How to use java 8 merge function for n number of hashmaps

Question

I need to merge n hashmaps ideally in a loop like shown below using java 8 merge or something:

Map 1 : {Name:XXX,Phn:123,Work:""}

Map 2 : {Name:XXX,Phn:456,Work: xyz}

Map 3 : {Name:XXX,Phn:789,Work:""}

I would like to get an output like below :

{
   Name:XXX,        // if all values for a key are same take one
   Phn:123/456/789  // merge all non null values for same key
   Work:xyz         // make sure non-null values are never replaced
}

When I try to use putall like this

public Map<String,String> mergeOriginalDataMaps(List<Integer> indexList, List<Map<String,String>> originalData) {   
    Map<String,String> tmpMap = new HashMap<String,String> ();
    for (int index : indexList ) {
        tmpMap.putAll(originalData.get(index));
    }
    return tmpMap;
}

If the value of duplicate key is "", the previous value is replaced with the new one. I need to concatenate the values rather than replace them.

Answer 1

If you have a List<Map<String, String>>, which represents a list of maps to merge, you can have

Map<String, String> result =
    maps.stream()
        .flatMap(m -> m.entrySet().stream())
        .collect(Collectors.groupingBy(
            Map.Entry::getKey,
            Collectors.mapping(
                Map.Entry::getValue,
                Collectors.collectingAndThen(Collectors.<String>toSet(), s -> String.join("", s))
            )
        ));

This flat maps each map into a Stream of its entries. Then the Stream is grouped by the value of each entry and all distinct elements with the same key are mapped to their value, joined.

The distinct part is made possible by first collecting all the values inside a Set.

---

Sample code:

public static void main(String[] args) {
    List<Map<String, String>> maps = new ArrayList<>();
    maps.add(new HashMap<String, String>(){{ put("Name", "XXX"); put("Phn", "123"); put("Work", ""); }});
    maps.add(new HashMap<String, String>(){{ put("Name", "XXX"); put("Phn", "456"); put("Work", "xyz"); }});
    maps.add(new HashMap<String, String>(){{ put("Name", "XXX"); put("Phn", "789"); put("Work", ""); }});

    Map<String, String> result =
        maps.stream()
            .flatMap(m -> m.entrySet().stream())
            .collect(Collectors.groupingBy(
                Map.Entry::getKey,
                Collectors.mapping(
                    Map.Entry::getValue,
                    Collectors.collectingAndThen(Collectors.<String>toSet(), s -> String.join("", s))
                )
            ));

    System.out.println(result); // prints "{Phn=123456789, Work=xyz, Name=XXX}"
}