playing with collectors, requires guava 21.0

Mapping.java

import com.google.common.collect.*;

import java.util.HashSet;
import java.util.Map;
import java.util.Set;

import static com.google.common.collect.ImmutableSet.toImmutableSet;
import static java.util.stream.Collectors.*;

public class Mapping {

    static BiMap<Character,ImmutableSet<String>> indexUsingImmutableSet(Map<String,Character> input) {
        BiMap<Character,ImmutableSet<String>> result = input.entrySet().stream()
                .collect(groupingBy(Map.Entry::getValue, HashBiMap::create,
                        mapping(Map.Entry::getKey,
                                toImmutableSet())));
        return result;
    }
    
    static BiMap<Character,Set<String>> indexUsingMutableSet(Map<String,Character> input) {
        BiMap<Character,Set<String>> result = input.entrySet().stream()
                .collect(groupingBy(Map.Entry::getValue, HashBiMap::create,
                        mapping(Map.Entry::getKey,
                                toSet())));
        return result;
    }
    
    static BiMap<Character,Set<String>> indexUsingIntermediateCollector(Map<String,Character> input) {
        BiMap<Character,Set<String>> result = input.entrySet().stream()
                .collect(groupingBy(Map.Entry::getValue, HashBiMap::create, 
                        collectingAndThen(
                                mapping(Map.Entry<String,Character>::getKey,
                                        toSet()),
                                HashSet<String>::new)));
        return result;
    }
    
    static BiMap<Character,Set<String>> indexUsingMapThenBimap(Map<String,Character> input) {
        BiMap<Character,Set<String>> result = input.entrySet().stream()
                .collect(collectingAndThen(
                        groupingBy(Map.Entry::getValue,
                                mapping(Map.Entry::getKey,
                                        toSet())),
                        HashBiMap::create));
        return result;
    }
    
    public static void main(String[] args) {
        Map<String,Character> before = ImmutableMap.of("cleaning",'A',"cooking",'A',"washing",'C',"ironing",'C');
        Map<String,Character> after  = ImmutableMap.of("cleaning",'B',"cooking",'B',"washing",'D',"ironing",'E');
        BiMap<Character,? extends Set<String>> indexUsingImmutableSetBefore = indexUsingImmutableSet(before);
        BiMap<Character,? extends Set<String>> indexUsingImmutableSetAfter  = indexUsingImmutableSet(after);
        System.out.println(Sets.intersection(indexUsingImmutableSetBefore.values(), indexUsingImmutableSetAfter.values()));
        BiMap<Character,? extends Set<String>> indexUsingMutableSetBefore = indexUsingMutableSet(before);
        BiMap<Character,? extends Set<String>> indexUsingMutableSetAfter  = indexUsingMutableSet(after);
        System.out.println(Sets.intersection(indexUsingMutableSetBefore.values(), indexUsingMutableSetAfter.values()));
        BiMap<Character,? extends Set<String>> indexUsingIntermediateCollectorBefore = indexUsingIntermediateCollector(before);
        BiMap<Character,? extends Set<String>> indexUsingIntermediateCollectorAfter  = indexUsingIntermediateCollector(after);
        System.out.println(Sets.intersection(indexUsingIntermediateCollectorBefore.values(), indexUsingIntermediateCollectorAfter.values()));
        BiMap<Character,? extends Set<String>> indexUsingMapThenBimapBefore = indexUsingMapThenBimap(before);
        BiMap<Character,? extends Set<String>> indexUsingMapThenBimapAfter  = indexUsingMapThenBimap(after);
        System.out.println(Sets.intersection(indexUsingMapThenBimapBefore.values(), indexUsingMapThenBimapAfter.values()));
        /* prints:
        [[cleaning, cooking]]
        []
        [[cleaning, cooking]]
        [[cleaning, cooking]]
         */
    }

}