How to do map inversion with Guava with non-unique values?

user_234877 0
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How can we do that with Guava? Notice the presence of List<K> in the return type since many keys can map to the same value in any normal map.

public static <K, V> Map<V, List<K>> inverse(Map<K, V> map){
    Map<V, List<K>> result = new LinkedHashMap<V, List<K>>();
    for (Map.Entry<K, V> entry : map.entrySet()) {
        if(!result.containsKey(entry.getValue())){
            result.put(entry.getValue(), new ArrayList<K>());                
        }
        result.get(entry.getValue()).add(entry.getKey());
    }        
    return result;        
}

BiMap seems to insist on the unicity of the values, but I don't have this luxury.

guava collections java
3 Answers

In case someone stumbles here now (well within Java's Stream era), here are two single-expression Stream-based solutions:

1) Immutable version based on ImmutableListMultimap + toImmutableListMultimap collector

ImmutableListMultimap<V, K> output = inputMap.entrySet().stream()
        .collect(ImmutableListMultimap.toImmutableListMultimap(Map.Entry::getValue, Map.Entry::getKey));

2) Mutable version based on ArrayListMultimap + Multimaps.toMultimap collector

ListMultimap<V, K> output = inputMap.entrySet().stream()
        .collect(Multimaps.toMultimap(Map.Entry::getValue, Map.Entry::getKey, ArrayListMultimap::create));
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