/**
 * The Deque interface specifies a collection of elements that are added
 * and removed both at the beginning and the end of the double-ended queue.
 *
 * @version     1.1    June 7, 2001
 * @author      Franck van Breugel
 */
public interface Deque 
{
    /** 
     * Returns the size of this deque. 
     *
     * @return The size of this deque.
     */
    public int size();

    /** 
     * Tests if this deque is empty. 
     *
     * @return true if this deque is empty, false otherwise.
     */
    public boolean isEmpty();

    /** 
     * Returns the first element of this deque.
     * Throws a DequeEmptyException if this deque is empty. 
     *
     * @return The first element of this deque.
     * @exception DequeEmptyException if this deque is empty. 
     */
    public Object first() throws DequeEmptyException;

    /** 
     * Returns the last element of this deque.
     * Throws a DequeEmptyException if this deque is empty. 
     *
     * @return The last element of this deque.
     * @exception DequeEmptyException if this deque is empty. 
     */
    public Object last() throws DequeEmptyException;

    /** 
     * Adds the specified element at the beginning of this deque.
     *
     * @param element The element added to this deque. 
     */
    public void insertFirst(Object element);

    /** 
     * Adds the specified element at the end of this deque.
     *
     * @param element The element added to this deque. 
     */
    public void insertLast(Object element);

    /** 
     * Removes the first element of this deque and returns that element.
     * Throws a DequeEmptyException if this deque is empty. 
     *
     * @return The first element of this deque.
     * @exception DequeEmptyException if this deque is empty. 
     */
    public Object removeFirst() throws DequeEmptyException;

    /** 
     * Removes the last element of this deque and returns that element.
     * Throws a DequeException if this deque is empty. 
     *
     * @return The last element of this deque.
     * @exception DequeException if this deque is empty. 
     */
    public Object removeLast() throws DequeEmptyException;
}