Java program to convert a given binary tree to doubly linked list

public class BinaryTreeToDLL {    
       //Represent a node of the binary tree    
    public static class Node{    
        int data;    
        Node left;    
        Node right;    
            
        public Node(int data) {    
            this.data = data;    
            this.left = null;    
            this.right = null;    
        }    
    }    
        
    //Represent the root of the binary tree    
    
    public Node root;    
          
    //Represent the head and tail of the doubly linked list    
    Node head, tail = null;    
        
    //convertbtToDLL() will convert the given binary tree to corresponding doubly linked list    
    public void convertbtToDLL(Node node) {    
        //Checks whether node is null    
        if(node == null)    
            return;    
            
        //Convert left subtree to doubly linked list    
        convertbtToDLL(node.left);    
            
        //If list is empty, add node as head of the list    
        if(head == null) {    
            //Both head and tail will point to node    
            head = tail = node;    
        }    
        //Otherwise, add node to the end of the list    
        else {    
            //node will be added after tail such that tail's right will point to node    
            tail.right = node;    
            //node's left will point to tail    
            node.left = tail;    
            //node will become new tail    
            tail = node;    
        }    
            
        //Convert right subtree to doubly linked list    
        convertbtToDLL(node.right);    
    }    
        
    //display() will print out the nodes of the list    
    public void display() {    
        //Node current will point to head    
        Node current = head;    
        if(head == null) {    
            System.out.println("List is empty");    
            return;    
        }    
        System.out.println("Nodes of generated doubly linked list: ");    
        while(current != null) {    
            //Prints each node by incrementing the pointer.    
    
            System.out.print(current.data + " ");    
            current = current.right;    
        }    
        System.out.println();    
    }    
        
    public static void main(String[] args) {    
            
        BinaryTreeToDLL bt = new BinaryTreeToDLL();    
        //Add nodes to the binary tree    
        bt.root = new Node(1);    
        bt.root.left = new Node(2);    
        bt.root.right = new Node(3);    
        bt.root.left.left = new Node(4);    
        bt.root.left.right = new Node(5);    
        bt.root.right.left = new Node(6);    
        bt.root.right.right = new Node(7);    
            
        //Converts the given binary tree to doubly linked list    
        bt.convertbtToDLL(bt.root);    
            
        //Displays the nodes present in the list    
        bt.display();    
    }    
}