java computeIfAbsent 处理mysql获取树形的数据

左手的ㄟ右手 2024-04-07 14:28 37阅读 0赞

学习了《[java 处理mysql获取树形的数据][java _mysql]》，这个还是比较复杂，在学了map的 [computeIfAbsent][]的方法，发现可以用来处理树形数据

直接上代码：

### 代码： ###

#### map: ####

public static void main(String args[]) {
            JSONArray datas = initData();
            List<Map<String, Object>> treeDatas = parseTreeModel(datas);
            System.out.println(JSON.toJSONString(treeDatas));
        }
    
        public static List<Map<String, Object>> parseTreeModel(JSONArray datas){
            Map<String, List<Map<String, Object>>> map = new HashMap<>();
            List<Map<String, Object>> results = ListUtils.emptyIfNull(datas).stream().map(e -> (Map<String, Object>) e).collect(Collectors.toList());
            ListUtils.emptyIfNull(results).stream().map(e -> (JSONObject)e).forEach(x -> {
                String parentCode = MapUtils.getString(x, "parentCode");
                if (StringUtils.isBlank(parentCode)) {
                    parentCode = "-1";
                }
                // 如果 parentCode 没有值，生成空数组，
                List<Map<String, Object>> list = map.computeIfAbsent(parentCode, k -> new ArrayList<>());
                // 如果有值，就把值取出来，相同的parentCode 放到一起
                list.add(x);
            });
            System.out.println("======= computeIfAbsent data =======");
            System.out.println(JSON.toJSONString(map));
            // 从-1开始遍历
            List<Map<String, Object>> beiginResult = map.getOrDefault("-1", new ArrayList<>());
            parseChildrenTreeModel(map, beiginResult);
            System.out.println("======= parseChildrenTreeModel =======");
    
            return beiginResult;
        }
    
        // getOrDefault() 方法获取指定 key 对应对 value，如果找不到 key ，则返回设置的默认值。
        private static void parseChildrenTreeModel(Map<String, List<Map<String, Object>>> map, List<Map<String, Object>> datas) {
            datas.forEach(x -> {
                x.put("children", map.getOrDefault(MapUtils.getString(x, "code"), new ArrayList<>()));
                List<Map<String, Object>> children = (List<Map<String, Object>>) x.get("children");
                if (!CollectionUtils.isEmpty(children)) {
                    parseChildrenTreeModel(map, children);
                }
            });
        }
    
    
    
        private static JSONArray initData(){
            String data = "[{\n" +
                    "\t\t\"code\": 59,\n" +
                    "\t\t\"parentCode\": -1,\n" +
                    "\t\t\"name\": \"福建\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 591,\n" +
                    "\t\t\"parentCode\": 59,\n" +
                    "\t\t\"name\": \"福州\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 59101,\n" +
                    "\t\t\"parentCode\": 591,\n" +
                    "\t\t\"name\": \"鼓楼区\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 59195,\n" +
                    "\t\t\"parentCode\": 591,\n" +
                    "\t\t\"name\": \"晋安区\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 59196,\n" +
                    "\t\t\"parentCode\": 591,\n" +
                    "\t\t\"name\": \"马尾区\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 59197,\n" +
                    "\t\t\"parentCode\": 591,\n" +
                    "\t\t\"name\": \"台江区\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 59198,\n" +
                    "\t\t\"parentCode\": 591,\n" +
                    "\t\t\"name\": \"金山区\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 592,\n" +
                    "\t\t\"parentCode\": 59,\n" +
                    "\t\t\"name\": \"厦门\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 59201,\n" +
                    "\t\t\"parentCode\": 592,\n" +
                    "\t\t\"name\": \"思明区\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 59202,\n" +
                    "\t\t\"parentCode\": 592,\n" +
                    "\t\t\"name\": \"同安\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 59203,\n" +
                    "\t\t\"parentCode\": 592,\n" +
                    "\t\t\"name\": \"杏林\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 59204,\n" +
                    "\t\t\"parentCode\": 592,\n" +
                    "\t\t\"name\": \"海沧区\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 59205,\n" +
                    "\t\t\"parentCode\": 592,\n" +
                    "\t\t\"name\": \"同安区\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 59206,\n" +
                    "\t\t\"parentCode\": 592,\n" +
                    "\t\t\"name\": \"翔安区\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 593,\n" +
                    "\t\t\"parentCode\": 59,\n" +
                    "\t\t\"name\": \"宁德\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 59301,\n" +
                    "\t\t\"parentCode\": 593,\n" +
                    "\t\t\"name\": \"宁德\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 59302,\n" +
                    "\t\t\"parentCode\": 593,\n" +
                    "\t\t\"name\": \"古田\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 59303,\n" +
                    "\t\t\"parentCode\": 593,\n" +
                    "\t\t\"name\": \"屏南\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 59304,\n" +
                    "\t\t\"parentCode\": 593,\n" +
                    "\t\t\"name\": \"寿宁\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 59305,\n" +
                    "\t\t\"parentCode\": 593,\n" +
                    "\t\t\"name\": \"周宁\"\n" +
                    "\t}]";
            return JSON.parseArray(data);
    
        }

#### 对象类型： ####

public static void main(String[] args){
            List<TreeModel> areaList = initData();
            // 构建树形
            List<TreeModel> treeModels = parseTreeModel(areaList);
            System.out.println("result: "+ JSON.toJSONString(treeModels));
        }
    
        private static List<TreeModel> parseTreeModel(List<TreeModel> treeModels) {
            if (CollectionUtils.isEmpty(treeModels)) {
                return Collections.emptyList();
            }
            Map<String, List<TreeModel>> map = new HashMap<>();
            treeModels.forEach(x -> {
                String parentCode = x.getParentCode();
                if (StringUtils.isBlank(parentCode)) {
                    parentCode = "-1";
                }
                // 如果 parentCode 没有值，生成空数组，
                List<TreeModel> list = map.computeIfAbsent(parentCode, k -> new ArrayList<>());
                // 如果有值，就把值取出来，相同的parentCode 放到一起
                list.add(x);
            });
            // 从-1开始遍历
            List<TreeModel> result = map.getOrDefault("-1", new ArrayList<>());
            parseChildrenTreeModel(map, result);
            return result;
        }
    
        private static void parseChildrenTreeModel(Map<String, List<TreeModel>> map, List<TreeModel> treeModels) {
            treeModels.forEach(x -> {
                x.setChildren(map.getOrDefault(x.getCode(), new ArrayList<>()));
                if (!CollectionUtils.isEmpty(x.getChildren())) {
                    parseChildrenTreeModel(map, x.getChildren());
                }
            });
        }
    
        private static List<TreeModel> initData(){
            String areaInfo = "[{\n" +
                    "\t\t\"code\": 59,\n" +
                    "\t\t\"parentCode\": -1,\n" +
                    "\t\t\"name\": \"福建\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 591,\n" +
                    "\t\t\"parentCode\": 59,\n" +
                    "\t\t\"name\": \"福州\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 59101,\n" +
                    "\t\t\"parentCode\": 591,\n" +
                    "\t\t\"name\": \"鼓楼区\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 59195,\n" +
                    "\t\t\"parentCode\": 591,\n" +
                    "\t\t\"name\": \"晋安区\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 59196,\n" +
                    "\t\t\"parentCode\": 591,\n" +
                    "\t\t\"name\": \"马尾区\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 59197,\n" +
                    "\t\t\"parentCode\": 591,\n" +
                    "\t\t\"name\": \"台江区\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 59198,\n" +
                    "\t\t\"parentCode\": 591,\n" +
                    "\t\t\"name\": \"金山区\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 592,\n" +
                    "\t\t\"parentCode\": 59,\n" +
                    "\t\t\"name\": \"厦门\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 59201,\n" +
                    "\t\t\"parentCode\": 592,\n" +
                    "\t\t\"name\": \"思明区\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 59202,\n" +
                    "\t\t\"parentCode\": 592,\n" +
                    "\t\t\"name\": \"同安\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 59203,\n" +
                    "\t\t\"parentCode\": 592,\n" +
                    "\t\t\"name\": \"杏林\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 59204,\n" +
                    "\t\t\"parentCode\": 592,\n" +
                    "\t\t\"name\": \"海沧区\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 59205,\n" +
                    "\t\t\"parentCode\": 592,\n" +
                    "\t\t\"name\": \"同安区\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 59206,\n" +
                    "\t\t\"parentCode\": 592,\n" +
                    "\t\t\"name\": \"翔安区\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 593,\n" +
                    "\t\t\"parentCode\": 59,\n" +
                    "\t\t\"name\": \"宁德\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 59301,\n" +
                    "\t\t\"parentCode\": 593,\n" +
                    "\t\t\"name\": \"宁德\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 59302,\n" +
                    "\t\t\"parentCode\": 593,\n" +
                    "\t\t\"name\": \"古田\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 59303,\n" +
                    "\t\t\"parentCode\": 593,\n" +
                    "\t\t\"name\": \"屏南\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 59304,\n" +
                    "\t\t\"parentCode\": 593,\n" +
                    "\t\t\"name\": \"寿宁\"\n" +
                    "\t},\n" +
                    "\t{\n" +
                    "\t\t\"code\": 59305,\n" +
                    "\t\t\"parentCode\": 593,\n" +
                    "\t\t\"name\": \"周宁\"\n" +
                    "\t}]";
            JSONArray areaArr = JSONArray.parseArray(areaInfo);
            return ListUtils.emptyIfNull(areaArr).stream().map(e -> (JSONObject) e)
                    .map(e -> JSONObject.parseObject(e.toString(), TreeModel.class)).collect(Collectors.toList());
        }

### 分析： ###

// 如果 parentCode 没有值，生成空数组，
    List<Map<String, Object>> list = map.computeIfAbsent(parentCode, k -> new ArrayList<>());
    // 如果有值，就把值取出来，相同的parentCode 放到一起
    list.add(x);

先把有相同parentCode的内容放到一起，这样在遍历的时候，就可以直接map.get("code")获取到对应的子集了，再不断进行调用。

### 总结： ###

使用 map.computeIfAbsent对数据进行汇集，这样处理树形的数据就变得简单了。如果是对象类型的数据，处理起来就更简单了。

[java _mysql]: https://blog.csdn.net/qq_35461948/article/details/112665917
[computeIfAbsent]: https://www.runoob.com/java/java-hashmap-computeifabsent.html