Question 46: Implement Queue using Linked List in java .

 There are two most important operations of Queue:

Queue is abstract data type which demonstrates First in first out (FIFO) behaviour. We will implement same behaviour using Array.

Although java provides implementation for  all abstract data types such as Stack , Queue and LinkedList but it is always good idea to understand basic data structures and implement them yourself

Please note that LinkedList implementation of Queue is dynamic in nature.
There are two most important operations of Queue:

enqueue : It is operation when we insert element into the queue.
dequeue : It is operation when we remove element from the queue. Read more at

Java Program to implement Queue using Linked List:

public class QueueUsingLinkedListMain {
    private Node front, rear;
    private int currentSize; // number of items

    //class to define linked node
    private class Node {
        int data;
        Node next;
    }

    //Zero argument constructor
    public QueueUsingLinkedListMain() {
        front = null;
        rear = null;
        currentSize = 0;
    }

    public boolean isEmpty() {
        return (currentSize == 0);
    }

    //Remove item from the beginning of the list.
    public int dequeue() {
        int data = front.data;
        front = front.next;
        if (isEmpty()) {
            rear = null;
        }
        currentSize--;
        System.out.println(data + " removed from the queue");
        return data;
    }

    //Add data to the end of the list.
    public void enqueue(int data) {
        Node oldRear = rear;
        rear = new Node();
        rear.data = data;
        rear.next = null;
        if (isEmpty()) {
            front = rear;
        } else {
            oldRear.next = rear;
        }
        currentSize++;
        System.out.println(data + " added to the queue");
    }
    public static void main(String a[]) {

        QueueUsingLinkedListMain queue = new QueueUsingLinkedListMain();
        queue.enqueue(6);
        queue.dequeue();
        queue.enqueue(3);
        queue.enqueue(99);
        queue.enqueue(56);
        queue.dequeue();
        queue.enqueue(43);
        queue.dequeue();
        queue.enqueue(89);
        queue.enqueue(77);
        queue.dequeue();
        queue.enqueue(32);
        queue.enqueue(232);
    }
}
 

When you run above program, you will get below output:

6 added to the queue
6 removed from the queue
3 added to the queue
99 added to the queue
56 added to the queue
3 removed from the queue
43 added to the queue
99 removed from the queue
89 added to the queue
77 added to the queue
56 removed from the queue
32 added to the queue
232 added to the queue