Merge multiple map with java stream api

Question

I am trying to merge multiple map with Java stream API. The reqiurements like ...

• this method can receive multiple map, does not care hot many of it
• support 2 merge method: intersection, union
• if key duplicates, it overwrites with later value

Here is basic code I wrote,

public class MapMergeHelper {
 public static Map<String, String> merge (MapMergeOperation mergeOperation, Map<String, String> ... maps) {
 Map<String, String> merged;
 switch (mergeOperation) {
 case INTERSECTION -> {
 Set<Map.Entry<String, String>> entrySet = maps[0].entrySet();
 for (int i = 1; i < maps.length; i++) {
 final int current = i;
 entrySet = entrySet.stream().filter(entry -> maps[current].containsKey(entry.getKey()))
 .collect(Collectors.toSet());
 }
 merged = entrySet.stream()
 .collect(Collectors.toMap(entry -> entry.getKey(), entry -> entry.getValue()));
 }
 default -> {
 // UNION -> If key duplicates, then override
 merged = Stream.of(maps)
 .flatMap(entry -> entry.entrySet().stream())
 .collect(Collectors.toMap(entry -> entry.getKey(), entry -> entry.getValue(), (o, o2) -> o2));
 }
 }
 return merged;
 }
}

MapMergeOperation is simple Enum like this

public enum MapMergeOperation {
 UNION,
 INTERSECTION
}

And here is the testcase

public class MapMergeHelperTest {
 @Test
 public void testMerge() {
 Map<String, String> testEntryMapA = Map.of("key1", "a", "key2", "b", "key3", "c", "key4", "d");
 Map<String, String> testEntryMapB = Map.of("key1", "b");
 Map<String, String> testEntryMapC = Map.of("key1", "a", "key5", "e");
 {
 // test intersection
 Map<String, String> actual = MapMergeHelper.merge(MapMergeOperation.INTERSECTION, testEntryMapA, testEntryMapB, testEntryMapC);
 assertThat(actual.keySet()).containsExactly("key1");
 assertThat(actual.get("key1")).isEqualTo("a"); // override
 }
 {
 // test union
 Map<String, String> actual = MapMergeHelper.merge(MapMergeOperation.UNION, testEntryMapA, testEntryMapB, testEntryMapC);
 assertThat(actual.keySet()).containsExactlyInAnyOrder("key1", "key2", "key3", "key4", "key5");
 }
 }
}

The problem is, I think it is not efficient enough because calling collect(Collections.toSet()) every time inside of for loop.

I am looking for a way to achieve goal in better readable, efficient way!

• TorbenPutkonen: Regarding the intersection, there is no need for streams as you can just do map[0].keySet().retainAll(map[i].keySet()); Loop in reverse order to retain the value in last map.

Answer 1

Some basic tips:

• Split this into 2 methods, drop the enum. It doesn't do you any good unless something is really forcing you to have it there. It will be a lot more readable and easier to have 2 separate functions intersect and union. Then you can worry about them separately, make separate tests for each of those as individual parts.
• Don't mix streams and for loops. Choose one and stick with it. This makes the code confusing and less readable.
• No point of this line final int current = i;, just use i directly or rename i to current.
• Collecting to set is fine, but your logic could be improved. Take at look at Set.retainAll
• Variable merged is unnecessary, you always assign it only to return in the next line. Just return the result already.