java：list<Map<String,Object>>树形结构怎么生成，不用for循环嵌套

你可以使用递归来生成树形结构，而不使用嵌套的循环。下面是一个示例代码：

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class TreeGenerator {
    public static List<Map<String, Object>> generateTree(List<Map<String, Object>> flatList, String parentIdKey, String idKey) {
        List<Map<String, Object>> treeList = new ArrayList<>();
        Map<Object, List<Map<String, Object>>> childMap = new HashMap<>();

        // 将所有节点按照父节点分组
        for (Map<String, Object> node : flatList) {
            Object parentId = node.get(parentIdKey);
            childMap.computeIfAbsent(parentId, k -> new ArrayList<>()).add(node);
        }

        // 递归生成树形结构
        generateChildren(childMap, treeList, null, idKey);

        return treeList;
    }

    private static void generateChildren(Map<Object, List<Map<String, Object>>> childMap,
                                         List<Map<String, Object>> parentList,
                                         Object parentId,
                                         String idKey) {
        List<Map<String, Object>> children = childMap.get(parentId);
        if (children != null) {
            for (Map<String, Object> child : children) {
                Object childId = child.get(idKey);
                List<Map<String, Object>> grandChildren = new ArrayList<>();
                child.put("children", grandChildren);
                generateChildren(childMap, grandChildren, childId, idKey);
            }
            parentList.addAll(children);
        }
    }

    public static void main(String[] args) {
        // 示例用法
        List<Map<String, Object>> flatList = new ArrayList<>();
        Map<String, Object> node1 = new HashMap<>();
        node1.put("id", 1);
        node1.put("parentId", null);
        flatList.add(node1);

        Map<String, Object> node2 = new HashMap<>();
        node2.put("id", 2);
        node2.put("parentId", 1);
        flatList.add(node2);

        Map<String, Object> node3 = new HashMap<>();
        node3.put("id", 3);
        node3.put("parentId", 1);
        flatList.add(node3);

        Map<String, Object> node4 = new HashMap<>();
        node4.put("id", 4);
        node4.put("parentId", 2);
        flatList.add(node4);

        List<Map<String, Object>> treeList = generateTree(flatList, "parentId", "id");
        System.out.println(treeList);
    }
}

这个示例代码中，`generateTree` 方法接受一个扁平的节点列表 `flatList`，以及两个键的名称：`parentIdKey` 和 `idKey`。它首先将节点按照父节点进行分组，然后通过递归调用 `generateChildren` 方法来构建树形结构。最后返回生成的树形结构列表。

你可以根据自己的数据结构和需求进行相应的修改。希望这能帮到你！