Count int occurrences with Java8

Try:

 Map<Integer, Long> counters = persons.stream()
     .collect(Collectors.groupingBy(p -> p.getBirthday().getMonthValue(), 
         Collectors.counting()));

There's a few variations this could take.

You can use Collectors.summingInt() to use Integer instead of the Long in the count.

If you wanted to skip the primitive int array, you could store the counts directly to a List in one iteration.

Count the birth months as Integers

Map<Integer, Integer> monthsToCounts = 
        people.stream().collect(
                Collectors.groupingBy(p -> p.getBirthday().getMonthValue(), 
                Collectors.summingInt(a -> 1)));

Store the birth months in a 0-based array

int[] monthCounter = new int[12];
people.stream().collect(Collectors.groupingBy(p -> p.getBirthday().getMonthValue(), 
                        Collectors.summingInt(a -> 1)))
                        .forEach((month, count) -> monthCounter[month-1]=count);

Skip the array and directly store the values to a list

List<Integer> counts = people.stream().collect(
        Collectors.groupingBy(p -> p.getBirthday().getMonthValue(), 
        Collectors.summingInt(a -> 1)))
        .values().stream().collect(Collectors.toList());