Java – how to reverse a map

Let's look at a map:

> A – > {1,2,3} > B – > {3,4,5} > C – > {2,3,5}

I need to reverse this map and get:

>1 – > {one} > 2 – > {a, C} > 3 – > {a, B, C} > 4 – > {B} > 5 – > {B, C}

I did it with this Code:

public static <U,V> Map<V,Set<U>> reverseMap(Map<U,Set<V>> map) {
  Map<V,Set<U>> result = Maps.newHashMap();
  for(Map.Entry<U,Set<V>> entry : map.entrySet()) {
    for(V value : entry.getValue()) {

      Set<U> set = result.get(value);
      if(set == null) {
        set = Sets.newHashSet();
        result.put(value,set);
      }
      set.add(entry.getKey());
      result.put(value,set);
    }

  }
  return result;
}

But this is just a reverse index, so I think there may be a predefined method to do this

Does anyone know such a library? Guava method?

Solution

If you replace HashMap < u, set < V > > with hashmultimap < u, V > (they are equivalent and Multimap is easier to use), you can now use multimaps Invertfrom() to populate the Multimap < V, u >

Note that, as mentioned in Javadoc, if you use immutable Multimap, you can call immutable Multimap directly inverse().