collecting HashMap<String, List<String>> java 8

Question

I want to be able to convert a List to a HashMap where the key is the elementName and the values is a list of something random (in this case its the Element Name). So in short I want (A->List(A), B->List(B), C-> List(C)). I tried using toMap() and passing it the keyMapper and ValueMapper but I get a compilation error. I would really appreciate if someone can help me out.

Thanks!

public static void main(String[] args) {
    // TODO Auto-generated method stub
    List<String> list = Arrays.asList("A","B","C","D");
    Map<String, List<String>> map = list.stream().map((element)->{
        Map<String, List<String>> map = new HashMap<>();
        map.put(element, Arrays.asList(element));
        return map;
    }).collect(??);
}


Function<Map<String, String>, String> key = (map) -> {
    return map.keySet().stream().findFirst().get();
};

Function<Map<String, String>, String> value = (map) -> {
    return map.values().stream().findFirst().get();
};

=== This worked for me

Thanks for all the help guys! @izstas "they should operate on the elements" helped a lot :). Actually this is what I was looking for to be exact

public static void test2 (){
    Function<Entry<String, List<String>>, String> key = (entry) -> {
        return entry.getKey();
    };
    Function<Entry<String, List<String>>, List<String>> value = (entry) -> {
        return new ArrayList<String>(entry.getValue());
    };
    BinaryOperator<List<String>> merge = (old, latest)->{
        old.addAll(latest);
        return old;
    };

    Map<String, List<String>> map1 = new HashMap<>();
    map1.put("A", Arrays.asList("A1", "A2"));
    map1.put("B", Arrays.asList("B1"));
    map1.put("D", Arrays.asList("D1"));

    Map<String, List<String>> map2 = new HashMap<>();
    map2.put("C", Arrays.asList("C1","C2"));
    map2.put("D", Arrays.asList("D2"));

    Stream<Map<String, List<String>>> stream =Stream.of(map1, map2);
    System.out.println(stream.flatMap((map)->{
        return map.entrySet().stream(); 
    }).collect(Collectors.toMap(key, value, merge)));
}

Answer 1

You can use the groupingBy method to manage aggregation, for example:

public static void main(String[] args) {
    List<String> list = Arrays.asList("A", "B", "C", "D", "A");
    Map<String, List<String>> map = list.stream().collect(groupingBy(Function.identity()));
}

If you want more flexibility (for example to map the value and return a Set instead of a List) you can always use the groupingBy method with more parameters as specified in javadoc:

Map<City, Set<String>> namesByCity = people.stream().collect(groupingBy(Person::getCity, mapping(Person::getLastName, toSet())));