What is the most efficient way to compare keys of two HashMaps?

1. Get the keySet of both maps (O(n)).
2. Compute the intersection among them (O(n)).
3. For the common keys found in above set, get the values from both maps and sum them (O(n)).

Total time complexity - O(n).

Set<Integer> commonKeys = new HashSet<>(map1.keySet());
commonKeys.retainAll(map2.keySet());

Map<Integer, Integer> result = commonKeys.stream()
            .collect(Collectors.toMap(Function.identity(), 
                    k -> map1.get(k) + map2.get(k)));