java8 Stream List如何在分组后隐藏Map by

Question

我从mysql中选择了一些数据。然后我想格式化它。数据如下所示：

var json = [
  {
    "date": "2016-01",
    "number": "1",
    "code": "420000",
    "type": "false"
  },
  {
    "date": "2016-01",
    "number": "2",
    "code": "440000",
    "type": "false"
  },
  {
    "date": "2016-02",
    "number": "3",
    "code": "420000",
    "type": "false"
  },
  {
    "date": "2016-03",
    "number": "4",
    "code": "420000",
    "type": "true"
  },
  {
    "date": "2016-03",
    "number": "5",
    "code": "410000",
    "type": "false"
  },
  {
    "date": "2016-03",
    "number": "6",
    "code": "440000",
    "type": "true"
  },
  {
    "date": "2016-04",
    "number": "7",
    "code": "420000",
    "type": "false"
  },
  {
    "date": "2016-04",
    "number": "8",
    "code": "440000",
    "type": "false"
  }
];
console.log(json);

我想groupingBydate，和partitioningBytype，并将缩小为地图。在格式化数据之后，就像

var json = {
  "2016-01": {
    "false": {
      "420000": "1",
      "440000": "2"
    }
  },
  "2016-02": {
    "false": {
      "420000": "3"
    }
  },
  "2016-03": {
    "false": {
      "410000": "5"
    },
    "true": {
      "420000": "4",
      "440000": "6"
    }
  },
  "2016-04": {
    "false": {
      "420000": "7",
      "440000": "8"
    }
  }
};
console.log(json);

我编码java8流

public static void main(String[] args) {
    List<Map<String, String>> postmans = new ArrayList<>();

    // "420000" : post area code, "false": just paper(without goods)
    postmans.add(createMap("2016-01", "1", "420000", "false"));
    postmans.add(createMap("2016-01", "2", "440000", "false"));
    postmans.add(createMap("2016-02", "3", "420000", "false"));
    postmans.add(createMap("2016-03", "4", "420000", "true"));
    postmans.add(createMap("2016-03", "5", "410000", "false"));
    postmans.add(createMap("2016-03", "6", "440000", "true"));
    postmans.add(createMap("2016-04", "7", "420000", "false"));
    postmans.add(createMap("2016-04", "8", "440000", "false"));

    // before, I use fastjson Library
    System.out.println(JSONObject.toJSONString(postmans));


    Map<String, Map<Boolean, Map>> data = postmans.stream()
            .collect(Collectors.groupingBy(d -> d.get("date"), TreeMap::new,
                    Collectors.partitioningBy(d-> d.get("type").equals("true"),
                            Collectors.reducing(new HashMap(), (d1, d2)->{
                                Object code = d2.get("code");
                                Object number = d2.get("number");
                                // I think bug in reducing
                                d1.put(code, number);
                                return d1;
                            }))));
    // after, I use fastjson Library
    System.out.println(JSONObject.toJSONString(data));

}

private static Map<String,String> createMap(String date, String number, String code, String type){
    Map<String, String> map = new HashMap<>();
    map.put("date", date);
    map.put("number", number);
    map.put("code", code);
    map.put("type", type);
    return map;
}

但是，真正的结果是

var json = {
  "2016-01": {
    "false": {
      "410000": "5",
      "420000": "7",
      "440000": "8"
    },
    "true": {
      "410000": "5",
      "420000": "7",
      "440000": "8"
    }
  },
  "2016-02": {
    "false": {
      "410000": "5",
      "420000": "7",
      "440000": "8"
    },
    "true": {
      "410000": "5",
      "420000": "7",
      "440000": "8"
    }
  },
  "2016-03": {
    "false": {
      "410000": "5",
      "420000": "7",
      "440000": "8"
    },
    "true": {
      "410000": "5",
      "420000": "7",
      "440000": "8"
    }
  },
  "2016-04": {
    "false": {
      "410000": "5",
      "420000": "7",
      "440000": "8"
    },
    "true": {
      "410000": "5",
      "420000": "7",
      "440000": "8"
    }
  }
};
console.log(json);

我认为这个bug的原因在reduing。当分区通过得到一个新的List

我只想partitioningByresult，每个列表

我很抱歉代码太长了，我的英语很差。谢谢你帮我！




Answer 1

这里有两个主要错误。Collectors.reducing，因为这是一个缩减，需要一直返回一个新实例，基本上第二个是你的逻辑有缺陷：

Map<String, Map<Boolean, Map<String, String>>> data = postmans.stream()
            .collect(Collectors.groupingBy(
                    d -> d.get("date"),
                    TreeMap::new,
                    Collectors.partitioningBy(
                            d -> d.get("type").equals("true"),
                            Collectors.reducing(
                                    new HashMap<>(),
                                    (left, right) -> {

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

                                        String leftCode = left.get("code");
                                        String leftNumber = left.get("number");

                                        if (leftCode == null) {
                                            map.putAll(left);
                                        } else {
                                            map.put(leftCode, leftNumber);
                                        }

                                        String rightCode = right.get("code");
                                        String rightNumber = right.get("number");

                                        map.put(rightCode, rightNumber);

                                        return map;

                                    }))));

您还存在将日期存储为字符串和执行树映射：：新建-基本上对字符串排序的问题，这可能不是您想要的排序。一般来说，将所有内容都作为字符串的整个方法使得代码几乎不可读。