Java - Doubly Linked List

A doubly linked list is a linear data structure, in which the elements are stored in the form of a node. Each node contains three sub-elements. A data part that stores the value of the element, the previous part that stores the link to the previous node, and the next part that stores the link to the next node as shown in the below image:

The first node also known as HEAD is always used as a reference to traverse the list. The previous of head node and next of last node points to NULL. A doubly linked list can be visualized as a chain of nodes, where every node points to previous and next node.

Implementation of Doubly Linked List

Representation:

In Java, circular doubly linked list can be represented as a class and a Node as a separate class. The LinkedList class contains a reference of Node class type.

//node structure
class Node {
    int data;
    Node next;
    Node prev;
};

class LinkedList {
  Node head;
  //constructor to create an empty LinkedList
  LinkedList(){
    head = null;
  }
};

Create a Doubly Linked List

Let us create a simple doubly linked list which contains three data nodes.

//node structure
class Node {
    int data;
    Node next;
    Node prev;
};

class LinkedList {
  Node head;
  //constructor to create an empty LinkedList
  LinkedList(){
    head = null;
  }  
};

// test the code 
public class Implementation { 
  public static void main(String[] args) {
    //create an empty LinkedList
    LinkedList MyList = new LinkedList();

    //Add first node.
    Node first = new Node();
    first.data = 10;
    first.next = null; 
    first.prev = null;
    //linking with head node
    MyList.head = first;

    //Add second node.
    Node second = new Node();
    second.data = 20;
    second.next = null; 
    //linking with first node
    second.prev = first;
    first.next = second;

    //Add third node.
    Node third = new Node();
    third.data = 30;
    third.next = null; 
    //linking with second node
    third.prev = second;
    second.next = third;
  }
}

Traverse a Doubly Linked List

A doubly linked list can be traversed using a temp node. Keep on moving the temp node to the next one and displaying its content. At the end of the list, the temp node will become NULL.

//node structure
class Node {
    int data;
    Node next;
    Node prev;
};

class LinkedList {
  Node head;
  //constructor to create an empty LinkedList
  LinkedList(){
    head = null;
  }

  //display the content of the list
  void PrintList() {
    Node temp = new Node();
    temp = this.head;
    if(temp != null) {
      System.out.print("The list contains: ");
      while(temp != null) {
        System.out.print(temp.data + " ");
        temp = temp.next;
      }
      System.out.println();
    } else {
      System.out.println("The list is empty.");
    }
  }    
};

// test the code 
public class Implementation { 
  public static void main(String[] args) {
    //create an empty LinkedList
    LinkedList MyList = new LinkedList();

    //Add first node.
    Node first = new Node();
    first.data = 10;
    first.next = null; 
    first.prev = null;
    //linking with head node
    MyList.head = first;

    //Add second node.
    Node second = new Node();
    second.data = 20;
    second.next = null; 
    //linking with first node
    second.prev = first;
    first.next = second;

    //Add third node.
    Node third = new Node();
    third.data = 30;
    third.next = null; 
    //linking with second node
    third.prev = second;
    second.next = third;

    //print the content of list
    MyList.PrintList();    
  }
}

The above code will give the following output:

The list contains: 10 20 30