Merge Map<String, List<String> Java 8 Stream

user_3568904 0
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I would like to merge two Map with JAVA 8 Stream:

Map<String, List<String>> mapGlobal = new HashMap<String, List<String>>();
Map<String, List<String>> mapAdded = new HashMap<String, List<String>>();

I try to use this implementation:

mapGlobal = Stream.of(mapGlobal, mapAdded)
                .flatMap(m -> m.entrySet().stream())
                .collect(Collectors.groupingBy(Map.Entry::getKey,
                        Collectors.mapping(Map.Entry::getValue,        
                                           Collectors.toList())
                ));

However, this implementation only create a result like:

Map<String, List<Object>>

If one key is not contained in the mapGlobal, it would be added as a new key with the corresponding List of String. If the key is duplicated in mapGlobal and mapAdded, both list of values will be merge as: A = {1, 3, 5, 7} and B = {1, 2, 4, 6} then A ∪ B = {1, 2, 3, 4, 5, 6, 7}.

java-stream java-8 collections lambda java
6 Answers

The original implementation doesn't create result like Map<String, List<Object>>, but Map<String, List<List<String>>>. You need additional Stream pipeline on it to produce Map<String, List<String>>.

user_86973760 
Map<String, List<String>> result = new HashMap<String, List<String>>();

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

for(Map.Entry<String, List<String>> entry: map1.entrySet()) {
   result.put(entry.getKey(), new ArrayList<>(entry.getValue());
}

for(Map.Entry<String, List<String>> entry: map2.entrySet()) {
   if(result.contains(entry.getKey())){
       result.get(entry.getKey()).addAll(entry.getValue());
   } else {
       result.put(entry.getKey(), new ArrayList<>(entry.getValue());
   }

}

This solution creates independent result map without any reference to map1 and map2 lists.

user_42474560

Using StreamEx

Map<String, List<String>> mergedMap =
        EntryStream.of(mapGlobal)
                .append(EntryStream.of(mapAdded))
                .toMap((v1, v2) -> {
                    List<String> combined = new ArrayList<>();
                    combined.addAll(v1);
                    combined.addAll(v2);
                    return combined;
                });

If you have even more maps to merge just append to the stream

                .append(EntryStream.of(mapAdded2))
                .append(EntryStream.of(mapAdded3))
user_17816110

Here is the full code to Iterate Two HashMap which has values stored in the form of a list. Merging all the keys and values in first hashmap. Below is the example.

HashMap<String, List<String>> hmap1 = new HashMap<>();
      List<String> list1 = new LinkedList<>();
      list1.add("000");
      list1.add("111");
      List<String> list2 = new LinkedList<>();
      list2.add("222");
      list2.add("333");
      hmap1.put("Competitor", list1);
      hmap1.put("Contractor", list2);
      //  System.out.println(hmap1);


      HashMap<String, List<String>> hmap2 = new HashMap<>();
      List<String> list3 = new LinkedList<>();
      list3.add("aaa");
      list3.add("bbb");
      List<String> list4 = new LinkedList<>();
      list4.add("ccc");
      list4.add("ddd");
      hmap2.put("Competitor", list3);
      hmap2.put("Contractor", list4);


//******* Java 8 Feature *****
hmap1.forEach((k, v) -> hmap2.merge(k, v, (v1, v2) -> {
          List<String> li = new LinkedList<>(v1);
          li.addAll(v2);
          hmap2.put(k,li);
          return new ArrayList<>(li);
      }));
      System.out.println(hmap2);

Output:

{Competitor=[aaa, bbb, 000, 111], Contractor=[ccc, ddd, 222, 333]}
user_116037960
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