How to implement Doubly Linked List in Java

In this source code example, we will write a Java program to demonstrate how to implement Doubly Linked List in Java.

A doubly linked list is a linear data structure that consists of a sequence of nodes, where each node stores a reference to an element and references to the previous and next nodes in the sequence. The nodes are linked together in both directions, so it is called a doubly linked list.

Doubly Linked List Overview

Doubly Linked List Implementation in Java

import java.util.NoSuchElementException;

public class DoublyLinkedList {

   private ListNode head;
   private ListNode tail;
   private int length;

   private class ListNode {
      private int data; // Can be any generic type
      private ListNode next;
      private ListNode previous;

      public ListNode(int data) {
         this.data = data;
      }
   }

   public DoublyLinkedList() {
      this.head = null;
      this.tail = null;
      this.length = 0;
   }

   public boolean isEmpty() {
      return length == 0; // or head == null
   }

   public int length() {
      return length;
   }

   public void displayForward() {
      if (head == null) {
         return;
      }

      ListNode temp = head;
      while (temp != null) {
         System.out.print(temp.data + " --> ");
         temp = temp.next;
      }
      System.out.println("null");
   }

   public void displayBackward() {
      if (head == null) {
         return;
      }

      ListNode temp = tail;
      while (temp != null) {
         System.out.print(temp.data + " --> ");
         temp = temp.previous;
      }
      System.out.println("null");
   }

   public void insertFirst(int value) {
      ListNode newNode = new ListNode(value);
      if (isEmpty()) {
         tail = newNode;
      } else {
         head.previous = newNode;
      }
      newNode.next = head;
      head = newNode;
      length++;
   }

   public void insertEnd(int value) {
      ListNode newNode = new ListNode(value);
      if (isEmpty()) {
         head = newNode;
      } else {
         tail.next = newNode;
         newNode.previous = tail;
      }
      tail = newNode;
      length++;
   }

   public ListNode deleteFirst() {
      if (isEmpty()) {
         throw new NoSuchElementException();
      }

      ListNode temp = head;
      if (head == tail) {
         tail = null;
      } else {
         head.next.previous = null;
      }
      head = head.next;
      temp.next = null;
      length--;
      return temp;
   }

   public ListNode deleteLast() {
      if (isEmpty()) {
         throw new NoSuchElementException();
      }

      ListNode temp = tail;
      if (head == tail) {
         head = null;
      } else {
         tail.previous.next = null;
      }
      tail = tail.previous;
      temp.previous = null;
      length--;
      return temp;
   }

   public static void main(String[] args) {
      DoublyLinkedList dll = new DoublyLinkedList();
      dll.insertEnd(1);
      dll.insertEnd(2);
      dll.insertEnd(3);

      dll.displayForward();

      dll.deleteLast();
      dll.deleteLast();

      dll.displayForward();
   }
}

Output:

1 --> 2 --> 3 --> null
1 --> null

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