Queue Implementation Using Linked List in Java

Last Updated : 27 May, 2024

Queue is the linear data structure that follows the First In First Out(FIFO) principle where the elements are added at the one end, called the rear, and removed from the other end, called the front. Using the linked list to implement the queue allows for dynamic memory utilization, avoiding the constraints of the fixed-size data structure like an array-based queue.

Organization of Queue Using Linked List

In the linked list-based queue, each node contains two parts: the data and the pointer to the next node. The queue has two main pointers:

Front: The front can point to the first node of the linked list.
Rear: Rear can point to the last node of the linked list.

Initially, when the queue is empty, both the front and rear point to the null. As the new nodes are added, the rear is to be updated to the point to the latest node and when the nodes are removed, the front is to be updated to the point to the next node in line.

Basic Operations of Queue

Operation	Description	Algorithm
1. Enqueue(adding an elements)	The enqueue operation can be used to add the element to the rear of the queue.	Create the new node with the given value. If the queue is empty then set both the front and rear pointers to the new node. Otherwise set the next pointer of a current rear node to the new node and update the rear pointer to the new node.
2. Dequeue(Removing an element)	The dequeue operation can be used to removes the element from the front of the queues.	Check if the queue is empty then return the message indicating the queue is empty. Store the value of the front node. Update the front pointer to next node. If front becomes NULL set the real to NULL. Delete the old front node. Return the stored value.
3. Peek	The peek operation can be used to allow the look at the element at the front of queue without removing it.	Check if the queue is empty then return the message indicating the queue is empty. Return the data of the front node.
4. IsEmpty	This operation checks the whether the queue is empty which is determined by the checking if the front pointer is null.	Return whether the front pointer is NULL.
5. Size	This operation can be used to calculating the size of the queue involves the traversing from the front to the rear and counting the number of the nodes.	Initialize the count variable to 0. Traverse the queue from the front to the rear. For the each node increment the count by 1. Return the count.

Implementation of the Queue using Linked List

To the implement the queue using the linked list in java, We need two classes: Node and LinkedListQueue.

Node: This class can represents the individual elements of the queue.
LinkedListQueue: This class will be handle the queue operations using the Node objects.

Example program:

Java

// Java Program to Implement Queue
// Using Linked List

// Node Class
class Node {
    int data;
    Node next;

      // Constructor
    public Node(int data) {
        this.data = data;
        this.next = null;
    }
}

// Queue Class
class Queue {
    Node front, rear;
      int length;

      // Constructor
    public Queue() {
        this.front = this.rear = null;
          this.length=0;
    }

      // Adding Element in the LinkedList/Queue
    void enqueue(int key) {
          this.length++;
        Node temp = new Node(key);
        
          if (this.rear == null) {
            this.front = this.rear = temp;
            return;
        }
          
        this.rear.next = temp;
        this.rear = temp;
    }

      // Removing Element from the top
    void deque() {
        if (this.front == null)
            return;
          
        Node temp = this.front;
        this.front = this.front.next;
          
          // If Queue is Empty
        if (this.front == null)
            this.rear = null;
      
          // Removing the Front Element
        temp.next = null;  
    }

      // Checking on Front Element
    int peek() {
        if (this.front != null)
            return this.front.data;
      
          // Return Exception
        return Integer.MIN_VALUE;  
    }
  
      // Return the Size of Queue
      int size(){
          return this.length;
    }
  
      // Printing the Elements of Queue
      void printQueue(){
          Node temp=this.front;
          
          System.out.print("Element of Queue : ");
      
          while(temp!=null){
            System.out.print(temp.data+" ");
              temp=temp.next;
        }
          System.out.println();
    }
}

// Driver Class
public class Main {
      // Main Function
    public static void main(String[] args) {
        Queue q = new Queue();
        q.enqueue(10);
        q.enqueue(20);
          
          q.printQueue();
          System.out.println("Size :" + q.size());
        q.deque();
        q.deque();
        q.enqueue(30);
        q.enqueue(40);
        q.enqueue(50);
          q.printQueue();
          System.out.println("Size :" + q.size());
        System.out.println("Front item is: " + q.peek());
    }
}

Output

Element of Queue : 10 20 
Size :2
Element of Queue : 30 40 50 
Size :5
Front item is: 30

Time and Space Complexity

Time complexity: O(1)
Space complexity: O(n), where n is number of nodes in the linked list.

Applications of the Queue

It can applied in Event-driven simulation and Queues are used to the manage events in the time-based simulation systems.
It can applies on the Job scheduling and it can used to queues the job scheduling tasks in the operating system.
It can applies on Data buffering. Queue serve as the buffer in the data processing and transmission systems.

Queue element() method in Java

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Article Tags :

Practice Tags :

Java

Queue Implementation Using Linked List in Java

Organization of Queue Using Linked List

Basic Operations of Queue

Operation

Description

Algorithm

1. Enqueue(adding an elements)

2. Dequeue(Removing an element)

3. Peek

4. IsEmpty

5. Size

Implementation of the Queue using Linked List

Time and Space Complexity

Applications of the Queue

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