public class Tree {
    private List<Tree> leaves = new LinkedList<Tree>();
    private Tree parent = null;
    private String data;

    public Tree(String data, Tree parent) {
        this.data = data;
        this.parent = parent;
    }
}

显然，您可以添加实用工具方法来添加/删除子元素。

不使用Collection框架的Tree的自定义树实现。 它包含Tree实现所需的不同基本操作。

class Node {

    int data;
    Node left;
    Node right;

    public Node(int ddata, Node left, Node right) {
        this.data = ddata;
        this.left = null;
        this.right = null;      
    }

    public void displayNode(Node n) {
        System.out.print(n.data + " "); 
    }

}

class BinaryTree {

    Node root;

    public BinaryTree() {
        this.root = null;
    }

    public void insertLeft(int parent, int leftvalue ) {
        Node n = find(root, parent);
        Node leftchild = new Node(leftvalue, null, null);
        n.left = leftchild;
    }

    public void insertRight(int parent, int rightvalue) {
        Node n = find(root, parent);
        Node rightchild = new Node(rightvalue, null, null);
        n.right = rightchild;
    }

    public void insertRoot(int data) {
        root = new Node(data, null, null);
    }

    public Node getRoot() {
        return root;
    }

    public Node find(Node n, int key) {     
        Node result = null;

        if (n == null)
            return null;

        if (n.data == key)
            return n;

        if (n.left != null)
            result = find(n.left, key);

        if (result == null)
            result = find(n.right, key);

        return result;
    } 

    public int getheight(Node root){
        if (root == null)
            return 0;

        return Math.max(getheight(root.left), getheight(root.right)) + 1; 
    }

    public void printTree(Node n) {     
        if (n == null)
            return;

        printTree(n.left);
        n.displayNode(n);
        printTree(n.right);             
    }

}

import java.util.Collection;
import java.util.LinkedList;
import java.util.function.BiConsumer;
import java.util.function.Function;

/**
 * @author changjin wei(魏昌进)
 * @since 2021/7/15
 */
public class TreeUtils {

    private TreeUtils() {
    }

    /**
     * @param collection this is a collection of elements
     * @param getId this is a getId Function
     * @param getParentId this is a getParentId Function
     * @param setNode this is a setNode BiConsumer
     * @param <E> the type of elements in this collection
     * @param <R> the type of the result of the function
     *
     * @return Collection
     */
    public static <E, R> Collection<E> tree(Collection<E> collection, Function<E, R> getId, Function<E, R> getParentId, BiConsumer<E, Collection<E>> setNode) {
        Collection<E> root = new LinkedList<>();
        for (E node : collection) {
            R parentId = getParentId.apply(node);
            R id = getId.apply(node);
            Collection<E> elements = new LinkedList<>();
            boolean isParent = true;
            for (E element : collection) {
                if (id.equals(getParentId.apply(element))) {
                    elements.add(element);
                }
                if (isParent && getId.apply(element).equals(parentId)) {
                    isParent = false;
                }
            }
            if (isParent) {
                root.add(node);
            }
            setNode.accept(node, elements);
        }
        return root;
    }
}

例如:

import java.util.ArrayList;
import java.util.List;



/**
 * 
 * @author X2
 *
 * @param <T>
 */
public class HisTree<T> 
{
    private Node<T> root;

    public HisTree(T rootData) 
    {
        root = new Node<T>();
        root.setData(rootData);
        root.setChildren(new ArrayList<Node<T>>());
    }

}

class Node<T> 
{

    private T data;
    private Node<T> parent;
    private List<Node<T>> children;

    public T getData() {
        return data;
    }
    public void setData(T data) {
        this.data = data;
    }
    public Node<T> getParent() {
        return parent;
    }
    public void setParent(Node<T> parent) {
        this.parent = parent;
    }
    public List<Node<T>> getChildren() {
        return children;
    }
    public void setChildren(List<Node<T>> children) {
        this.children = children;
    }
}

public class Tree {
    private List<Tree> leaves = new LinkedList<Tree>();
    private Tree parent = null;
    private String data;

    public Tree(String data, Tree parent) {
        this.data = data;
        this.parent = parent;
    }
}

显然，您可以添加实用工具方法来添加/删除子元素。

这个呢?

import java.util.ArrayList;
import java.util.Collection;
import java.util.HashMap;

/**
  * @author ycoppel@google.com (Yohann Coppel)
  * 
  * @param <T>
  *          Object's type in the tree.
*/
public class Tree<T> {

  private T head;

  private ArrayList<Tree<T>> leafs = new ArrayList<Tree<T>>();

  private Tree<T> parent = null;

  private HashMap<T, Tree<T>> locate = new HashMap<T, Tree<T>>();

  public Tree(T head) {
    this.head = head;
    locate.put(head, this);
  }

  public void addLeaf(T root, T leaf) {
    if (locate.containsKey(root)) {
      locate.get(root).addLeaf(leaf);
    } else {
      addLeaf(root).addLeaf(leaf);
    }
  }

  public Tree<T> addLeaf(T leaf) {
    Tree<T> t = new Tree<T>(leaf);
    leafs.add(t);
    t.parent = this;
    t.locate = this.locate;
    locate.put(leaf, t);
    return t;
  }

  public Tree<T> setAsParent(T parentRoot) {
    Tree<T> t = new Tree<T>(parentRoot);
    t.leafs.add(this);
    this.parent = t;
    t.locate = this.locate;
    t.locate.put(head, this);
    t.locate.put(parentRoot, t);
    return t;
  }

  public T getHead() {
    return head;
  }

  public Tree<T> getTree(T element) {
    return locate.get(element);
  }

  public Tree<T> getParent() {
    return parent;
  }

  public Collection<T> getSuccessors(T root) {
    Collection<T> successors = new ArrayList<T>();
    Tree<T> tree = getTree(root);
    if (null != tree) {
      for (Tree<T> leaf : tree.leafs) {
        successors.add(leaf.head);
      }
    }
    return successors;
  }

  public Collection<Tree<T>> getSubTrees() {
    return leafs;
  }

  public static <T> Collection<T> getSuccessors(T of, Collection<Tree<T>> in) {
    for (Tree<T> tree : in) {
      if (tree.locate.containsKey(of)) {
        return tree.getSuccessors(of);
      }
    }
    return new ArrayList<T>();
  }

  @Override
  public String toString() {
    return printTree(0);
  }

  private static final int indent = 2;

  private String printTree(int increment) {
    String s = "";
    String inc = "";
    for (int i = 0; i < increment; ++i) {
      inc = inc + " ";
    }
    s = inc + head;
    for (Tree<T> child : leafs) {
      s += "\n" + child.printTree(increment + indent);
    }
    return s;
  }
}