import java.util.*;
import java.io.*;
import java.util.stream.*;
class Ideone
{
	public static void main(String[] args) {
        Map<String, Set<String>> map1 = new HashMap<>();
        Set<String> set1 = new HashSet<>();
        set1.add("AB");
        set1.add("BC");
        map1.put("ABCD", set1);
        Map<String, Set<String>> map2 = new HashMap<>();
        Set<String> set2 = new HashSet<>();
        set2.add("CD");
        set2.add("EF");
        map2.put("ABCD", set2);
        final Map<String, Set<String>> map3 = Stream.concat(map1.entrySet().stream(), map2.entrySet().stream())
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue,
                        (a, b) -> Stream.concat(a.stream(), b.stream()).collect(Collectors.toSet())));
        System.out.println(map3);
    }
}