Java 8 中的 Stream 轻松遍历树形结构

在平时的需求开发中，我们经常会遍历一些树形结构，数据库中使用父id来关联，为了降低数据库的查询压力，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。
实体类：Menu.java

package com.example.tree;import java.util.List;/*** 菜单实体类* @date: 2023/12/28**/
public class Menu {private Integer id;private String name;private Integer parentId;private List<Menu> childList;public Menu(Integer id, String name, Integer parentId) {this.id = id;this.name = name;this.parentId = parentId;}public Menu(Integer id, String name, Integer parentId, List<Menu> childList) {this.id = id;this.name = name;this.parentId = parentId;this.childList = childList;}public Integer getId() {return id;}public void setId(Integer id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}public Integer getParentId() {return parentId;}public void setParentId(Integer parentId) {this.parentId = parentId;}public List<Menu> getChildList() {return childList;}public void setChildList(List<Menu> childList) {this.childList = childList;}
}

模拟查看数据库所有数据到List里面
测试类：testTree.java

package com.example.tree;import com.alibaba.fastjson.JSON;import java.util.Arrays;
import java.util.List;
import java.util.Objects;
import java.util.stream.Collectors;/*** 测试树* @date: 2023/12/28**/
public class testTree {public static void main(String[] args) {List<Menu> menuList = Arrays.asList(new Menu(1,"根节点",0),new Menu(2,"子节点1",1),new Menu(3,"子节点1.1",2),new Menu(4,"子节点1.2",2),new Menu(5,"根节点1.3",2),new Menu(6,"根节点2",1),new Menu(7,"根节点2.1",6),new Menu(8,"根节点2.2",6),new Menu(9,"根节点2.2.1",7),new Menu(10,"根节点2.2.2",7),new Menu(11,"根节点3",1),new Menu(12,"根节点3.1",11));//获取父节点List<Menu> collect = menuList.stream().filter(m -> m.getParentId() == 0).map((m) -> {m.setChildList(getChildrens(m,menuList));return m;}).collect(Collectors.toList());System.out.println("============转json输出结果：");System.out.println(JSON.toJSON(collect));}/*** 递归查询子节点* @param root 根节点* @param all 所有节点* @return 根节点信息* @date 2023/12/28*/private static List<Menu> getChildrens(Menu root, List<Menu> all) {List<Menu> children = all.stream().filter(m -> {return Objects.equals(m.getParentId(),root.getId());}).map((m) -> {m.setChildList(getChildrens(m,all));return m;}).collect(Collectors.toList());return children;}
}

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":"根节点1.3","childList":[],"id":5,"parentId":2}],"id":2,"parentId":1},{"name":"根节点2","childList":[{"name":"根节点2.1","childList":[{"name":"根节点2.2.1","childList":[],"id":9,"parentId":7},{"name":"根节点2.2.2","childList":[],"id":10,"parentId":7}],"id":7,"parentId":6},{"name":"根节点2.2","childList":[],"id":8,"parentId":6}],"id":6,"parentId":1},{"name":"根节点3","childList":[{"name":"根节点3.1","childList":[],"id":12,"parentId":11}],"id":11,"parentId":1}],"id":1,"parentId":0}
]