Map<String, List<Model>> map = new HashMap<>();

// 如何获取相同的num 的对象做运算

        for (Model model6 : list) {
            String s = model6.getNum();
            if (map.containsKey(s)) {
                map.get(s).add(model6);
            } else {
                ArrayList<Model> NumList = new ArrayList<Model>();
                NumList.add(model6);
                map.put(s, NumList);
            }
        }

 Map<String, List<Model>> result = list.stream().collect(Collectors.groupingBy(e -> e.getB()));