• 一、前言
• 二、创建返回前端的实体类VO
• 三、List转树形方法
• 四、测试

Java8 Stream

一、前言

在日常开发中，必不可少的就是树形结构。有的时候可以使用Mybatis的ResultMap进行转化，有的时候需要进行Java进行手动转化为树结构。这里分享一个JDK8新特性Stream来实现List到树形结构的转化！

二、创建返回前端的实体类VO

import lombok.Data;
import java.util.List;
/**
 * @date 2022/2/28 10:28
 */
@Data
public class NodeVO {
    private String id;
    private String name;
    private String pid;
    private List<NodeVO> children;
    public NodeVO(String id, String name, String pid) {
        this.id = id;
        this.name = name;
        this.pid = pid;
    }
}

三、List转树形方法

public static List<NodeVO> streamToTree(List<NodeVO> treeList, String parentId) {
    List<NodeVO> list = treeList.stream()
            // 过滤父节点
            .filter(parent -> parent.getPid().equals(parentId))
            // 把父节点children递归赋值成为子节点
            .map(child -> {
                child.setChildren(streamToTree(treeList,child.getId()));
                return child;
            }).collect(Collectors.toList());
    return list;
}

四、测试

public static void main(String[] args) {
    NodeVO NodeVO1 = new NodeVO("1","山东省","0");
    NodeVO NodeVO2 = new NodeVO("2","青岛市","1");
    NodeVO NodeVO3 = new NodeVO("3","市北区","2");
    NodeVO NodeVO4 = new NodeVO("4","济南市","1");
    NodeVO NodeVO5 = new NodeVO("5","浙江省","0");
    NodeVO NodeVO6 = new NodeVO("6","杭州市","5");
    NodeVO NodeVO7 = new NodeVO("7","西湖区","6");
    List<NodeVO> list = new ArrayList<>();
    list.add(NodeVO1);
    list.add(NodeVO2);
    list.add(NodeVO3);
    list.add(NodeVO4);
    list.add(NodeVO5);
    list.add(NodeVO6);
    list.add(NodeVO7);
    // 默认父节点id为0
    List<NodeVO> nodeVOList = streamToTree(list,"0");
    System.out.println(JSON.toJSONString(nodeVOList));
}

结果展示：

[
    {
        "children": [
            {
                "children": [
                    {
                        "children": [],
                        "id": "3",
                        "name": "市北区",
                        "pid": "2"
                    }
                ],
                "id": "2",
                "name": "青岛市",
                "pid": "1"
            },
            {
                "children": [],
                "id": "4",
                "name": "济南市",
                "pid": "1"
            }
        ],
        "id": "1",
        "name": "山东省",
        "pid": "0"
    },
    {
        "children": [
            {
                "children": [
                    {
                        "children": [],
                        "id": "7",
                        "name": "西湖区",
                        "pid": "6"
                    }
                ],
                "id": "6",
                "name": "杭州市",
                "pid": "5"
            }
        ],
        "id": "5",
        "name": "浙江省",
        "pid": "0"
    }
]