Implementation of Stack in Java

Stack.java

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

public class Stack {

	/**
	 * Properties of stack.
	 */
	private StackNode top = null;

	/**
	 * Element of stack.
	 */
	class StackNode {

		public int key;

		public StackNode next = null;

		StackNode(int key) {
			this.key = key;
		}
	}

	/**
	 * Push method.
	 *
	 * Add an element to the top of the stack.
	 *
	 * @param int key
	 *  Integer key.
	 * @return void
	 */
	public void push(int key) {

		StackNode node = new StackNode(key);
		node.next = this.top;
		this.top = node;
	}

	/**
	 * Pop method.
	 *
	 * Remove the element on the top of stack.
	 *
	 * @return int
	 *  Stack element key of -5 if is empty.
	 */
	public int pop() {

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

		int key = this.top.key;

		this.top = this.top.next;

		return key;
	}

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

		return this.top == null;
	}

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

		if (this.is_empty()) return 0;

		StackNode p = this.top;

		int i = 0;

		while(p != null) {

			i++;
			p = p.next;
		}

		return i;
	}

	/**
	 * Top method.
	 *
	 * Gets the element key in the top of stack, without remove it.
	 *
	 * @return int
	 *  Stack element key on the top or -5 if is empty.
	 */
	public int top() {

		return (this.is_empty()) ? -5 : this.top.key;
	}

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

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

		StackNode p = this.top;

		String description = "Stack: ";

		while(p != null) {

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

		return description;
	}
}