/*

  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

  *

  * This code is free software; you can redistribute it and/or modify it

  * under the terms of the GNU General Public License version 2 only, as

  * published by the Free Software Foundation.  Oracle designates this

  * particular file as subject to the "Classpath" exception as provided

  * by Oracle in the LICENSE file that accompanied this code.

  *

  * This code is distributed in the hope that it will be useful, but WITHOUT

  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

  * version 2 for more details (a copy is included in the LICENSE file that

  * accompanied this code).

  *

  * You should have received a copy of the GNU General Public License version

  * 2 along with this work; if not, write to the Free Software Foundation,

  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

  *

  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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  */

 /*

  * This file is available under and governed by the GNU General Public

  * License version 2 only, as published by the Free Software Foundation.

  * However, the following notice accompanied the original version of this

  * file:

  *

  * 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/publicdomain/zero/1.0/

  */

 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

      * (<a href="Collection.html#optional-restrictions">optional</a>)

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

      *         deque does not permit null elements

      * (<a href="Collection.html#optional-restrictions">optional</a>)

      */

     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

      * (<a href="Collection.html#optional-restrictions">optional</a>)

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

      *         deque does not permit null elements

      * (<a href="Collection.html#optional-restrictions">optional</a>)

      */

     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

      * (<a href="Collection.html#optional-restrictions">optional</a>)

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

      *         deque does not permit null elements

      * (<a href="Collection.html#optional-restrictions">optional</a>)

      */

     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

      * (<a href="Collection.html#optional-restrictions">optional</a>)

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

      *         deque does not permit null elements

      * (<a href="Collection.html#optional-restrictions">optional</a>)

      */

     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();

 }