Merging of two independent maps into third map

Question

below I have two independent map coming which i have stored in individual maps , I want to add them into the third final map, so the final map will contain the sum of maps named m1 and m2 , please advise how to achieve the same

    map1=(Map<String, List<NTM>>) m1.exceute(jobCode, clientlogo);
     map2=(Map<String, List<NTM>>) m1.exceute(jobCode, clientlogo);
private Map<String, List<NTM>> finalMap;

        public Map<String, List<NTM>>  invoke (String jobCode, String clientlogo)
    {
        maestroCardBusinessCNFRuleImpl m1 = new maestroCardBusinessCNFRuleImpl(); 
        maestroCardBusinessANFRuleImpl m2 = new maestroCardBusinessANFRuleImpl();
        try {
            map1=(Map<String, List<NTM>>) m1.exceute(jobCode, clientlogo);
            map2=(Map<String, List<NTM>>) m1.exceute(jobCode, clientlogo);

            // *** logic to add the sum of both the maps ****
        } catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

If you google there are a lot of answers. e.g. https://stackoverflow.com/a/42240509/3959856 — Jack Flamp, Jun 05 '18 at 10:40
I suggest you copy `map1` to the final map, then iterate over the entries of `map2`, adding or merging the values as needed. — Peter Lawrey, Jun 05 '18 at 10:50
@PeterLawrey Thanks but i am still not able to grasp , request you to please post the code so that i can grasp more — sdsd dsds, Jun 05 '18 at 10:55
@PeterLawrey what I have tried is map1.putAll(finalMap); map2.putAll(finalMap); as i want to retain duplicates keys also — sdsd dsds, Jun 05 '18 at 10:58
@sdsddsds what you get after merging like this? Are you getting the UNION or the JOIN of the two sets ? — NitinSingh, Jun 05 '18 at 11:22
Possible duplicate of [Merge maps in java](https://stackoverflow.com/questions/11010392/merge-maps-in-java) — Jonathan Laliberte, Jun 05 '18 at 11:55

davidxxx · Answer 1 · 2018-06-05T11:26:52.607

In Java 7, you could add all entries of the first map in the merged map.
Then iterate on the second map to add entries (as the key is not present) and update entries (as the key is already present) in the merged map.

Map<String,  List<NTM>> mergedMap = new HashMap<>();
mergedMap.putAll(m1);
for (Entry<String, List<NTM>> entry : m2.entrySet()) {
    String key = entry.getKey();
    List<NTM> list = entry.getValue();

    List<NTM> mergedList = mergedMap.get(key);

    // key not existing in the mergedMap
    if (mergedList == null) {
        mergedMap.put(key, list);
    }

    // key existing in the mergedMap
    else {
        mergedList.addAll(list);
    }
}

With Java 8 you could use Collectors.toMap() to provide a merge function that merges the List of NTM as you have duplicate keys in your maps :

Map<String, List<NTM>> mergedMap = Stream.of(m1.entrySet(), m2.entrySet())
                                           .flatMap(Collection::stream)
                                           .collect(Collectors.toMap(Entry::getKey, Entry::getValue, (v1, v2) -> {
                                               v1.addAll(v2);
                                               return v1;
                                           }));

Note that the merge function will change the original list of the map referenced by v1.
If it is not desirable, you could create a new ArrayList and return it such as :

.collect(Collectors.toMap(Entry::getKey, Entry::getValue, (v1, v2) -> {
                                                   List<NTM> mergedList = new 
                                                    ArrayList<>(v1);
                                                   mergedList.addAll(v2);
                                                   return mergedList;
                                               }));

please advise will key will be overridden if the second map contains the same key present in first map — sdsd dsds, Jun 05 '18 at 11:20

score 0 · Answer 2 · answered Jun 05 '18 at 10:57

0

Map<String, List<NTM>> map3 = new HashMap<>();
map3.putAll(map1);
map3.putAll(map2);

answered Jun 05 '18 at 10:57

Rafał Sokalski

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please advise what will happen if both the maps have same key , in that case second map will override the first one as the second map key is also same which was there in the first one – sdsd dsds Jun 05 '18 at 11:01
Duplicate key will be replaced in this situation – Rafał Sokalski Jun 05 '18 at 11:04
It means that e.g map1 contains key: "key1" with value "value1" and map2 key: "key1" with value "value2" so if you merge it there will be key: "key1" with value "value2" in map3 – Rafał Sokalski Jun 05 '18 at 11:09
I want to keep the duplicated key , please advise how to achieve that – sdsd dsds Jun 05 '18 at 11:10
you cannot have duplicate key in one map – Rafał Sokalski Jun 05 '18 at 11:11
please advise i want to have complete backup of data of both maps – sdsd dsds Jun 05 '18 at 11:11
https://stackoverflow.com/questions/1062960/map-implementation-with-duplicate-keys – Rafał Sokalski Jun 05 '18 at 11:11

Merging of two independent maps into third map

2 Answers2