collecting HashMap java 8

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(Collectors.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(Collectors.groupingBy(Person::getCity, mapping(Person::getLastName, toSet())));