreeMap<Integer, Collection<String>> map = 
    new TreeMap<>(
        Multimaps.invertFrom(
           Multimaps.forMap(programCounts),
           ArrayListMultimap.create()
        ).asMap()
    );

edited Nov 16 '11 at 21:32

answered Nov 16 '11 at 19:12

Mark Peters

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But doesn't this create two maps, so it's a bit slower? – Fabian Zeindl Nov 17 '11 at 16:08
@Fabian: It creates a Multimap and a TreeMap. It doesn't create two Multimaps; `forMap` returns a *view* of the map. – Mark Peters Nov 17 '11 at 16:20
1

To put it another way: Multimaps.forMap just creates a wrapper around programCounts, and an inexpensive one at that. It's a constant-time, constant-memory operation. – Louis Wasserman Nov 17 '11 at 18:37

Guava: construct a Multimap by inverting a Map

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