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

Accepted answer throws a java.lang.StackOverflowError when calling the setParent or addChild methods.

Here's a slightly simpler implementation without those bugs:

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

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

    public void addChild(MyTreeNode child) {
        child.setParent(this);
        this.children.add(child);
    }

    public void addChild(T data) {
        MyTreeNode<T> newChild = new MyTreeNode<>(data);
        this.addChild(newChild);
    }

    public void addChildren(List<MyTreeNode> children) {
        for(MyTreeNode t : children) {
            t.setParent(this);
        }
        this.children.addAll(children);
    }

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

    public T getData() {
        return data;
    }

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

    private void setParent(MyTreeNode parent) {
        this.parent = parent;
    }

    public MyTreeNode getParent() {
        return parent;
    }
}

Some examples:

MyTreeNode<String> root = new MyTreeNode<>("Root");

MyTreeNode<String> child1 = new MyTreeNode<>("Child1");
child1.addChild("Grandchild1");
child1.addChild("Grandchild2");

MyTreeNode<String> child2 = new MyTreeNode<>("Child2");
child2.addChild("Grandchild3");

root.addChild(child1);
root.addChild(child2);
root.addChild("Child3");

root.addChildren(Arrays.asList(
        new MyTreeNode<>("Child4"),
        new MyTreeNode<>("Child5"),
        new MyTreeNode<>("Child6")
));

for(MyTreeNode node : root.getChildren()) {
    System.out.println(node.getData());
}

Answer 2

In the accepted answer

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

should be

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

otherwise the parent does not have the child in its children list

Answer 3

The process of assembling tree nodes is similar to the process of assembling lists. We have a constructor for tree nodes that initializes the instance variables.

public Tree (Object cargo, Tree left, Tree right) { 
    this.cargo = cargo; 
    this.left = left; 
    this.right = right; 
} 

We allocate the child nodes first:

Tree left = new Tree (new Integer(2), null, null); 
Tree right = new Tree (new Integer(3), null, null); 

We can create the parent node and link it to the children at the same time:

Tree tree = new Tree (new Integer(1), left, right); 

Answer 4

Here is my implementation in java for your requirement. In the treeNode class i used generic array to store the tree data. we can also use arraylist or dynamic array to store the tree value.

public class TreeNode<T> {
   private T value = null;
   private TreeNode[] childrens = new TreeNode[100];
   private int childCount = 0;

    TreeNode(T value) {
        this.value = value;
    }

    public TreeNode addChild(T value) {
        TreeNode newChild = new TreeNode(value, this);
        childrens[childCount++] = newChild;
        return newChild;
    }

    static void traverse(TreeNode obj) {
        if (obj != null) {
            for (int i = 0; i < obj.childCount; i++) {
                System.out.println(obj.childrens[i].value);
                traverse(obj.childrens[i]);
            }
        }
        return;
    }

    void printTree(TreeNode obj) {
        System.out.println(obj.value);
        traverse(obj);
    }
}

And the client class for the above implementation.

public class Client {
    public static void main(String[] args) {
        TreeNode menu = new TreeNode("Menu");
        TreeNode item = menu.addChild("Starter");
            item = item.addChild("Veg");
                item.addChild("Paneer Tikka");
                item.addChild("Malai Paneer Tikka");
            item = item.addChild("Non-veg");
                item.addChild("Chicken Tikka");
                item.addChild("Malai Chicken Tikka");
        item = menu.addChild("Main Course");
            item = item.addChild("Veg");
                item.addChild("Mili Juli Sabzi");
                item.addChild("Aloo Shimla Mirch");
            item = item.addChild("Non-veg");
                item.addChild("Chicken Do Pyaaza");
                item.addChild("Chicken Chettinad");
        item = menu.addChild("Desserts");
                item = item.addChild("Cakes");
                        item.addChild("Black Forest");
                        item.addChild("Black Current");
                item = item.addChild("Ice Creams");
                        item.addChild("chocolate");
                        item.addChild("Vanilla");
        menu.printTree(menu);
    }
}

OUTPUT

Menu                                                                     
Starter                                                                 
Veg                                                                
Paneer Tikka                                                        
Malai Paneer Tikka                                                    
Non-veg                                                              
Chicken Tikka                                                      
Malai Chicken Tikka                                                 
Main Course                                                      
Veg                                                             
Mili Juli Sabzi                                                   
Aloo Shimla Mirch                                                  
Non-veg                                                               
Chicken Do Pyaaza                                                   
Chicken Chettinad                                                    
Desserts                                                         
Cakes                                                            
Black Forest                                                     
Black Current                                                   
Ice Creams                                                      
chocolate                                                       
Vanilla           

Answer 5

This tree is not a binary tree, so you need an array of the children elements, like List.

public Node(Object data, List<Node> children) {
    this.data = data;
    this.children = children;
}

Then create the instances.

Answer 6

In answer ,it creates circular dependency.This can be avoided by removing parent inside Child nodes. i.e,

public class MyTreeNode<T>{     


    private T data = null;
    private List<MyTreeNode> children = new ArrayList<>();


    public MyTreeNode(T data) {
        this.data = data;

    }

    public void addChild(MyTreeNode child) {
        this.children.add(child);
    }

    public void addChild(T data) {
        MyTreeNode<T> newChild = new MyTreeNode<>(data);
        children.add(newChild);
    }

    public void addChildren(List<MyTreeNode> children) {
        this.children.addAll(children);
    }

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

    public T getData() {
        return data;
    }

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


}

Using the same example specified above,the output will be like this:

{ "data": "Root", "children": [ { "data": "Child1", "children": [ { "data": "Grandchild1", "children": [] }, { "data": "Grandchild2", "children": [] } ] }, { "data": "Child2", "children": [ { "data": "Grandchild3", "children": [] } ] }, { "data": "Child3", "children": [] }, { "data": "Child4", "children": [] }, { "data": "Child5", "children": [] }, { "data": "Child6", "children": [] } ] }