1. 将Map转换为List, ArrayList>>，并重写比较器，按照value排序，最后再转换回Map。

Map, ArrayList> map = new HashMap<>();
// 添加数据到map
List, ArrayList>> list =
    new LinkedList<>(map.entrySet());
// 重写Comparator，按照value排序
Collections.sort(list, new Comparator, ArrayList>>() {
  public int compare(Map.Entry, ArrayList> o1,
      Map.Entry, ArrayList> o2) {
    return (o1.getValue().size() > o2.getValue().size() ? -1 :
      (o1.getValue().size() == o2.getValue().size() ? 0 : 1));
  }
});
// 转换回Map
Map, ArrayList> sortedMap = new LinkedHashMap<>();
for (Map.Entry, ArrayList> entry : list) {
  sortedMap.put(entry.getKey(), entry.getValue());
}

2. 使用Java 8的Stream和Lambda表达式，进行排序

Map, ArrayList> sortedMap = 
    map.entrySet()
       .stream()
       .sorted(Collections.reverseOrder(Comparator.comparing(
           (Map.Entry, ArrayList> e) -> e.getValue().size())))
       .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, 
           (oldv, newv) -> oldv, LinkedHashMap::new));