Merge two maps with Java 8

Question

I have two maps like this:

map1 = new Map();
map2 = new Map();

MyObject {
   Integer mark1;
   Integer mark2         


        

Answer 1

Case 1: Given a List of Maps. Then Join the maps according to the Key

    public class Test14 {

        public static void main(String[] args) {

            Map> m1 = new HashMap<>();
            Map> m2 = new HashMap<>();
            m1.put("a", List.of(1));
            m1.put("b", List.of(2, 3));
            m2.put("a", List.of(12, 115));
            m2.put("b", List.of(2, 5));
            m2.put("c", List.of(6));

            System.out.println("map1 => " + m1);
            System.out.println("map2 => " + m2);

            ArrayList>> maplist = new ArrayList>>();
            maplist.add(m1);
            //   map1 => {a=[1], b=[2, 3]}
            maplist.add(m2);
            //      map2 => {a=[12, 115], b=[2, 5], c=[6]}

            System.out.println("maplist => " + maplist);
         //    maplist => [{a=[1], b=[2, 3]}, {a=[12, 115], b=[2, 5], c=[6]}]


            //  flatmap does omitted {} 
            List>> collect11 = 
                    maplist
                    .stream()
                    .flatMap(map -> map.entrySet().stream())
                    .collect(Collectors.toList());
            System.out.println(" collect11  => " + collect11);
            // collect11  => [a=[1], b=[2, 3], a=[12, 115], b=[2, 5], c=[6]]

            // That's why we will use this flatmap


            Map> map2 = maplist.stream()
            .flatMap(map -> map.entrySet().stream())

            .collect(
                    Collectors.toMap(
                            //keyMapper 
                            Map.Entry::getKey,


                            //valueMapper
                            Map.Entry::getValue,

                            (list_a,list_b) -> Stream.concat(list_a.stream(), list_b.stream())
                            .collect(Collectors.toList())


                            )//tomap

                    );
//{a=[1, 12, 115], b=[2, 3, 2, 5], c=[6]}

            System.out.println("After joining the maps according the key  => " + map2);
            // After joining the maps according the key  => {a=[1, 12, 115], b=[2, 3, 2, 5], c=[6]}

            /*
             OUTPUT :

             After joining the maps according the key  => {a=[1, 12, 115], b=[2, 3, 2, 5], c=[6]}
             */

        }// main

    }