JAVA - Array Queue

 

run:

Element at front :  18

Element removed from front : 18

Queue is Empty : true

Size of the queue : 2

Element removed from front end : 79

BUILD SUCCESSFUL (total time: 0 seconds)

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package arrayqueue;

import java.util.*;

public class ArrayQueue < E > {

    private E[] data; // generic array used for storage

    // constructors

    private int frontIndex;

    private int queueSize;

    public ArrayQueue(int capacity) { // constructs queue with given capacity

        data = (E[]) new Object[capacity]; // safe cast; compiler may give warning

        queueSize = 0; // current number of elements

        frontIndex = 0; // index of the front element

        

        

    }

    public ArrayQueue() {

        this(1000);

    } // constructs queue with default capacity

    // methods

    /* Returns the number of elements in the queue. */

    public int size() {

        return queueSize;

    }

    /* Tests whether the queue is empty. */

    public boolean isEmpty() {

        return (queueSize == 0);

    }

    /* Inserts an element at the rear of the queue. */

    public void enqueue(E e) throws IllegalStateException {

        if (queueSize == data.length) throw new IllegalStateException("Queue is full");

        int avail = (frontIndex + queueSize) % data.length; // use modular arithmetic

        data[avail] = e;

        queueSize++;

    }

    /* Returns, but does not remove, the first element of the queue (null if empty). */

    public E first() throws IllegalStateException {

        if (queueSize == data.length) throw new IllegalStateException("Queue is empty");

        return data[frontIndex];

    }

    /* pops and returns the first element of the queue (null if empty). */

    public E dequeue() throws IllegalStateException {

        if (queueSize == data.length) throw new IllegalStateException("Queue is empty");

        E answer = data[frontIndex];

        data[frontIndex] = null; // dereference to help garbage collection

        frontIndex = (frontIndex + 1) % data.length;

        queueSize--;

        return answer;

    }

    public static void main(String[] args) {

        ArrayQueue queue = new ArrayQueue();

        queue.enqueue(18); //a

        System.out.println("Element at front :  " + queue.first()); //b

        System.out.println("Element removed from front : " + queue.dequeue()); //c  

        System.out.println("Queue is Empty : " + queue.isEmpty()); //d

        queue.enqueue(79); //e

        queue.enqueue(90); //f

        System.out.println("Size of the queue : " + queue.size()); //g

        System.out.println("Element removed from front end : " + queue.dequeue());

        //h    

    }

}