Java implements public entity class to Tree structure

import .*;
import ;
import ;
 
// Tree node class, used to store each element in the treeclass TreeNode<T> {
    private T data; // Data stored by nodes    private List<TreeNode<T>> children = new ArrayList<>(); // List of child nodes 
    // Constructor, initialize node data    public TreeNode(T data) {
         = data;
    }
 
    // Add child nodes    public void addChild(TreeNode<T> child) {
        (child);
    }
 
    // Get the data of the current node    public T getData() {
        return data;
    }
 
    // Get all children of the current node    public List<TreeNode<T>> getChildren() {
        return children;
    }
}
 
// Tree builder class, responsible for building tree structures from listsclass TreeBuilder<T> {
    // buildTree method to build a tree structure    public TreeNode<T> buildTree(List<T> items, Function<T, String> idExtractor, Function<T, String> parentIdExtractor) {
        // Create a map to map the ID of each node to the corresponding TreeNode object        Map<String, TreeNode<T>> nodeMap = ()
                .map(item -> new TreeNode<>(item)) // Convert each element to TreeNode                .collect((node -> (()), ()));
 
        TreeNode<T> root = null; // Used to store the root node        
        // traverse each element and establish a parent-child relationship        for (T item : items) {
            String id = (item); // Get the ID of the current node            String parentId = (item); // Get the parent node's ID            TreeNode<T> node = (id); // Get the current node            
            if (parentId == null) {
                root = node; // Assume that there is only one root node            } else {
                TreeNode<T> parentNode = (parentId); // Get the parent node                if (parentNode != null) {
                    (node); // Add the current node to the parent node's child node list                }
            }
        }
        
        return root; // Return the root node of the built tree    }
}

import ;
import ;
 
// Entity class, used to represent the data of the tree nodeclass Entity {
    private String id;         // Node ID    private String parentId;   // Parent node ID    private String name;       // Node name 
    // Constructor    public Entity(String id, String parentId, String name) {
         = id;
         = parentId;
         = name;
    }
 
    // Get the node ID    public String getId() {
        return id;
    }
 
    // Get the parent node ID    public String getParentId() {
        return parentId;
    }
 
    // Get the node name    public String getName() {
        return name;
    }
}
 
// Main programpublic class Main {
    public static void main(String[] args) {
        // Create entity list and simulate tree structure        List<Entity> entities = (
            new Entity("1", null, "Root"),         // Root node            new Entity("2", "1", "Child 1"),       // Child 1, the parent node is Root            new Entity("3", "1", "Child 2"),       // Child 2, the parent node is Root            new Entity("4", "2", "Grandchild 1")   // Grandchild 1, the parent node is Child 1        );
 
        // Create a tree builder instance        TreeBuilder<Entity> treeBuilder = new TreeBuilder<>();
        
        // Build a tree, pass in the entity list and extract the ID and parent ID functions        TreeNode<Entity> tree = (
            entities,
            Entity::getId,        // Extract ID            Entity::getParentId   // Extract parent ID        );
 
        // Print tree structure        printTree(tree, 0);
    }
 
    // Helper method for recursively printing tree structure    private static void printTree(TreeNode<Entity> node, int level) {
        if (node == null) return; // If the node is empty, return directly 
        // Print the name of the current node and indent the display level        ("  ".repeat(level) + ().getName());
        
        // Recursively print all child nodes        for (TreeNode<Entity> child : ()) {
            printTree(child, level + 1);
        }
    }
}