List<String> list1 = new ArrayList<>(Arrays.asList("A", "B", "C"));
List<String> list2 = Arrays.asList("D", "E");

list1.addAll(list2);
System.out.println(list1); // [A, B, C, D, E]

List<String> mergedList = Stream.concat(list1.stream(), list2.stream())
                                .collect(Collectors.toList());
System.out.println(mergedList); // [A, B, C, D, E]

List<String> list3 = Arrays.asList("F", "G");

List<String> mergedList = Stream.of(list1, list2, list3)
                                .flatMap(Collection::stream)
                                .collect(Collectors.toList());

System.out.println(mergedList); // [A, B, C, D, E, F, G]

List<String> mergedList = List.copyOf(Stream.concat(list1.stream(), list2.stream())
                                           .collect(Collectors.toList()));
System.out.println(mergedList); // [A, B, C, D, E]

Set<Integer> set1 = new HashSet<>(Arrays.asList(1, 2, 3));
Set<Integer> set2 = new HashSet<>(Arrays.asList(3, 4, 5));

set1.addAll(set2);
System.out.println(set1); // [1, 2, 3, 4, 5]

Set<Integer> mergedSet = Stream.concat(set1.stream(), set2.stream())
                               .collect(Collectors.toSet());
System.out.println(mergedSet); // [1, 2, 3, 4, 5]

Set<Integer> set3 = new HashSet<>(Arrays.asList(6, 7));

Set<Integer> mergedSet = Stream.of(set1, set2, set3)
                               .flatMap(Set::stream)
                               .collect(Collectors.toSet());

System.out.println(mergedSet); // [1, 2, 3, 4, 5, 6, 7]

Map<String, Integer> map1 = new HashMap<>();
map1.put("A", 1);
map1.put("B", 2);

Map<String, Integer> map2 = new HashMap<>();
map2.put("B", 3);
map2.put("C", 4);

map1.putAll(map2);
System.out.println(map1); // {A=1, B=3, C=4}

map2.forEach((key, value) ->
    map1.merge(key, value, Integer::sum)
);
System.out.println(map1); // {A=1, B=5, C=4}

map2.forEach((key, value) ->
    map1.compute(key, (k, v) -> (v == null) ? value : v + value)
);
System.out.println(map1); // {A=1, B=5, C=4}

Map<String, Integer> mergedMap = Stream.of(map1, map2)
    .flatMap(m -> m.entrySet().stream())
    .collect(Collectors.toMap(
        Map.Entry::getKey, 
        Map.Entry::getValue, 
        Integer::sum));

System.out.println(mergedMap); // {A=1, B=5, C=4}