思路

1. 我们首先要知道数据中，哪两列能够体现父子级
2. 我们需要找到最顶层父 id 是什么，因为只有知道最顶层的父 id，我们才能进行递归
3. 我们要在不改变数据的原有结构下，而转换为 Tree 结构，那么就需要创建新的结构

代码

/*** Date: 2023/10/28* Author: PuKun* Description: 树形数据ID*/
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface TreeId {}

/*** Date: 2023/10/28* Author: PuKun* Description: 树形数据的父类id*/
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface TreeParentId {}

/*** Date: 2023/10/28* Author: PuKun* Description: Tree工具类*/
public class TreeUtils<T> {private Class<T> aClass;private Field treeIdField;private Field treeParentIdField;private final String CHILDREN_KEY = "children";public TreeUtils(Class<T> aClass) {if (aClass == null) {throw new RuntimeException("传入的class为NULL");}this.aClass = aClass;init();}private void init() {Field[] fields = this.aClass.getDeclaredFields();for (Field f : fields) {if (f.isAnnotationPresent(TreeId.class)) {this.treeIdField = f;} else if (f.isAnnotationPresent(TreeParentId.class)) {this.treeParentIdField = f;}// 这一步也没什么用，我的想法是如果这两列提前找到，那么就提前结束循环if (this.treeIdField != null && this.treeParentIdField != null) {break;}}if (this.treeIdField == null || this.treeParentIdField == null) {throw new RuntimeException("没有找到相应的注解");}// 避免因字段为private时，无法访问this.treeIdField.setAccessible(true);this.treeParentIdField.setAccessible(true);}public List<HashMap<String, Object>> getTreeData(List<T> data) throws IllegalAccessException {Object topParentId = getTopParentId(data);return buildTreeData(data, topParentId);}private Object getTopParentId(List<T> data) throws IllegalAccessException {// 获取所有parentId的值Map<Object, Integer> parentIdMap = new HashMap<>();for (T o : data) {Object v = this.treeParentIdField.get(o);parentIdMap.put(v, 0);}// 计算每个parentId的数量for (T o : data) {Object v = this.treeIdField.get(o);if (parentIdMap.containsKey(v)) {int keyValue = parentIdMap.get(v);parentIdMap.put(v, ++keyValue);}}// 当parentId的数量等于0时，说明当前的parentId没有找到对应的treeId，那么就是最顶层的parentIdObject topParentId = null;Set<Map.Entry<Object, Integer>> entries = parentIdMap.entrySet();for (Map.Entry<Object, Integer> e : entries) {if (e.getValue() == 0) {topParentId = e.getKey();}}return topParentId;}private List<HashMap<String, Object>> buildTreeData(List<T> data, Object parentId) throws IllegalAccessException {List<HashMap<String, Object>> list = new ArrayList<>();for (T o : data) {Object pid = this.treeParentIdField.get(o);Object tid = this.treeIdField.get(o);if (pid.equals(parentId)) {HashMap<String, Object> map = new HashMap<>();Field[] fields = this.aClass.getDeclaredFields();for (Field f : fields) {f.setAccessible(true);map.put(f.getName(), f.get(o));}List<HashMap<String, Object>> children = buildTreeData(data, tid);if (children.size() > 0) {map.put(CHILDREN_KEY, children);}list.add(map);}}return list;}
}

/*** Date: 2023/11/14* Author: PuKun* Description: 树形工具类测试*/
class TreeUtilsTest {class Dept {@TreeIdprivate int id;private String deptName;@TreeParentIdprivate int parentId;public Dept(int id, String deptName, int parentId) {this.id = id;this.deptName = deptName;this.parentId = parentId;}public int getId() {return id;}public void setId(int id) {this.id = id;}public String getDeptName() {return deptName;}public void setDeptName(String deptName) {this.deptName = deptName;}public int getParentId() {return parentId;}public void setParentId(int parentId) {this.parentId = parentId;}}class Dept2 {@TreeIdprivate String id;private String deptName;@TreeParentIdprivate String parentId;public Dept2(String id, String deptName, String parentId) {this.id = id;this.deptName = deptName;this.parentId = parentId;}public String getId() {return id;}public void setId(String id) {this.id = id;}public String getDeptName() {return deptName;}public void setDeptName(String deptName) {this.deptName = deptName;}public String getParentId() {return parentId;}public void setParentId(String parentId) {this.parentId = parentId;}}@Testvoid getTreeData() {// List<Dept> list = new ArrayList<>();
//        list.add(new Dept(1, "闲简居有限公司", 0));
//        list.add(new Dept(2, "开发部", 1));
//        list.add(new Dept(4, "Java组", 2));
//        list.add(new Dept(5, "前端组", 2));
//        list.add(new Dept(3, "测试部", 1));//        list.add(new Dept(2, "开发部", 1));
//        list.add(new Dept(4, "Java组", 2));
//        list.add(new Dept(5, "前端组", 2));
//        list.add(new Dept(3, "测试部", 1));//        TreeUtils<Dept> treeUtils = new TreeUtils<>(Dept.class);
//        List<HashMap<String, Object>> mapList = null;
//        try {
//            mapList = treeUtils.getTreeData(list);
//        } catch (IllegalAccessException e) {
//            Assertions.fail(e.getMessage());
//        }List<Dept2> list = new ArrayList<>();list.add(new Dept2("1", "闲简居有限公司", "0"));list.add(new Dept2("2", "开发部", "1"));list.add(new Dept2("4", "Java组", "2"));list.add(new Dept2("5", "前端组", "2"));list.add(new Dept2("3", "测试部", "1"));TreeUtils<Dept2> treeUtils = new TreeUtils<>(Dept2.class);List<HashMap<String, Object>> mapList = null;try {mapList = treeUtils.getTreeData(list);} catch (IllegalAccessException e) {Assertions.fail(e.getMessage());}Assertions.assertNotEquals(null, mapList);System.out.println(mapList);}
}