Queue implementation using linked list

Queue Implementation using Linked List

Last Updated: 16 November, 2024 🕓 4 Mins Read

A Queue is a linear data structure to store and manipulate the data elements. A queue follows the concept of "First in, First out" (FIFO), where the first element inserted into the queue is the first one to be deleted from the queue.

A queue is open at both ends which enables insert operations to be performed at one end called REAR or TAIL and delete operations to be performed at another end called FRONT or HEAD.

Operations in Queue:

Example: Queue Implementation using Linked List in Java

  class Node {
    int data;       // Store value
    Node next;      // Stroe address of next node
  
    // Initializing a node with data in the constructor
    public Node(int data) {
      this.data = data;
      this.next = null;
    }
  }

  class Queue {
    // Pointer to the front and the rear of the linked list
    Node front, rear;
  
    // Constructor to initialize the front and rear
    Queue() {
      front = rear = null;
    }
  
    // Function to check the queue is empty
    boolean isEmpty() {
      if (front == null && rear == null) {
        System.out.println("Queue is Empty.");
        return true;
      }
      return false;
    }
  
    // Function to add an element to the queue
    void enqueue(int data) {
  
      // Create a new node
      Node node = new Node(data);
  
      // If queue is empty, the new node is both the front and rear
      if (rear == null) {
        front = rear = node;
        return;
      }
  
      // Add the new node at the end of the queue and change rear
      rear.next = node;
      rear = node;
    }
  
    // Function to remove an element from the queue
    void dequeue() {
      if (isEmpty()) {
        return;
      }
  
      // Store previous front and move front one node ahead
      front = front.next;
  
      // If front becomes null, then change rear also to null
      if (front == null) {
        rear = null;
      }
    }
  
    // Function to get the front element of the queue
    int getFront() {
      if (isEmpty()) {
        return Integer.MIN_VALUE;
      }
      return front.data;
    }
  
    // Function to get the rear element of the queue
    int getRear() {
      if (isEmpty()) {
        return Integer.MIN_VALUE;
      }
      return rear.data;
    }
  
    // Function to print queue elements
    void printQueue() {
      if (isEmpty()) {
        return;
      }
      while (front != null) {
        System.out.print(front.data + " ");
        // Move to the next node
        front = front.next;
      }
    }
  
  }
  
  public class QueueImplUsingLinkedList {
    public static void main(String[] args) {
      Queue queue = new Queue();
  
      // Enqueue (Insert) an elements into the queue
      queue.enqueue(10);
      queue.enqueue(20);
  
      System.out.println("Queue Front: " + queue.getFront());
      System.out.println("Queue Rear: " + queue.getRear());
  
      // Dequeue (Delete) an elements from the queue
      queue.dequeue();
      queue.dequeue();
  
      // Enqueue (Insert) an elements into the queue
      queue.enqueue(30);
      queue.enqueue(40);
      queue.enqueue(50);
  
      System.out.println("Queue Front: " + queue.getFront());
      System.out.println("Queue Rear: " + queue.getRear());
  
      // Print queue all elements of queue
      System.out.println("Queue elements: ");
      queue.printQueue();
    }
  }