Since we're talking Map, I think a more utility would be gained by including putIfAbsent (see java.util.concurrent) because I see many lines of code that do that idiom all the time.
This idiom is especially prevalent when the value is itself a nested collection. Map<Object, List<Object>> map = ... List<Object> collection = map.get(key); if (collection == null) { collection = new LinkedList<Object>(); map.put(key, collection); } collection.add(value); Paul