JAVA - LinkedList Queue

 

run:

Element at front :  18

Element removed from front : 18

Queue is Empty : true

Size of the queue : 2

Element removed from front end : 79

BUILD SUCCESSFUL (total time: 0 seconds)

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https://www.scaler.com/topics/java/stack-and-queue-in-java/

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package linkedlistqueue;

import java.util.*;

public class LinkedListQueue {

    private Node frontNode, rearNode;

    private int queueSize; // queue size  

    //linked list node  

    private class Node {

        int data;

        Node next;

    }

    //default constructor - initially front & rear are null; size=0; queue is empty  

    public LinkedListQueue() {

        frontNode = null;

        rearNode = null;

        queueSize = 0;

    }

    //check if the queue is empty  

    public boolean isEmpty() {

        return (queueSize == 0);

    }

    //Remove the item from the front of the queue.  

    public int dequeue() throws IllegalStateException {

        if (isEmpty()) throw new IllegalArgumentException("Queue is Empty");

        int data = frontNode.data;

        frontNode = frontNode.next;

        if (isEmpty()) {

            rearNode = null;

        }

        queueSize--;

        return data;

    }

    public int first() throws IllegalStateException {

        if (isEmpty()) throw new IllegalArgumentException("Queue is Empty");

        return frontNode.data;

    }

    public int size() {

        return queueSize;

    }

    //Add data at the rear of the queue.  

    public void enqueue(int data) {

        Node oldRear = rearNode;

        rearNode = new Node();

        rearNode.data = data;

        rearNode.next = null;

        if (isEmpty()) {

            frontNode = rearNode;

        } else {

            oldRear.next = rearNode;

        }

        queueSize++;

    }

    public static void main(String[] args) {

        LinkedListQueue queue = new LinkedListQueue();

        queue.enqueue(18);

        System.out.println("Element at front :  " + queue.first());

        System.out.println("Element removed from front : " + queue.dequeue());

        System.out.println("Queue is Empty : " + queue.isEmpty());

        queue.enqueue(79);

        queue.enqueue(90);

        System.out.println("Size of the queue : " + queue.size());

        System.out.println("Element removed from front end : " + queue.dequeue());

    }

}