Java8 中的 Stream 轻松遍历树形结构

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我们在写代码的时候，一般会遇到构建树结构接口，大部分我们都是通过多个嵌套循环来实现，可读性非常差，在代码规范中我们要极可能避免嵌套代码。在这里我们可以使用 Java8 中的 Stream 流一次性把数据查出来，然后通过流式处理，这样代码简单已读，非常好维护。

一、创建实体类

这里我们创建一个需要生成树状的实体类。

package com.piao.document;

import lombok.Data;

import java.io.Serializable;
import java.util.List;

/**
 * @author piao
 * @date 2024/1/12
 */
@Data
public class Tree implements Serializable {

    private static final long serialVersionUID = 1L;

    private Integer id;

    private String name;

    private Integer parentId;

    private List<Tree> childList;

    public Tree(Integer id, String name, Integer parentId) {
        this.id = id;
        this.name = name;
        this.parentId = parentId;
    }

    public Tree(Integer id, String name, Integer parentId, List<Tree> childList) {
        this.id = id;
        this.name = name;
        this.parentId = parentId;
        this.childList = childList;
    }

}

二、编写树结构代码

这里我们就简单使用 mian 方法来跑。

package com.piao.document;

import com.alibaba.fastjson.JSON;

import java.util.Arrays;
import java.util.List;
import java.util.Objects;
import java.util.stream.Collectors;

/**
 * @author piao
 * @date 2024/1/12
 */
public class MyStart {

    public static void main(String[] args) {
        List<Tree> trees = Arrays.asList(
                new Tree(1, "根节点", 0),
                new Tree(2, "子节点1", 1),
                new Tree(3, "子节点1-1", 2),
                new Tree(4, "子节点1-2", 2),
                new Tree(5, "子节点1-3", 2),
                new Tree(6, "子节点2", 1),
                new Tree(7, "子节点2-1", 6),
                new Tree(8, "子节点2-2", 6),
                new Tree(9, "子节点2-2-1", 7),
                new Tree(10, "子节点3", 1),
                new Tree(11, "子节点3-1", 10),
                new Tree(12, "子节点3-1-1", 11),
                new Tree(13, "子节点3-1-2", 11)
        );

        List<Tree> list = trees.stream()
                .filter(t -> 0 == t.getParentId())
                .map(t -> {
                    t.setChildList(getChildrens(t, trees));
                    return t;
                }).collect(Collectors.toList());

        System.out.println(JSON.toJSON(list));
    }

    private static List<Tree> getChildrens(Tree root, List<Tree> all) {
        List<Tree> children = all.stream()
                .filter(t -> Objects.equals(t.getParentId(), root.getId()))
                .map(t -> {
                    t.setChildList(getChildrens(t, all));
                    return t;
                }).collect(Collectors.toList());

        return children;
    }

}

三、格式化打印结果

[
  {
    "name": "根节点",
    "childList": [
      {
        "name": "子节点1",
        "childList": [
          {
            "name": "子节点1-1",
            "childList": [],
            "id": 3,
            "parentId": 2
          },
          {
            "name": "子节点1-2",
            "childList": [],
            "id": 4,
            "parentId": 2
          },
          {
            "name": "子节点1-3",
            "childList": [],
            "id": 5,
            "parentId": 2
          }
        ],
        "id": 2,
        "parentId": 1
      },
      {
        "name": "子节点2",
        "childList": [
          {
            "name": "子节点2-1",
            "childList": [
              {
                "name": "子节点2-2-1",
                "childList": [],
                "id": 9,
                "parentId": 7
              }
            ],
            "id": 7,
            "parentId": 6
          },
          {
            "name": "子节点2-2",
            "childList": [],
            "id": 8,
            "parentId": 6
          }
        ],
        "id": 6,
        "parentId": 1
      },
      {
        "name": "子节点3",
        "childList": [
          {
            "name": "子节点3-1",
            "childList": [
              {
                "name": "子节点3-1-1",
                "childList": [],
                "id": 12,
                "parentId": 11
              },
              {
                "name": "子节点3-1-2",
                "childList": [],
                "id": 13,
                "parentId": 11
              }
            ],
            "id": 11,
            "parentId": 10
          }
        ],
        "id": 10,
        "parentId": 1
      }
    ],
    "id": 1,
    "parentId": 0
  }
]