Java List与树的互转

　　平时工作中都会遇到包含层级关系的List数据转换成树形结构，或者数据已是树形结构了，需要我们处理成普通的单层list结构。以下代码均为本人实际开发所写代码，可能不是最优解、复杂度也比较高，在此和大家一起分享学习！

　　注：该工具类支持将list转换成树/森林。可自行测试，有疑问或更优方案，可私聊我。

 

　　TreeNode

@Data
@JsonInclude(JsonInclude.Include.NON_NULL)
public class TreeNode<T> {

    private String id;

    private String key;

    private String value;

　　 private String name;

    private String parentId;

    private String createName;

    private LocalDateTime createTime;

    private List<TreeNode<T>> children;

    private boolean hasParent = false;

    private boolean hasChildren = false;

    public void initChildren() {
        this.children = new ArrayList<>();
    }

}

TreeUtil

public class TreeUtil {

    protected TreeUtil() {

    }

    private final static String TOP_NODE_ID = "0";



    /**
     * 使用递归方法建树
     *
     * @param nodes
     * @return
     */
    public static List<TreeNode<T>> buildByRecursive(List<TreeNode<T>> nodes) {
        List<TreeNode<T>> trees = new ArrayList<>();
        for (TreeNode<T> treeNode : nodes) {
            if (TOP_NODE_ID.equals(treeNode.getParentId())) {
                trees.add(findChildren(treeNode,nodes));
            }
        }
        return trees;
    }

    /**
     * 递归查找子节点
     *
     * @param treeNodes
     * @return
     */
    public static TreeNode<T> findChildren(TreeNode<T> treeNode, List<TreeNode<T>> treeNodes) {
        for (TreeNode<T> it : treeNodes) {
            if (treeNode.getId().equals(it.getParentId())) {
                if (treeNode.getChildren() == null) {
                    treeNode.setChildren(new ArrayList<TreeNode<T>>());
                }
                treeNode.getChildren().add(findChildren(it, treeNodes));
            }
        }
        return treeNode;
    }

    public static void buildTrees(List<TreeNode<T>> trees, List<T> list) {
        list.forEach(entity -> {
            TreeNode<T> tree = new TreeNode<>();
            tree.setId(entity.getId());
            tree.setKey(entity.getId());
            tree.setValue(entity.getId());
            tree.setName(entity.getName());
            tree.setParentId(entity.getParentId());
            tree.setCreateTime(entity.getCreateTime());
            tree.setCreateName(entity.getCreateName());
            trees.add(tree);
        });
    }

    public static <T> List<TreeNode<T>> buildTreeList(List<TreeNode<T>> nodes) {
        if (nodes == null) {
            return null;
        }
        List<TreeNode<T>> topNodes = new ArrayList<>();
        //利用两层循环，组装树/森林结构
        nodes.forEach(first -> {
            String pid = first.getParentId();
            first.setHasParent(true);
            if (pid == null || TOP_NODE_ID.equals(pid)) {
                topNodes.add(first);
                first.setHasParent(false);
            } else {
                boolean isExistPid = false;
                for (TreeNode<T> temp : nodes) {
                    if (temp.getId().equals(pid)) {
                        isExistPid = true;
                    }
                }
                if (!isExistPid) {
                    topNodes.add(first);
                    first.setHasParent(false);
                }
            }
            for (TreeNode<T> second : nodes) {
                String id = second.getId();
                if (id != null && id.equals(pid)) {
                    if (second.getChildren() == null) {
                        second.initChildren();
                    }
                    second.getChildren().add(first);
                    first.setHasParent(true);
                    second.setHasChildren(true);
                    second.setHasParent(true);
                    return;
                }
            }
        });
        return topNodes;
    }

    public static <T> TreeNode<T> buildTree(List<TreeNode<T>> nodes) {
        if (nodes == null) {
            return null;
        }
        List<TreeNode<T>> topNodes = new ArrayList<>();
        //利用两层循环，组装树/森林结构
        nodes.forEach(first -> {
            String pid = first.getParentId();
            first.setHasParent(true);
            if (pid == null || TOP_NODE_ID.equals(pid)) {
                topNodes.add(first);
                first.setHasParent(false);
            } else {
                boolean isExistPid = false;
                for (TreeNode<T> temp : nodes) {
                    if (temp.getId().equals(pid)) {
                        isExistPid = true;
                    }
                }
                if (!isExistPid) {
                    topNodes.add(first);
                    first.setHasParent(false);
                }
            }
            for (TreeNode<T> second : nodes) {
                String id = second.getId();
                if (id != null && id.equals(pid)) {
                    if (second.getChildren() == null) {
                        second.initChildren();
                    }
                    second.getChildren().add(first);
                    first.setHasParent(true);
                    second.setHasChildren(true);
                    second.setHasParent(true);
                    return;
                }
            }
        });
        TreeNode<T> root = new TreeNode<>();
        root.setId("0");
        root.setParentId("");
        root.setHasParent(false);
        root.setHasChildren(true);
        root.setChildren(topNodes);
        root.setName("root");
        return root;
    }


    public static <T> List<TreeNode<T>> tree2List(List<TreeNode<T>>  treeList) {
        List<TreeNode<T>> list = new ArrayList<>();
        for (TreeNode<T> tree : treeList) {
            List<TreeNode<T>> child = tree.getChildren();
            list.add(tree);
            if (child != null && child.size() > 0) {
                list.addAll(tree2List(child));
                tree.setChildren(null);
            }
        }
        return list;
    }
}