java 8 stream api将List＜T＞转换成树形结构

1、新建实体类

package com.example.springboot3.entity;

import lombok.Builder;
import lombok.Data;

import java.util.List;

@Data
@Builder
public class Menu {
    /**
     * id
     */
    public Integer id;
    /**
     * 名称
     */
    public String name;
    /**
     * 父id ，根节点为0
     */
    public Integer parentId;
    /**
     * 子节点信息
     */
    public List children;


    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 children) {
        this.id = id;
        this.name = name;
        this.parentId = parentId;
        this.children = children;
    }
}

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2、controller测试，正式环境可到service进行分层

package com.example.springboot3.controller;

import com.example.springboot3.entity.Menu;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

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

@RestController
public class MenuController {

    @GetMapping("/getMenuTree")
    public List getMenuTree(){
        //模拟从数据库查询出来
        List menus = 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 collect = menus.stream().filter(m -> m.getParentId() == 0).map(
                (m) -> {
                    m.setChildren(getChildrens(m, menus));
                    return m;
                }
        ).collect(Collectors.toList());
        return collect;
    }

    private List getChildrens(Menu root, List all) {
        List children = all.stream().filter(m ->
                Objects.equals(m.getParentId(), root.getId())
        ).map(
                (m) -> {
                    m.setChildren(getChildrens(m, all));
                    return m;
                }
        ).collect(Collectors.toList());
        return children;
    }


}

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3、结果