public static ArrayList<Object> frequency (ArrayList<Object> input) {
        if (input.size() == 0) {
            System.out.println("No input was defined");
            return null;
        }
        HashMap<Object, Integer> map = new HashMap<Object, Integer>();
        for (Object obj : input) {
            if (map.containsKey(obj))
                map.put(obj, map.get(obj) + 1);
            else
                map.put(obj, 1);
        }
        ArrayList<Object> output = sortList(map);
        return output;
    }

    private static ArrayList<Object> sortList(HashMap<Object, Integer> map) {
        ArrayList<Object> output = new ArrayList<Object>();
        for (Entry<Object, Integer> entry : map.entrySet()) {
            if (output.size() == 0) {
                output.add(entry.getKey());
            }
            else
                for (int i = 0; i<output.size(); i++) {
                    if (entry.getValue()>map.get(output.get(i))) {
                        output.add(i, entry.getKey());
                        //Not using break here would cause the list to grow for each iteration
                        //which gives an infinite loop. The same goes for line 57.
                        break;
                    }
                    if (i == output.size()-1) {
                        output.add(entry.getKey());
                        break;
                    }
                }

        }
        return output;
    }