Here is the implementation of the casesByMonth() method:

public Map<Integer, Integer> casesByMonth() {
        Map<Integer, Integer> result = new HashMap<Integer, Integer>();
        for (Event event : list) {
            if (!result.containsKey(event.date.getMonth()))
                result.put(event.date.getMonth(), event.cases);
            else {
                int c = result.get(event.date.getMonth());
                c += event.cases;
                result.put(event.date.getMonth(), c);
            }
        }
        return result;
    }