Implementation of Deque data structure in Java

Deque.java

/**
 * @file
 *  Deque (Double ended queue).
 *
 * Implementation with doubly linked list, through the nested class DequeNode.
 * It's used nested class to improve encapsulation.
 *
 * @author Pedro Furtado
 */

public class Deque {

	/**
	 * Properties of deque.
	 */
	private DequeNode begin = null;

	private DequeNode end = null;

	/**
	 * Element of deque.
	 */
	class DequeNode {

		public int key;

		public DequeNode previous = null;

		public DequeNode next = null;

		DequeNode(int key) {

			this.key = key;
		}
	}

	/**
	 * Add to begin method.
	 *
	 * Add an element to the beginning of deque.
	 *
	 * @param int key
	 *  Integer key.
	 * @return void
	 */
	public void add_to_begin(int key) {

		DequeNode node = new DequeNode(key);

		if (this.is_empty()) {

			this.end = node;
		}
		else {

			this.begin.previous = node;
		}

		node.next = this.begin;
		this.begin = node;
	}

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

		DequeNode node = new DequeNode(key);

		if (this.is_empty()) {

			this.begin = node;
		}
		else {

			this.end.next = node;
		}
		
		node.previous = this.end;
		this.end = node;
	}

	/**
	 * Remove to begin method.
	 *
	 * Remove an element to the beginning of deque.
	 *
	 * @param int key
	 *  Integer key.
	 * @return void
	 */
	public int remove_to_begin() {

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

		int key = this.begin.key;

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

			this.begin = this.begin.next;
			this.begin.previous = null;
		}

		return key;
	}

	/**
	 * Remove to end method.
	 *
	 * Remove an element to the end of deque.
	 *
	 * @param int key
	 *  Integer key.
	 * @return void
	 */
	public int remove_to_end() {

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

		int key = this.begin.key;

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

			this.end = this.end.previous;
			this.end.next = null;
		}

		return key;
	}

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

		return (this.begin == null) && (this.end == null);
	}

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

		if (this.is_empty()) return 0;

		int i = 0;

		DequeNode p = this.begin;

		while(p != null) {

			i++;
			p = p.next;
		}

		return i;
	}

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

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

		DequeNode p = this.begin;

		String description = "Deque: ";

		while(p != null) {

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

		description += " Size : " + this.size();

		return description;
	}
}