Queue.java

public class Queue {
    int size = 20;
    Integer[] queue = new Integer[size];
    int front = -1;
    int rear = -1;

    public static void main(String[] args){
        Queue queue = new Queue();

        System.out.println("Queue is empty? " + queue.isEmpty());

        System.out.println("Enqueuing 5...");
        queue.enqueue(5);

        System.out.println("Enqueuing 15...");
        queue.enqueue(15);

        System.out.println("Enqueuing 25...");
        queue.enqueue(25);

        System.out.println("Queue is empty? " + queue.isEmpty());

        System.out.println("Dequeuing...");
        queue.dequeue();

        System.out.println("Dequeuing...");
        queue.dequeue();

        System.out.println("Queue is empty? " + queue.isEmpty());

        System.out.println("Dequeuing...");
        queue.dequeue();

        System.out.println("Queue is empty? " + queue.isEmpty());

        System.out.println("Trying one more dequeue (should trigger underflow)...");
        queue.dequeue();

    }
    private void enqueue(int data){
        if(rear >= size - 1){
            System.out.println("Queue Overflow");
            return;
        }
        if(front == -1){
            front = 0;
        }
        rear++;
        queue[rear] = data;
    }
    private void dequeue(){
        if(rear <= -1){
            System.out.println("Queue Underflow");
            return;
        }
        System.out.println(queue[front] + " has been removed");
        queue[front] = null;
        front++;

        if (front > rear) {
            front = rear = -1;
        }

    }
    private boolean isEmpty(){
        return rear == -1 || front >  rear;
    }

}