java list 转树 tree

在做目录树的时候通常是将一个目录存在数据库的List全部返回来，再根据节点id和parentId组装成一颗树。

List转成Tree有两种方式，一种是常用的递归，一种是双层循环。(建议递归)

package com.gysoft.utils.tree;

import lombok.Data;
import lombok.ToString;

import java.util.List;

/**
 * @Description:
 * @Date: 2019/9/19 17:20
 * @Author: WangLingJi
 */
@Data
@ToString
public class TreeNode {
    private String id;

    private String parentId;

    private String name;

    private List<TreeNode> children;

    public TreeNode(String id, String name, String parentId) {
        this.id = id;
        this.parentId = parentId;
        this.name = name;
    }
    public TreeNode(String id, String name, TreeNode parent) {
        this.id = id;
        this.parentId = parent.getId();
        this.name = name;
    }


}

    /**
     * 双重for循环
     */
public static List<TreeNode> foreachTest(List<TreeNode> treeNodes) {
        List<TreeNode> trees = new ArrayList<>();
        treeNodes.forEach(treeNode -> {
            if ("0".equals(treeNode.getParentId())) {
                trees.add(treeNode);
            }
            treeNodes.forEach(treeNode1 -> {
                if (treeNode1.getParentId().equals(treeNode.getId())) {
                    if (treeNode.getChildren() == null) {
                        treeNode.setChildren(new ArrayList<>());
                    }
                    treeNode.getChildren().add(treeNode1);
                }
            });

        });
        return trees;
    }

     /**
     * 使用递归方法建树
     */
    public static List<TreeNode> recursiveTest(List<TreeNode> treeNodes) {
        List<TreeNode> trees = new ArrayList<>();
        treeNodes.forEach(treeNode -> {
            if ("0".equals(treeNode.getParentId())) {
                trees.add(findChildren(treeNode, treeNodes));
            }
        });
        return trees;
    }

    /**
     * 递归查找子节点
     */
   public static TreeNode findChildren(TreeNode treeNode, List<TreeNode> treeNodes) {
        treeNodes.forEach(treeNode1 -> {
            if (treeNode.getId().equals(treeNode1.getParentId())) {
                if (treeNode.getChildren() == null) {
                    treeNode.setChildren(new ArrayList<>());
                }
                treeNode.getChildren().add(findChildren(treeNode1, treeNodes));
            }
        });
        return treeNode;
    }

public static void main(String[] args) throws Exception{
        TreeNode treeNode1 = new TreeNode("1", "广州", "0");
        TreeNode treeNode2 = new TreeNode("2", "深圳", "0");

        TreeNode treeNode3 = new TreeNode("3", "天河区", treeNode1);
        TreeNode treeNode4 = new TreeNode("4", "越秀区", treeNode1);
        TreeNode treeNode5 = new TreeNode("5", "黄埔区", treeNode1);
        TreeNode treeNode6 = new TreeNode("6", "石牌", treeNode3);
        TreeNode treeNode7 = new TreeNode("7", "百脑汇", treeNode6);

        TreeNode treeNode8 = new TreeNode("8", "南山区", treeNode2);
        TreeNode treeNode9 = new TreeNode("9", "宝安区", treeNode2);
        TreeNode treeNode10 = new TreeNode("10", "科技园", treeNode8);

        List<TreeNode> list= Stream.of(treeNode1,treeNode2,treeNode3,treeNode4,treeNode5,treeNode6,treeNode7,treeNode8,treeNode9,treeNode10).collect(Collectors.toList());

        List<TreeNode> trees = foreachTest(list);
        trees.forEach(System.out::println);
        System.out.println("------------------华丽的分割线-----------------------");
        List<TreeNode> trees2 = recursiveTest(list);
        trees2.forEach(System.out::println);
    }