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/*

 * Written by Doug Lea and Josh Bloch with assistance from members of

 * JCP JSR-166 Expert Group and released to the public domain, as explained

 * at http://creativecommons.org/licenses/publicdomain

 */

package java.util;

/**

 * A linear collection that supports element insertion and removal at

 * both ends.  The name <i>deque</i> is short for "double ended queue"

 * and is usually pronounced "deck".  Most <tt>Deque</tt>

 * implementations place no fixed limits on the number of elements

 * they may contain, but this interface supports capacity-restricted

 * deques as well as those with no fixed size limit.

 *

 * <p>This interface defines methods to access the elements at both

 * ends of the deque.  Methods are provided to insert, remove, and

 * examine the element.  Each of these methods exists in two forms:

 * one throws an exception if the operation fails, the other returns a

 * special value (either <tt>null</tt> or <tt>false</tt>, depending on

 * the operation).  The latter form of the insert operation is

 * designed specifically for use with capacity-restricted

 * <tt>Deque</tt> implementations; in most implementations, insert

 * operations cannot fail.

 *

 * <p>The twelve methods described above are summarized in the

 * following table:

 *

 * <p>

 * <table BORDER CELLPADDING=3 CELLSPACING=1>

 *  <tr>

 *    <td></td>

 *    <td ALIGN=CENTER COLSPAN = 2> <b>First Element (Head)</b></td>

 *    <td ALIGN=CENTER COLSPAN = 2> <b>Last Element (Tail)</b></td>

 *  </tr>

 *  <tr>

 *    <td></td>

 *    <td ALIGN=CENTER><em>Throws exception</em></td>

 *    <td ALIGN=CENTER><em>Special value</em></td>

 *    <td ALIGN=CENTER><em>Throws exception</em></td>

 *    <td ALIGN=CENTER><em>Special value</em></td>

 *  </tr>

 *  <tr>

 *    <td><b>Insert</b></td>

 *    <td>{@link #addFirst addFirst(e)}</td>

 *    <td>{@link #offerFirst offerFirst(e)}</td>

 *    <td>{@link #addLast addLast(e)}</td>

 *    <td>{@link #offerLast offerLast(e)}</td>

 *  </tr>

 *  <tr>

 *    <td><b>Remove</b></td>

 *    <td>{@link #removeFirst removeFirst()}</td>

 *    <td>{@link #pollFirst pollFirst()}</td>

 *    <td>{@link #removeLast removeLast()}</td>

 *    <td>{@link #pollLast pollLast()}</td>

 *  </tr>

 *  <tr>

 *    <td><b>Examine</b></td>

 *    <td>{@link #getFirst getFirst()}</td>

 *    <td>{@link #peekFirst peekFirst()}</td>

 *    <td>{@link #getLast getLast()}</td>

 *    <td>{@link #peekLast peekLast()}</td>

 *  </tr>

 * </table>

 *

 * <p>This interface extends the {@link Queue} interface.  When a deque is

 * used as a queue, FIFO (First-In-First-Out) behavior results.  Elements are

 * added at the end of the deque and removed from the beginning.  The methods

 * inherited from the <tt>Queue</tt> interface are precisely equivalent to

 * <tt>Deque</tt> methods as indicated in the following table:

 *

 * <p>

 * <table BORDER CELLPADDING=3 CELLSPACING=1>

 *  <tr>

 *    <td ALIGN=CENTER> <b><tt>Queue</tt> Method</b></td>

 *    <td ALIGN=CENTER> <b>Equivalent <tt>Deque</tt> Method</b></td>

 *  </tr>

 *  <tr>

 *    <td>{@link java.util.Queue#add add(e)}</td>

 *    <td>{@link #addLast addLast(e)}</td>

 *  </tr>

 *  <tr>

 *    <td>{@link java.util.Queue#offer offer(e)}</td>

 *    <td>{@link #offerLast offerLast(e)}</td>

 *  </tr>

 *  <tr>

 *    <td>{@link java.util.Queue#remove remove()}</td>

 *    <td>{@link #removeFirst removeFirst()}</td>

 *  </tr>

 *  <tr>

 *    <td>{@link java.util.Queue#poll poll()}</td>

 *    <td>{@link #pollFirst pollFirst()}</td>

 *  </tr>

 *  <tr>

 *    <td>{@link java.util.Queue#element element()}</td>

 *    <td>{@link #getFirst getFirst()}</td>

 *  </tr>

 *  <tr>

 *    <td>{@link java.util.Queue#peek peek()}</td>

 *    <td>{@link #peek peekFirst()}</td>

 *  </tr>

 * </table>

 *

 * <p>Deques can also be used as LIFO (Last-In-First-Out) stacks.  This

 * interface should be used in preference to the legacy {@link Stack} class.

 * When a deque is used as a stack, elements are pushed and popped from the

 * beginning of the deque.  Stack methods are precisely equivalent to

 * <tt>Deque</tt> methods as indicated in the table below:

 *

 * <p>

 * <table BORDER CELLPADDING=3 CELLSPACING=1>

 *  <tr>

 *    <td ALIGN=CENTER> <b>Stack Method</b></td>

 *    <td ALIGN=CENTER> <b>Equivalent <tt>Deque</tt> Method</b></td>

 *  </tr>

 *  <tr>

 *    <td>{@link #push push(e)}</td>

 *    <td>{@link #addFirst addFirst(e)}</td>

 *  </tr>

 *  <tr>

 *    <td>{@link #pop pop()}</td>

 *    <td>{@link #removeFirst removeFirst()}</td>

 *  </tr>

 *  <tr>

 *    <td>{@link #peek peek()}</td>

 *    <td>{@link #peekFirst peekFirst()}</td>

 *  </tr>

 * </table>

 *

 * <p>Note that the {@link #peek peek} method works equally well when

 * a deque is used as a queue or a stack; in either case, elements are

 * drawn from the beginning of the deque.

 *

 * <p>This interface provides two methods to remove interior

 * elements, {@link #removeFirstOccurrence removeFirstOccurrence} and

 * {@link #removeLastOccurrence removeLastOccurrence}.

 *

 * <p>Unlike the {@link List} interface, this interface does not

 * provide support for indexed access to elements.

 *

 * <p>While <tt>Deque</tt> implementations are not strictly required

 * to prohibit the insertion of null elements, they are strongly

 * encouraged to do so.  Users of any <tt>Deque</tt> implementations

 * that do allow null elements are strongly encouraged <i>not</i> to

 * take advantage of the ability to insert nulls.  This is so because

 * <tt>null</tt> is used as a special return value by various methods

 * to indicated that the deque is empty.

 *

 * <p><tt>Deque</tt> implementations generally do not define

 * element-based versions of the <tt>equals</tt> and <tt>hashCode</tt>

 * methods, but instead inherit the identity-based versions from class

 * <tt>Object</tt>.

 *

 * <p>This interface is a member of the <a

 * href="{@docRoot}/../technotes/guides/collections/index.html"> Java Collections

 * Framework</a>.

 *

 * @author Doug Lea

 * @author Josh Bloch

 * @since  1.6

 * @param <E> the type of elements held in this collection

 */

public interface Deque<E> extends Queue<E> {

    /**

     * Inserts the specified element at the front of this deque if it is

     * possible to do so immediately without violating capacity restrictions.

     * When using a capacity-restricted deque, it is generally preferable to

     * use method {@link #offerFirst}.

     *

     * @param e the element to add

     * @throws IllegalStateException if the element cannot be added at this

     *         time due to capacity restrictions

     * @throws ClassCastException if the class of the specified element

     *         prevents it from being added to this deque

     * @throws NullPointerException if the specified element is null and this

     *         deque does not permit null elements

     * @throws IllegalArgumentException if some property of the specified

     *         element prevents it from being added to this deque

     */

    void addFirst(E e);

    /**

     * Inserts the specified element at the end of this deque if it is

     * possible to do so immediately without violating capacity restrictions.

     * When using a capacity-restricted deque, it is generally preferable to

     * use method {@link #offerLast}.

     *

     * <p>This method is equivalent to {@link #add}.

     *

     * @param e the element to add

     * @throws IllegalStateException if the element cannot be added at this

     *         time due to capacity restrictions

     * @throws ClassCastException if the class of the specified element

     *         prevents it from being added to this deque

     * @throws NullPointerException if the specified element is null and this

     *         deque does not permit null elements

     * @throws IllegalArgumentException if some property of the specified

     *         element prevents it from being added to this deque

     */

    void addLast(E e);

    /**

     * Inserts the specified element at the front of this deque unless it would

     * violate capacity restrictions.  When using a capacity-restricted deque,

     * this method is generally preferable to the {@link #addFirst} method,

     * which can fail to insert an element only by throwing an exception.

     *

     * @param e the element to add

     * @return <tt>true</tt> if the element was added to this deque, else

     *         <tt>false</tt>

     * @throws ClassCastException if the class of the specified element

     *         prevents it from being added to this deque

     * @throws NullPointerException if the specified element is null and this

     *         deque does not permit null elements

     * @throws IllegalArgumentException if some property of the specified

     *         element prevents it from being added to this deque

     */

    boolean offerFirst(E e);

    /**

     * Inserts the specified element at the end of this deque unless it would

     * violate capacity restrictions.  When using a capacity-restricted deque,

     * this method is generally preferable to the {@link #addLast} method,

     * which can fail to insert an element only by throwing an exception.

     *

     * @param e the element to add

     * @return <tt>true</tt> if the element was added to this deque, else

     *         <tt>false</tt>

     * @throws ClassCastException if the class of the specified element

     *         prevents it from being added to this deque

     * @throws NullPointerException if the specified element is null and this

     *         deque does not permit null elements

     * @throws IllegalArgumentException if some property of the specified

     *         element prevents it from being added to this deque

     */

    boolean offerLast(E e);

    /**

     * Retrieves and removes the first element of this deque.  This method

     * differs from {@link #pollFirst pollFirst} only in that it throws an

     * exception if this deque is empty.

     *

     * @return the head of this deque

     * @throws NoSuchElementException if this deque is empty

     */

    E removeFirst();

    /**

     * Retrieves and removes the last element of this deque.  This method

     * differs from {@link #pollLast pollLast} only in that it throws an

     * exception if this deque is empty.

     *

     * @return the tail of this deque

     * @throws NoSuchElementException if this deque is empty

     */

    E removeLast();

    /**

     * Retrieves and removes the first element of this deque,

     * or returns <tt>null</tt> if this deque is empty.

     *

     * @return the head of this deque, or <tt>null</tt> if this deque is empty

     */

    E pollFirst();

    /**

     * Retrieves and removes the last element of this deque,

     * or returns <tt>null</tt> if this deque is empty.

     *

     * @return the tail of this deque, or <tt>null</tt> if this deque is empty

     */

    E pollLast();

    /**

     * Retrieves, but does not remove, the first element of this deque.

     *

     * This method differs from {@link #peekFirst peekFirst} only in that it

     * throws an exception if this deque is empty.

     *

     * @return the head of this deque

     * @throws NoSuchElementException if this deque is empty

     */

    E getFirst();

    /**

     * Retrieves, but does not remove, the last element of this deque.

     * This method differs from {@link #peekLast peekLast} only in that it

     * throws an exception if this deque is empty.

     *

     * @return the tail of this deque

     * @throws NoSuchElementException if this deque is empty

     */

    E getLast();

    /**

     * Retrieves, but does not remove, the first element of this deque,

     * or returns <tt>null</tt> if this deque is empty.

     *

     * @return the head of this deque, or <tt>null</tt> if this deque is empty

     */

    E peekFirst();

    /**

     * Retrieves, but does not remove, the last element of this deque,

     * or returns <tt>null</tt> if this deque is empty.

     *

     * @return the tail of this deque, or <tt>null</tt> if this deque is empty

     */

    E peekLast();

    /**

     * Removes the first occurrence of the specified element from this deque.

     * If the deque does not contain the element, it is unchanged.

     * More formally, removes the first element <tt>e</tt> such that

     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>

     * (if such an element exists).

     * Returns <tt>true</tt> if this deque contained the specified element

     * (or equivalently, if this deque changed as a result of the call).

     *

     * @param o element to be removed from this deque, if present

     * @return <tt>true</tt> if an element was removed as a result of this call

     * @throws ClassCastException if the class of the specified element

     *         is incompatible with this deque (optional)

     * @throws NullPointerException if the specified element is null and this

     *         deque does not permit null elements (optional)

     */

    boolean removeFirstOccurrence(Object o);

    /**

     * Removes the last occurrence of the specified element from this deque.

     * If the deque does not contain the element, it is unchanged.

     * More formally, removes the last element <tt>e</tt> such that

     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>

     * (if such an element exists).

     * Returns <tt>true</tt> if this deque contained the specified element

     * (or equivalently, if this deque changed as a result of the call).

     *

     * @param o element to be removed from this deque, if present

     * @return <tt>true</tt> if an element was removed as a result of this call

     * @throws ClassCastException if the class of the specified element

     *         is incompatible with this deque (optional)

     * @throws NullPointerException if the specified element is null and this

     *         deque does not permit null elements (optional)

     */

    boolean removeLastOccurrence(Object o);

    // *** Queue methods ***

    /**

     * Inserts the specified element into the queue represented by this deque

     * (in other words, at the tail of this deque) if it is possible to do so

     * immediately without violating capacity restrictions, returning

     * <tt>true</tt> upon success and throwing an

     * <tt>IllegalStateException</tt> if no space is currently available.

     * When using a capacity-restricted deque, it is generally preferable to

     * use {@link #offer(Object) offer}.

     *

     * <p>This method is equivalent to {@link #addLast}.

     *

     * @param e the element to add

     * @return <tt>true</tt> (as specified by {@link Collection#add})

     * @throws IllegalStateException if the element cannot be added at this

     *         time due to capacity restrictions

     * @throws ClassCastException if the class of the specified element

     *         prevents it from being added to this deque

     * @throws NullPointerException if the specified element is null and this

     *         deque does not permit null elements

     * @throws IllegalArgumentException if some property of the specified

     *         element prevents it from being added to this deque

     */

    boolean add(E e);

    /**

     * Inserts the specified element into the queue represented by this deque

     * (in other words, at the tail of this deque) if it is possible to do so

     * immediately without violating capacity restrictions, returning

     * <tt>true</tt> upon success and <tt>false</tt> if no space is currently

     * available.  When using a capacity-restricted deque, this method is

     * generally preferable to the {@link #add} method, which can fail to

     * insert an element only by throwing an exception.

     *

     * <p>This method is equivalent to {@link #offerLast}.

     *

     * @param e the element to add

     * @return <tt>true</tt> if the element was added to this deque, else

     *         <tt>false</tt>

     * @throws ClassCastException if the class of the specified element

     *         prevents it from being added to this deque

     * @throws NullPointerException if the specified element is null and this

     *         deque does not permit null elements

     * @throws IllegalArgumentException if some property of the specified

     *         element prevents it from being added to this deque

     */

    boolean offer(E e);

    /**

     * Retrieves and removes the head of the queue represented by this deque

     * (in other words, the first element of this deque).

     * This method differs from {@link #poll poll} only in that it throws an

     * exception if this deque is empty.

     *

     * <p>This method is equivalent to {@link #removeFirst()}.

     *

     * @return the head of the queue represented by this deque

     * @throws NoSuchElementException if this deque is empty

     */

    E remove();

    /**

     * Retrieves and removes the head of the queue represented by this deque

     * (in other words, the first element of this deque), or returns

     * <tt>null</tt> if this deque is empty.

     *

     * <p>This method is equivalent to {@link #pollFirst()}.

     *

     * @return the first element of this deque, or <tt>null</tt> if

     *         this deque is empty

     */

    E poll();

    /**

     * Retrieves, but does not remove, the head of the queue represented by

     * this deque (in other words, the first element of this deque).

     * This method differs from {@link #peek peek} only in that it throws an

     * exception if this deque is empty.

     *

     * <p>This method is equivalent to {@link #getFirst()}.

     *

     * @return the head of the queue represented by this deque

     * @throws NoSuchElementException if this deque is empty

     */

    E element();

    /**

     * Retrieves, but does not remove, the head of the queue represented by

     * this deque (in other words, the first element of this deque), or

     * returns <tt>null</tt> if this deque is empty.

     *

     * <p>This method is equivalent to {@link #peekFirst()}.

     *

     * @return the head of the queue represented by this deque, or

     *         <tt>null</tt> if this deque is empty

     */

    E peek();

    // *** Stack methods ***

    /**

     * Pushes an element onto the stack represented by this deque (in other

     * words, at the head of this deque) if it is possible to do so

     * immediately without violating capacity restrictions, returning

     * <tt>true</tt> upon success and throwing an

     * <tt>IllegalStateException</tt> if no space is currently available.

     *

     * <p>This method is equivalent to {@link #addFirst}.

     *

     * @param e the element to push

     * @throws IllegalStateException if the element cannot be added at this

     *         time due to capacity restrictions

     * @throws ClassCastException if the class of the specified element

     *         prevents it from being added to this deque

     * @throws NullPointerException if the specified element is null and this

     *         deque does not permit null elements

     * @throws IllegalArgumentException if some property of the specified

     *         element prevents it from being added to this deque

     */

    void push(E e);

    /**

     * Pops an element from the stack represented by this deque.  In other

     * words, removes and returns the first element of this deque.

     *

     * <p>This method is equivalent to {@link #removeFirst()}.

     *

     * @return the element at the front of this deque (which is the top

     *         of the stack represented by this deque)

     * @throws NoSuchElementException if this deque is empty

     */

    E pop();

    // *** Collection methods ***

    /**

     * Removes the first occurrence of the specified element from this deque.

     * If the deque does not contain the element, it is unchanged.

     * More formally, removes the first element <tt>e</tt> such that

     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>

     * (if such an element exists).

     * Returns <tt>true</tt> if this deque contained the specified element

     * (or equivalently, if this deque changed as a result of the call).

     *

     * <p>This method is equivalent to {@link #removeFirstOccurrence}.

     *

     * @param o element to be removed from this deque, if present

     * @return <tt>true</tt> if an element was removed as a result of this call

     * @throws ClassCastException if the class of the specified element

     *         is incompatible with this deque (optional)

     * @throws NullPointerException if the specified element is null and this

     *         deque does not permit null elements (optional)

     */

    boolean remove(Object o);

    /**

     * Returns <tt>true</tt> if this deque contains the specified element.

     * More formally, returns <tt>true</tt> if and only if this deque contains

     * at least one element <tt>e</tt> such that

     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.

     *

     * @param o element whose presence in this deque is to be tested

     * @return <tt>true</tt> if this deque contains the specified element

     * @throws ClassCastException if the type of the specified element

     *         is incompatible with this deque (optional)

     * @throws NullPointerException if the specified element is null and this

     *         deque does not permit null elements (optional)

     */

    boolean contains(Object o);

    /**

     * Returns the number of elements in this deque.

     *

     * @return the number of elements in this deque

     */

    public int size();

    /**

     * Returns an iterator over the elements in this deque in proper sequence.

     * The elements will be returned in order from first (head) to last (tail).

     *

     * @return an iterator over the elements in this deque in proper sequence

     */

    Iterator<E> iterator();

    /**

     * Returns an iterator over the elements in this deque in reverse

     * sequential order.  The elements will be returned in order from

     * last (tail) to first (head).

     *

     * @return an iterator over the elements in this deque in reverse

     * sequence

     */

    Iterator<E> descendingIterator();

}