Queue Fundamentals in Java

What is a Queue?

Queue is one of the many data structures in programming. One of the best example of queue is people standing in a row to get the movie tickets or train tickets. The person who comes first, will get the ticket first. For the same reason queue is also called First In First Out (FIFO) data structure.

Important functions in Queue

If we talk about very simple queue, then there are very limited functions a queue has.

enqueue(): This pushes the element at the end of the queue.

dequeue(): This removes and returns the element from the beginning of the queue.

size(): This returns the current size of the queue.

Queue Implementation

Queue can be implemented using array or linkedlist. We will use linkedlist to implement queue for below reasons:

1. No limit on queue capacity
2. Shifting elements is costly in array, we can avoid that when we dequeue elements

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

public class Queue {
    int size;
    Node front;
    Node rear;

    public Queue() {
        size = 0;
        front = null;
        rear = null;
    }

    public void enqueue(int data) {
        Node node = new Node(data);
        if(front == null) {
            front = node;
            rear = node;
            size = size+1;
        }
        rear.next = node;
        rear = rear.next;
        rear.next = null;
        size = size+1;
    }

    public int dequeue() {
        int data = front.data;
        front = front.next;
        size = size-1;
        return data;
    }
    public int size() {
        return size;
    }

    public void printQueue() {
        Node temp = front;
        while(temp != null) {
            System.out.print(temp.data+" ");
            temp = temp.next;
        }
    }

    public static void main(String[] args) {
        Queue queue = new Queue();
        queue.enqueue(10);
        queue.enqueue(20);
        queue.enqueue(30);
        queue.printQueue();
        System.out.println();
        System.out.println("de-queued = "+queue.dequeue());
        queue.printQueue();
        System.out.println();
        System.out.println("Current queue size="+ queue.size());
    }
}

Console output

10 20 30 
de-queued = 10
20 30 
Current queue size=3

Hope you liked this tutorial. You might also like other tutorials in this series here