Converting a List to a Tree Structure Using Java 8 Streams

In everyday development, handling hierarchical data structures is common, and while MyBatis resultMap can be used for conversion, a manual Java approach is sometimes needed; this article explores using the Java 8 Stream API to convert a list into a tree structure.

1. Define the VO class for the front‑end response

import lombok.Data;
import java.util.List;

/**
 * @author wangzhenjun
 * @date 2022/2/28 10:28
 */
@Data
public class NodeVO {
    private String id;
    private String name;
    private String pid;
    private List
children;

    public NodeVO(String id, String name, String pid) {
        this.id = id;
        this.name = name;
        this.pid = pid;
    }
}

2. Implement the list‑to‑tree conversion using Stream

public static List
streamToTree(List
treeList, String parentId) {
    List
list = treeList.stream()
        // filter parent nodes
        .filter(parent -> parent.getPid().equals(parentId))
        // recursively set children for each parent
        .map(child -> {
            child.setChildren(streamToTree(treeList, child.getId()));
            return child;
        })
        .collect(Collectors.toList());
    return list;
}

3. Test the conversion

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
list = new ArrayList<>();
    list.add(NodeVO1);
    list.add(NodeVO2);
    list.add(NodeVO3);
    list.add(NodeVO4);
    list.add(NodeVO5);
    list.add(NodeVO6);
    list.add(NodeVO7);
    // default parent id is 0
    List
nodeVOList = streamToTree(list, "0");
    System.out.println(JSON.toJSONString(nodeVOList));
}

4. Result output

[
  {
    "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"
  }
]

5. Summary

The Stream‑based approach provides a concise and elegant way to build hierarchical trees from flat lists, and although a simple double‑loop solution also works, the author prefers the readability and functional style of Java Streams.