Convert a HashMap<Integer, Long> ito HashMap<Integer, Integer>

Question

Collectors.counting() returns long values for each key in this method:

private static Map<Integer, Long> countDuplicates(HashSet<Card> cards) {
    return cards.stream().collect(Collectors.groupingBy(Card::getRankNumber, Collectors.counting()));
}

Is there a way to cast or convert the resulting Map from Map<Integer, Long> to Map<Integer, Integer>?

Direct casting gives this exception:

Type mismatch: cannot convert from Map<Integer,Integer> to Map<Integer,Long>

Note: The implementation of my class guarantees that cards has five objects in it, so there is no chance of overflow.

Answer 1

Try the following:

private static Map<Integer, Integer> countDuplicates(HashSet<Card> cards) {
    return cards.stream()
                .collect(Collectors.groupingBy(Card::getRankNumber, Collectors.summingInt(x -> 1)));
}

Instead of Collectors.counting() use Collectors.summingInt(x -> 1) so that you get immediately the value as an Integer.

Answer 2

Another way to get Map<Integer, Integer> is to use toMap collector along with Integer::sum method reference as a merge function:

private static Map<Integer, Integer> countDuplicates(HashSet<Card> cards) {
    return cards.stream()
                .collect(Collectors.toMap(Card::getRankNumber, x -> 1, Integer::sum));
}