Tree implementation in Java (root, parents and children)

Question

I need to create a tree structure similar as the attached image in Java. I\'ve found some questions related to this one but I haven\'t found a convincing and well explained

Answer 1

Since @Jonathan's answer still consisted of some bugs, I made an improved version. I overwrote the toString() method for debugging purposes, be sure to change it accordingly to your data.

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

/**
 * Provides an easy way to create a parent-->child tree while preserving their depth/history.
 * Original Author: Jonathan, https://stackoverflow.com/a/22419453/14720622
 */
public class TreeNode<T> {
    private final List<TreeNode<T>> children;
    private TreeNode<T> parent;
    private T data;
    private int depth;

    public TreeNode(T data) {
        // a fresh node, without a parent reference
        this.children = new ArrayList<>();
        this.parent = null;
        this.data = data;
        this.depth = 0; // 0 is the base level (only the root should be on there)
    }

    public TreeNode(T data, TreeNode<T> parent) {
        // new node with a given parent
        this.children = new ArrayList<>();
        this.data = data;
        this.parent = parent;
        this.depth = (parent.getDepth() + 1);
        parent.addChild(this);
    }

    public int getDepth() {
        return this.depth;
    }

    public void setDepth(int depth) {
        this.depth = depth;
    }

    public List<TreeNode<T>> getChildren() {
        return children;
    }

    public void setParent(TreeNode<T> parent) {
        this.setDepth(parent.getDepth() + 1);
        parent.addChild(this);
        this.parent = parent;
    }

    public TreeNode<T> getParent() {
        return this.parent;
    }

    public void addChild(T data) {
        TreeNode<T> child = new TreeNode<>(data);
        this.children.add(child);
    }

    public void addChild(TreeNode<T> child) {
        this.children.add(child);
    }

    public T getData() {
        return this.data;
    }

    public void setData(T data) {
        this.data = data;
    }

    public boolean isRootNode() {
        return (this.parent == null);
    }

    public boolean isLeafNode() {
        return (this.children.size() == 0);
    }

    public void removeParent() {
        this.parent = null;
    }

    @Override
    public String toString() {
        String out = "";
        out += "Node: " + this.getData().toString() + " | Depth: " + this.depth + " | Parent: " + (this.getParent() == null ? "None" : this.parent.getData().toString()) + " | Children: " + (this.getChildren().size() == 0 ? "None" : "");
        for(TreeNode<T> child : this.getChildren()) {
            out += "\n\t" + child.getData().toString() + " | Parent: " + (child.getParent() == null ? "None" : child.getParent().getData());
        }
        return out;
    }
}

And for the visualization:

import model.TreeNode;

/**
 * Entrypoint
 */
public class Main {
    public static void main(String[] args) {
        TreeNode<String> rootNode = new TreeNode<>("Root");
        TreeNode<String> firstNode = new TreeNode<>("Child 1 (under Root)", rootNode);
        TreeNode<String> secondNode = new TreeNode<>("Child 2 (under Root)", rootNode);
        TreeNode<String> thirdNode = new TreeNode<>("Child 3 (under Child 2)", secondNode);
        TreeNode<String> fourthNode = new TreeNode<>("Child 4 (under Child 3)", thirdNode);
        TreeNode<String> fifthNode = new TreeNode<>("Child 5 (under Root, but with a later call)");
        fifthNode.setParent(rootNode);

        System.out.println(rootNode.toString());
        System.out.println(firstNode.toString());
        System.out.println(secondNode.toString());
        System.out.println(thirdNode.toString());
        System.out.println(fourthNode.toString());
        System.out.println(fifthNode.toString());
        System.out.println("Is rootNode a root node? - " + rootNode.isRootNode());
        System.out.println("Is firstNode a root node? - " + firstNode.isRootNode());
        System.out.println("Is thirdNode a leaf node? - " + thirdNode.isLeafNode());
        System.out.println("Is fifthNode a leaf node? - " + fifthNode.isLeafNode());
    }
}

Example output:

Node: Root | Depth: 0 | Parent: None | Children: 
    Child 1 (under Root) | Parent: Root
    Child 2 (under Root) | Parent: Root
    Child 5 (under Root, but with a later call) | Parent: Root
Node: Child 1 (under Root) | Depth: 1 | Parent: Root | Children: None
Node: Child 2 (under Root) | Depth: 1 | Parent: Root | Children: 
    Child 3 (under Child 2) | Parent: Child 2 (under Root)
Node: Child 3 (under Child 2) | Depth: 2 | Parent: Child 2 (under Root) | Children: 
    Child 4 (under Child 3) | Parent: Child 3 (under Child 2)
Node: Child 4 (under Child 3) | Depth: 3 | Parent: Child 3 (under Child 2) | Children: None
Node: Child 5 (under Root, but with a later call) | Depth: 1 | Parent: Root | Children: None
Is rootNode a root node? - true
Is firstNode a root node? - false
Is thirdNode a leaf node? - false
Is fifthNode a leaf node? - true

Answer 2

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

public class Node<T> {
    private List<Node<T>> children = new ArrayList<Node<T>>();
    private Node<T> parent = null;
    private T data = null;

    public Node(T data) {
        this.data = data;
    }

    public Node(T data, Node<T> parent) {
        this.data = data;
        this.parent = parent;
    }

    public List<Node<T>> getChildren() {
        return children;
    }

    public void setParent(Node<T> parent) {
        parent.addChild(this);
        this.parent = parent;
    }

    public void addChild(T data) {
        Node<T> child = new Node<T>(data);
        child.setParent(this);
        this.children.add(child);
    }

    public void addChild(Node<T> child) {
        child.setParent(this);
        this.children.add(child);
    }

    public T getData() {
        return this.data;
    }

    public void setData(T data) {
        this.data = data;
    }

    public boolean isRoot() {
        return (this.parent == null);
    }

    public boolean isLeaf() {
        return this.children.size == 0;
    }

    public void removeParent() {
        this.parent = null;
    }
}

Example:

import java.util.List;

Node<String> parentNode = new Node<String>("Parent"); 
Node<String> childNode1 = new Node<String>("Child 1", parentNode);
Node<String> childNode2 = new Node<String>("Child 2");     

childNode2.setParent(parentNode); 

Node<String> grandchildNode = new Node<String>("Grandchild of parentNode. Child of childNode1", childNode1); 
List<Node<String>> childrenNodes = parentNode.getChildren();