Merge Map Java 8 Stream

Question

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

Map> mapGlobal = new HashMap>();
Map

        

Answer 1

Using foreach over Map can be used to merge given arraylist.

    public Map> merge(Map> map1, Map> map2) {
    Map> map = new HashMap<>();
    map.putAll(map1);

    map2.forEach((key , value) -> {
        //Get the value for key in map.
        ArrayList list = map.get(key);
        if (list == null) {
            map.put(key,value);
        }
        else {
            //Merge two list together
            ArrayList mergedValue = new ArrayList<>(value);
            mergedValue.addAll(list);
            map.put(key , mergedValue);
        }
    });
    return map;
}