Elegant way of counting occurrences in a java collection

Question

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.

Answer 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.