Elegant way of counting occurrences in a java collection

Given a collection of objects with possible duplicates, I'd like end up with a count of occurrences per object. I do it by initializing an empty Map, then iterating through the Collection and mapping the object to its count (incrementing the count each time the map already contains the object).

public Map<Object, Integer> countOccurrences(Collection<Object> list) {
    Map<Object, Integer> occurrenceMap = new HashMap<Object, Integer>();
    for (Object obj : list) {
        Integer numOccurrence = occurrenceMap.get(obj);
        if (numOccurrence == null) {
            //first count
            occurrenceMap.put(obj, 1);
        } else {
            occurrenceMap.put(obj, numOccurrence++);
        }
    }
    return occurrenceMap;
}

This looks too verbose for a simple logic of counting occurrences. Is there a more elegant/shorter way of doing this? I'm open to a completely different algorithm or a java language specific feature that allows for a shorter code.

Solution 1:

Check out Guava's Multiset. Pretty much exactly what you're looking for.

Unfortunately it doesn't have an addAll(Iterable iterable) function, but a simple loop over your collection calling add(E e) is easy enough.

EDIT

My mistake, it does indeed have an addAll method - as it must, since it implements Collection.

Solution 2:

Now let's try some Java 8 code:

static public Map<String, Integer> toMap(List<String> lst) {
    return lst.stream()
            .collect(HashMap<String, Integer>::new,
                    (map, str) -> {
                        if (!map.containsKey(str)) {
                            map.put(str, 1);
                        } else {
                            map.put(str, map.get(str) + 1);
                        }
                    },
                    HashMap<String, Integer>::putAll);
}

static public Map<String, Integer> toMap(List<String> lst) {
    return lst.stream().collect(Collectors.groupingBy(s -> s,
                                  Collectors.counting()));
}

I think this code is more elegant.