将数据库有关联的数据，树形展示

实体

public class Category {
    private String id;
    private String name;
    private String pid;
    private List<Category> children;
}

树形展示数据，id为整型

public List<Category> list(){
    List<Category> list= categoryMapper.list();
    return (createTree(null,list));//null 从第一级开始递归
}

//完全递归实现递归树
private List<Category> createTree(Integer pid, List<Category> categories) {
    List<Category> treeList = new ArrayList<>();
    for (Category category : categories) {
        // 如果pid为null，检查分类的父ID是否也为null，即第一级节点
        if ( pid==null ){
            if (category.getPid() == null){ //第一级节点
                treeList.add(category);
                // 递归调用createTree方法，为当前节点设置子节点
                category.setChildren(createTree(category.getId(),categories));
            }
        }else {  // 如果pid不为null，检查分类的父ID是否与传入的pid匹配
            if (pid.equals(category.getPid())) {
                treeList.add(category);
                category.setChildren(createTree(category.getId(), categories));
            }
        }
    }
    return treeList;
}

树形展示数据，id为字符串型，且一个可能有多个父id

public List<Category> list(){
    List<Category> list= categoryMapper.list();
    return (createTree(null,list));//null 从第一级开始递归
}

//完全递归实现递归树
private List<Category> createTree(String pid, List<Category> categories) {
    List<Category> treeList = new ArrayList<>();
    for (Category category : categories) {
        // 如果pid为null，检查分类的父ID是否也为null，即第一级节点
        if (pid == null) {
            if (category.getPid() == null) { // 第一级节点
                treeList.add(category);
                // 递归调用createTree方法，为当前节点设置子节点
                category.setChildren(createTree(category.getId(), categories));
            }
        } else {
            //先把id分开
            //逐个比对
                if (category.getPid()!= null) {
                    String PidS = category.getPid();
                    String[] pids = PidS.split(",");
                    for (String p : pids) {
                        if (p.equals(pid)) {
                            treeList.add(category);
                            category.setChildren(createTree(category.getId(), categories));
                            break; // 找到匹配的pid后，跳出循环
                        }
                    }
                }
            }
        }
        return treeList;
    }

结果展示

"data": [
        {
            "id": "4",
            "name": "B",
            "pid": null,
            "children": [
                {
                    "id": "6",
                    "name": "B2",
                    "pid": "4",
                    "children": [
                        {
                            "id": "10",
                            "name": "aabb",
                            "pid": "5,6",
                            "children": []
                        }
                    ]
                },
                {
                    "id": "5",
                    "name": "B1",
                    "pid": "4",
                    "children": [
                        {
                            "id": "10",
                            "name": "aabb",
                            "pid": "5,6",
                            "children": []
                        }
                    ]
                }
            ]
        },
        {
            "id": "1",
            "name": "A",
            "pid": null,
            "children": [
                {
                    "id": "3",
                    "name": "A2",
                    "pid": "1",
                    "children": [
                        {
                            "id": "8",
                            "name": "A12",
                            "pid": "2,3",
                            "children": []
                        }
                    ]
                },
                {
                    "id": "2",
                    "name": "A1",
                    "pid": "1",
                    "children": [
                        {
                            "id": "9",
                            "name": "A11a",
                            "pid": "7,2",
                            "children": []
                        },
                        {
                            "id": "8",
                            "name": "A12",
                            "pid": "2,3",
                            "children": []
                        },
                        {
                            "id": "7",
                            "name": "A11",
                            "pid": "2",
                            "children": [
                                {
                                    "id": "9",
                                    "name": "A11a",
                                    "pid": "7,2",
                                    "children": []
                                }
                            ]
                        }
                    ]
                }
            ]
        }
    ],

优化

public List<Category> list() {
        List<Category> list = categoryMapper.list();
        Map<String, Category> categoryMap = new HashMap<>();
        for (Category category : list) {
            categoryMap.put(category.getId(), category);
        }
        return createTree(null, categoryMap);
    }

    private List<Category> createTree(String pid, Map<String, Category> categoryMap) {
        List<Category> treeList = new ArrayList<>();
        for (Map.Entry<String, Category> entry : categoryMap.entrySet()) {
            Category category = entry.getValue();
            if (pid == null) {
                if (category.getPid() == null) { // 第一级节点
                    treeList.add(category);
                    category.setChildren(createTree(category.getId(), categoryMap));
                }
            } else {
                if (category.getPid()!= null) {
                    String PidS = category.getPid();
                    String[] pids = PidS.split(",");
                    for (String p : pids) {
                        if (p.equals(pid)) {
                            treeList.add(category);
                            category.setChildren(createTree(category.getId(), categoryMap));
                            break;
                        }
                    }
                }
            }
        }
        return treeList;
    }

继续优化，需要在实体类里加属性 private Set<String> pidSet;

 public List<Category> list() {
        List<Category> list = categoryMapper.list();
        Map<String, Category> categoryMap = list.stream()
                .collect(Collectors.toMap(Category::getId, category -> category));

        // 预处理 pid，转换为 Set
        for (Category category : list) {
            if (category.getPid() != null) {
                category.setPidSet(new HashSet<>(Arrays.asList(category.getPid().split(","))));
            }
        }

        return createTree(null, categoryMap);
    }

    private List<Category> createTree(String pid, Map<String, Category> categoryMap) {
        return categoryMap.values().stream()
                .filter(category -> isChildOf(category, pid))
                .peek(category -> category.setChildren(createTree(category.getId(), categoryMap)))
                .collect(Collectors.toList());
    }

    private boolean isChildOf(Category category, String pid) {
        if (pid == null) {
            return category.getPid() == null;
        }
        return category.getPidSet() != null && category.getPidSet().contains(pid);
    }