Java泛型：按值排序地图

"The method compareTo(V) is undefined for the type V"

请帮助完成这项工作！

public class CollectionsPlus<K,V> {

    /**
     * Sort map by value
     * @param map
     * @return
     */
    public static<K,V> Map<K,V> sortMapByValue(Map<K,V> map) {
        List<Map.Entry<K,V>> list = new LinkedList<Map.Entry<K,V>>(
                map.entrySet());
        Collections.sort(list,new Comparator<Map.Entry<K,V>>() {
                    public int compare(Map.Entry<K,V> o1,Map.Entry<K,V> o2) {
                        return (o2.getValue().compareTo(o1.getValue()));
                    }
                });

        Map<K,V> result = new LinkedHashMap<K,V>();
        for (Iterator<Map.Entry<K,V>> it = list.iterator(); it.hasNext();) {
            Map.Entry<K,V> entry = it.next();
            result.put(entry.getKey(),entry.getValue());
        }
        return result;
    }
}