Merge two maps with Java 8

Question

I have two maps like this:

map1 = new Map<String, MyObject>();
map2 = new Map<String, MyObject>();

MyObject {
   Integer mark1;
   Integer mark2;
}

What I want to do to is to merge the two maps into a map3 <String, MyObject> like this:

1. If map1.place is not in map2.place, then I add the entry to map3.
2. same if map2.place is not in map1.place, I add the entry to map3.
3. if map1.place is in map2.place, then I add this entry:

• map1.place, (map1.mark1, map2.mark2)

I have read about flatMap, but I really have a hard time using it. Any clue how to do this?

Answer 1

It can be done using the Stream API with the appropriate mergeFunction as next:

Map<String, MyObject> map3 = Stream.of(map1, map2)
    .flatMap(map -> map.entrySet().stream())
    .collect(
        Collectors.toMap(
            Map.Entry::getKey,
            Map.Entry::getValue,
            (v1, v2) -> new MyObject(v1.getMark1(), v2.getMark2())
        )
    );

This concatenates entries of map1 followed by the entries of map2, then convert everything as a Map with a merge function that will use mark1 from the first value (the one from map1) and mark2 from the second value (the one from map2) in case of duplicate keys.

---

Or it could also be done using a different Supplier<Map> that will propose a map that already contains the entries of map1 then we can focus only on adding the entries of map2 as next:

Map<String, MyObject> map3 = map2.entrySet()
    .stream()
    .collect(
        Collectors.toMap(
            Map.Entry::getKey,
            Map.Entry::getValue,
            (v1, v2) -> new MyObject(v1.getMark1(), v2.getMark2()),
            () -> new HashMap<>(map1)
        )
    );

Answer 2

Here is what I think would work

Map<String, MyObj> map3 = new HashMap<>(map1);
map2.forEach(
    (key, value) -> map3.merge(key, value, (v1, v2) -> new MyObject(v1.mark1,v2.mark2))
);

The merge function is what is taking care of your scenario 3, in that if the key already exists, it creates a new MyObject with v1.mark1 and v2.mark2