Implementation of Queue data structure in Java

Queue.java

/**
 * @file
 *  Queue.
 *
 * Implementation with singly linked list, through the nested class QueueNode.
 * It's used nested class to improve encapsulation.
 *
 * @author Pedro Furtado
 */

public class Queue {

	/**
	 * Properties of queue.
	 */
	private QueueNode begin = null;

	private QueueNode end = null;

	/**
	 * Element of queue.
	 */
	class QueueNode {

		public int key;

		public QueueNode next = null;

		QueueNode(int key) {

			this.key = key;
		}
	}

	/**
	 * Default constructor method.
	 */
	Queue() {

	}

	/**
	 * Overloading of Queue() constructor method.
	 *
	 * Allow to add to queue a single key on instantiation of the queue.
	 *
	 * @param int key
	 *  Integer key.
	 */
	Queue(int key) {

		this.queue(key);
	}

	/**
	 * Overloading of Queue() constructor method.
	 *
	 * Allow to add to queue a list of keys on instantiation of the queue.
	 *
	 * @param int[] keys
	 *  List of integer keys.
	 */
	Queue(int[] keys) {

		for (int key : keys) {

			this.queue(key);
		}
	}

	/**
	 * Enqueue method.
	 *
	 * Add an element to the end of the queue.
	 *
	 * @param int key
	 *  Integer key.
	 * @return void
	 */
	public void queue(int key) {

		QueueNode node = new QueueNode(key);

		if (this.is_empty()) {

			this.begin = node;
		}
		else {

			this.end.next = node;
		}

		this.end = node;
	}

	/**
	 * Dequeue method.
	 *
	 * Remove an element from the beginning of the queue.
	 *
	 * @return int 
	 *  Queue element or -5 if is empty.
	 */
	public int dequeue() {

		if (this.is_empty()) return -5;

		int key = this.begin.key;

		if (this.begin == this.end) {
			
			this.end = null;
		}

		this.begin = this.begin.next;

		return key;
	}

	/**
	 * Size method.
	 *
	 * Determines the size of queue, i.e., the number of elements in queue.
	 *
	 * @return int
	 *  Number of elements in queue.
	 */
	public int size() {

		QueueNode p = this.begin;

		int i = 0;

		while(p != null) {

			i++;
			p = p.next;
		}

		return i;
	}

	/**
	 * Empty method.
	 *
	 * Determines if the queue is empty, i.e., if it does not have elements inside.
	 *
	 * @return boolean
	 */
	public boolean is_empty() {

		return this.begin == null;
	}

	/**
	 * toString method.
	 *
	 * Provide some visual funcionality to see the elements inside the queue.
	 *
	 * @return String
	 *  Representation of the queue in the moment by a string.
	 */
	public String toString() {

		if (this.is_empty()) return "Empty queue.";

		QueueNode p = this.begin;

		String description = "Queue: ";

		while(p != null) {

			description += p.key + "  ";
			p = p.next;
		}

		return description;
	}
}