Merge two maps and keep null values

Question

I hope all is good for you.

I have two maps and I want to keep null values. I know that merge function can't manage null values but I need to keep null data and use the Binary Operation.

How I can manage my code to avoid NPE.

  private Map<String, Object> mergeMaps(
      Map<String, Object> map1, Map<String, Object> map2) {
    return Stream.of(map1, map2)
        .map(Map::entrySet)
        .flatMap(Collection::stream)
        .collect(
            Collectors.toMap(
                Map.Entry::getKey,
                Map.Entry::getValue,
                (v1, v2) -> v1 != null
                            && StringUtils.isNotEmpty(v1.toString()) 
                         ? v1 
                         : v2
                ));
  }

Sample test code:

  public static void main(String[] args) {
    Map<String, Object> map1 = new HashMap<>();
    map1.put("k1", null);
    map1.put("k2", "2");
    map1.put("k3", null);
    Map<String, Object> map2 = new HashMap<>();
    map2.put("k1", "1");
    map2.put("k2", null);
    map2.put("k3", null);

    var result = mergeMaps(map1, map2);

    System.out.println(result);
  }

should output something like {k1=1, k2=2, k3=null}; k1 having been replaced by map2, k2 staying from map1, and k3 being null in both.

Thanks a lot.

Answer 1

I'd go for

  Map<String, Object> mergeMaps(Map<String, Object> map1,
                                Map<String, Object> map2) {
    Map<String, Object> result = new HashMap<>(map1);
    map2.forEach((k, v) -> {
      var value = result.get(k);
      if (value == null || value.toString().isEmpty()) {
        result.put(k, v);
      }
    });
    return result;
  }

Which runs fine for the test case.