构建树形结构

@Data
public class TreeNode {
    private Long id;
    private Long pid;
    private String name;
    private List<TreeNode> children = new ArrayList<>();

    public TreeNode(Long id, Long pid) {
        this.id = id;
        this.pid = pid;
    }

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

遍历数据构建树

public class TreeUtil {

    public static List<TreeNode> buildTree(List<TreeNode> nodes) {
        //先选出非顶级的节点
        List<TreeNode> list = nodes.stream().filter(node -> node.getPid() != null).collect(Collectors.toList());

        //将这些非顶级节点的数据按pid进行分组
        Map<Long, List<TreeNode>> sub = list.stream().collect(Collectors.groupingBy(node -> node.getPid()));

        //循环设置对应的子节点（根据id = pid）
        nodes.forEach(node -> node.setChildren(sub.get(node.getId())));

        //只留下顶级节点，自动成树形结构
        List<TreeNode> collect = nodes.stream().filter(node -> node.getPid() == null).collect(Collectors.toList());
        return collect;
    }
}