CircularArrayQueue.java

import java.util.NoSuchElementException;

/**
 * An implementation of a queue as a circular array
 *
 * Author: AnxietyMedicine (GitHub)
 */
public class CircularArrayQueue {
    private Object[] elements;
    private int head, tail, currentSize;

    /**
     * Constructs an empty queue
     */
    public CircularArrayQueue() {
        final int INITIAL_SIZE = 10;
        elements = new Object[INITIAL_SIZE];
        head = tail = currentSize = 0;
    }

    /**
     * Adds an element to the tail of the queue
     * @param newElement new element to add
     */
    public void add(Object newElement) {
        growIfNecessary();
        currentSize++;

        elements[tail] = newElement;
        tail = (tail + 1) % elements.length;
    }

    /**
     * Removes an element from the head of the queue
     * @return removed element
     */
    public Object remove() {
        if (currentSize == 0) { throw new NoSuchElementException(); }

        Object removed = elements[head];
        head = (head + 1) % elements.length;

        return removed;
    }

    /**
     * Grows the element array if the current size equals the capacity
     */
    private void growIfNecessary() {
        if (currentSize == elements.length) {
            Object[] newElements = new Object[2 * elements.length];

            for (int i = 0; i < elements.length; i++) {
                newElements[i] = elements[(head + i) % elements.length];
            }

            elements = newElements;
            head = 0;
            tail = currentSize;
        }
    }

    /**
     * Checks whether queue is empty
     * @return true if queue is empty
     */
    public boolean empty() {
        return currentSize == 0;
    }
}