Queue Implementation

A Queue is a linear data structure that follows the FIFO (First In First Out) or LILO (Last In Last Out) principle. Elements are inserted at the rear and removed from the front.

Operations

• Enqueue (Add): Adds an item to the queue. O(1)
• Dequeue (Remove): Removes an item from the queue. O(1)
• Peek / Front: Gets the element at the front. O(1)

Visual Representation

Array Based Implementation in Java

public class QueueArray {
    private int capacity;
    private int front, rear, size;
    private int[] array;

    public QueueArray(int capacity) {
        this.capacity = capacity;
        front = this.size = 0;
        rear = capacity - 1;
        array = new int[capacity];
    }

    public boolean isFull() {
        return (size == capacity);
    }

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

    public void enqueue(int item) {
        if (isFull()) {
            System.out.println("Queue is Full");
            return;
        }
        rear = (rear + 1) % capacity;
        array[rear] = item;
        size = size + 1;
        System.out.println(item + " enqueued.");
    }

    public int dequeue() {
        if (isEmpty()) {
            System.out.println("Queue is Empty");
            return Integer.MIN_VALUE;
        }
        int item = array[front];
        front = (front + 1) % capacity;
        size = size - 1;
        return item;
    }

    public int front() {
        if (isEmpty()) return Integer.MIN_VALUE;
        return array[front];
    }

    public static void main(String[] args) {
        QueueArray queue = new QueueArray(3);
        queue.enqueue(10);
        queue.enqueue(20);
        queue.enqueue(30);

        System.out.println(queue.dequeue() + " dequeued from queue\n");
        System.out.println("Front item is " + queue.front());
    }
}

Note: Java's standard library provides a java.util.Queue interface which is typically implemented by LinkedList or PriorityQueue.