Simple Circular Queue using ArrayList in Java

A circular queue implemented with an ArrayList in Java uses fixed start and end points that wrap around the list, allowing efficient use of space by reusing array slots. It maintains the first-in-first-out (FIFO) order by incrementing head and tail indices modulo the array size.

Solutions:

import java.util.ArrayList;

class CircularQueue {
    private ArrayList<Integer> queue;
    private int maxSize;
    private int front;
    private int rear;
    private int currentSize;

    // Constructor
    public CircularQueue(int size) {
        this.maxSize = size;
        this.queue = new ArrayList<>(size);
        this.front = 0;
        this.rear = -1;
        this.currentSize = 0;

        // Initialize the array list with null values
        for (int i = 0; i < size; i++) {
            queue.add(null);
        }
    }

    // Enqueue operation
    public void enqueue(int data) {
        if (isFull()) {
            System.out.println("Queue is full. Cannot add " + data);
            return;
        }
        rear = (rear + 1) % maxSize;
        queue.set(rear, data);
        currentSize++;
    }

    // Dequeue operation
    public int dequeue() {
        if (isEmpty()) {
            System.out.println("Queue is empty. Cannot dequeue.");
            return -1;
        }
        int data = queue.get(front);
        queue.set(front, null); // Optional: Reset the value to null
        front = (front + 1) % maxSize;
        currentSize--;
        return data;
    }

    // Check if the queue is full
    public boolean isFull() {
        return currentSize == maxSize;
    }

    // Check if the queue is empty
    public boolean isEmpty() {
        return currentSize == 0;
    }

    // Display the queue
    public void display() {
        System.out.print("Queue: ");
        for (int i = 0; i < maxSize; i++) {
            if (queue.get(i) != null) {
                System.out.print(queue.get(i) + " ");
            } else {
                System.out.print("_ ");
            }
        }
        System.out.println();
    }
}
public class QueueAL {
    public static void main(String[] args) {
        CircularQueue queue = new CircularQueue(5);  // Create a circular queue of size 5

        queue.enqueue(10);  // Add 10
        queue.enqueue(20);  // Add 20
        queue.enqueue(30);  // Add 30
        queue.enqueue(40);  // Add 40
        queue.enqueue(50);  // Add 50

        System.out.println("Initial Queue:");
        queue.display();  // Display the queue

        queue.enqueue(60);  // Attempt to add 60 (queue is full)

        System.out.println("Dequeue operation: " + queue.dequeue());  // Remove and print front element
        System.out.println("Queue after one dequeue operation:");
        queue.display();  // Display the queue

        queue.enqueue(60);  // Add 60 (queue has space now)
        System.out.println("Queue after enqueue operation:");
        queue.display();  // Display the queue

        while (!queue.isEmpty()) {  // Dequeue all elements
            System.out.println("Dequeue operation: " + queue.dequeue());
        }

        System.out.println("Queue after dequeuing all elements:");
        queue.display();  // Display the queue

        queue.dequeue();  // Attempt to dequeue from empty queue
    }
}