java - Merging two Maps

Question

I have two maps whose keys are Strings and whose values are Set<MyObject>. Given two Maps, what is the easiest way to merge them such that if two keys are identical, the value is a union of the two sets. You can assume values are never null and if it is useful, we can make these Maps SortedMaps.

Answer 1

You can do this with a stream fairly easily:

Map<T, Set<U>> merged = Stream.of(first, second)
        .map(Map::entrySet)
        .flatMap(Set::stream)
        .collect(Collectors.toMap(Entry::getKey, Entry::getValue, (a, b) -> {
            HashSet<U> both = new HashSet<>(a);
            both.addAll(b);
            return both;
        }));

This splits the maps into their Entrys and then joins them with a Collector which resolves duplicates by adding both values to a new HashSet.

This also works for any number of maps.

Some variations which produce the same result:

Stream.of(first, second).flatMap(m -> m.entrySet().stream())
    .collect(...);
Stream.concat(first.entrySet().stream(), second.entrySet().stream())
    .collect(...); //from comment by Aleksandr Dubinsky

The third parameter for Collectors.toMap is not necessary if there are no duplicate keys.

There is another Collectors.toMap with a fourth parameter that lets you decide the type of the Map collected into.