[SOLVED] Finding common set of elements from a Map with Set values using Java streams

Finding common set of elements from a Map with Set values using Java streams

Let's say I've a HashMap which contains key as a String and value as a Set of Integers (Map<String, Set<Integer>>). And say that the map is populated with following values:

Map<String, Set<Integer>> map = new HashMap<>();
map.put("w1", Set.of(1,3,4,6,7));
map.put("w2", Set.of(2,3,4,5,7));
map.put("w3", Set.of(1,2,3,5,7));

How can I find common set of values across all keys using Streams in Java? e.g.: In this case, the common set of values across all keys is Set.of(3,7).

Solution

First note that using stream is not always the cleanest way.

My idea would be to get the first set and iterate over the rest to check if all of them contain it:

Set<Integer> res = map.values().iterator().next().stream()
            .filter(item -> map.values().stream().allMatch(set -> set.contains(item)))
            .collect(Collectors.toSet());

This is a concise solution but it checks the first set twice. You can also add a check to see if the map contains any entries.