Java8CN/java/util/Deque.java at master · Java8-CNAPI-Team/Java8CN

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 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 * 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 {@code Deque}
 * 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 {@code null} or {@code false}, depending on
 * the operation).  The latter form of the insert operation is
 * designed specifically for use with capacity-restricted
 * {@code Deque} implementations; in most implementations, insert
 * operations cannot fail.
 * <p>The twelve methods described above are summarized in the
 * following table:
 * <table BORDER CELLPADDING=3 CELLSPACING=1>
 * <caption>Summary of Deque methods</caption>
 *    <td></td>
 *    <td ALIGN=CENTER COLSPAN = 2> <b>First Element (Head)</b></td>
 *    <td ALIGN=CENTER COLSPAN = 2> <b>Last Element (Tail)</b></td>
 *    <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>
 *    <td><b>Insert</b></td>
 *    <td>{@link Deque#addFirst addFirst(e)}</td>
 *    <td>{@link Deque#offerFirst offerFirst(e)}</td>
 *    <td>{@link Deque#addLast addLast(e)}</td>
 *    <td>{@link Deque#offerLast offerLast(e)}</td>
 *    <td><b>Remove</b></td>
 *    <td>{@link Deque#removeFirst removeFirst()}</td>
 *    <td>{@link Deque#pollFirst pollFirst()}</td>
 *    <td>{@link Deque#removeLast removeLast()}</td>
 *    <td>{@link Deque#pollLast pollLast()}</td>
 *    <td><b>Examine</b></td>
 *    <td>{@link Deque#getFirst getFirst()}</td>
 *    <td>{@link Deque#peekFirst peekFirst()}</td>
 *    <td>{@link Deque#getLast getLast()}</td>
 *    <td>{@link Deque#peekLast peekLast()}</td>
 * </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 {@code Queue} interface are precisely equivalent to
 * {@code Deque} methods as indicated in the following table:
 * <table BORDER CELLPADDING=3 CELLSPACING=1>
 * <caption>Comparison of Queue and Deque methods</caption>
 *    <td ALIGN=CENTER> <b>{@code Queue} Method</b></td>
 *    <td ALIGN=CENTER> <b>Equivalent {@code Deque} Method</b></td>
 *    <td>{@link java.util.Queue#add add(e)}</td>
 *    <td>{@link #addLast addLast(e)}</td>
 *    <td>{@link java.util.Queue#offer offer(e)}</td>
 *    <td>{@link #offerLast offerLast(e)}</td>
 *    <td>{@link java.util.Queue#remove remove()}</td>
 *    <td>{@link #removeFirst removeFirst()}</td>
 *    <td>{@link java.util.Queue#poll poll()}</td>
 *    <td>{@link #pollFirst pollFirst()}</td>
 *    <td>{@link java.util.Queue#element element()}</td>
 *    <td>{@link #getFirst getFirst()}</td>
 *    <td>{@link java.util.Queue#peek peek()}</td>
 *    <td>{@link #peek peekFirst()}</td>
 * </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
 * {@code Deque} methods as indicated in the table below:
 * <table BORDER CELLPADDING=3 CELLSPACING=1>
 * <caption>Comparison of Stack and Deque methods</caption>
 *    <td ALIGN=CENTER> <b>Stack Method</b></td>
 *    <td ALIGN=CENTER> <b>Equivalent {@code Deque} Method</b></td>
 *    <td>{@link #push push(e)}</td>
 *    <td>{@link #addFirst addFirst(e)}</td>
 *    <td>{@link #pop pop()}</td>
 *    <td>{@link #removeFirst removeFirst()}</td>
 *    <td>{@link #peek peek()}</td>
 *    <td>{@link #peekFirst peekFirst()}</td>
 * </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 {@code Deque} implementations are not strictly required
 * to prohibit the insertion of null elements, they are strongly
 * encouraged to do so.  Users of any {@code Deque} 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
 * {@code null} is used as a special return value by various methods
 * to indicated that the deque is empty.
 * <p>{@code Deque} implementations generally do not define
 * element-based versions of the {@code equals} and {@code hashCode}
 * methods, but instead inherit the identity-based versions from class
 * {@code Object}.
 * <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
 * 一个线性数据容器类，支持首尾两端的元素插入和删除。
 * 名称 <i>deque</i>【译为：双端队列】是 "double ended queue"的缩写，
 * 通常拼作："deck"。 大多数{@code Deque}实现对它们包含的元素数量没有限制，
 * 但是该接口同时支持有容量限制和不限制大小的deque。
 * <p>本接口定义了方法去访问deque双端的元素。
 * 这些方法用以插入，删除和检查元素
 * 每一个方法存在两种格式：
 * 一种运行失败后抛出异常，
 * 另一种返回特定的值，(要么 {@code null} 要么 {@code false}，视运行而定)。
 * 后一种格式insert操作专门设计用于容量限制的的{@code Deque}实现；
 * 在多数实现中，插入操作不能失败。
 * <p>上述的12种方法总结在了下表之中：
 * <table BORDER CELLPADDING=3 CELLSPACING=1>
 * <caption>Summary of Deque methods</caption>
 *    <td></td>
 *    <td ALIGN=CENTER COLSPAN = 2> <b>First Element (Head)</b></td>
 *    <td ALIGN=CENTER COLSPAN = 2> <b>Last Element (Tail)</b></td>
 *    <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>
 *    <td><b>Insert</b></td>
 *    <td>{@link Deque#addFirst addFirst(e)}</td>
 *    <td>{@link Deque#offerFirst offerFirst(e)}</td>
 *    <td>{@link Deque#addLast addLast(e)}</td>
 *    <td>{@link Deque#offerLast offerLast(e)}</td>
 *    <td><b>Remove</b></td>
 *    <td>{@link Deque#removeFirst removeFirst()}</td>
 *    <td>{@link Deque#pollFirst pollFirst()}</td>
 *    <td>{@link Deque#removeLast removeLast()}</td>
 *    <td>{@link Deque#pollLast pollLast()}</td>
 *    <td><b>Examine</b></td>
 *    <td>{@link Deque#getFirst getFirst()}</td>
 *    <td>{@link Deque#peekFirst peekFirst()}</td>
 *    <td>{@link Deque#getLast getLast()}</td>
 *    <td>{@link Deque#peekLast peekLast()}</td>
 * </table>
 * <p>本接口继承了{@link Queue}接口。当deque用作FIFO的队列。
 * deque末端加入元素而前端删除元素。
 * 继承自{@code Queue}接口的方法正好相当于{@code Deque}在下列表中列出的方法。
 * <table BORDER CELLPADDING=3 CELLSPACING=1>
 * <caption>Comparison of Queue and Deque methods</caption>
 *    <td ALIGN=CENTER> <b>{@code Queue} Method</b></td>
 *    <td ALIGN=CENTER> <b>Equivalent {@code Deque} Method</b></td>
 *    <td>{@link java.util.Queue#add add(e)}</td>
 *    <td>{@link #addLast addLast(e)}</td>
 *    <td>{@link java.util.Queue#offer offer(e)}</td>
 *    <td>{@link #offerLast offerLast(e)}</td>
 *    <td>{@link java.util.Queue#remove remove()}</td>
 *    <td>{@link #removeFirst removeFirst()}</td>
 *    <td>{@link java.util.Queue#poll poll()}</td>
 *    <td>{@link #pollFirst pollFirst()}</td>
 *    <td>{@link java.util.Queue#element element()}</td>
 *    <td>{@link #getFirst getFirst()}</td>
 *    <td>{@link java.util.Queue#peek peek()}</td>
 *    <td>{@link #peek peekFirst()}</td>
 * </table>
 * <p>Deques同样可以用作LIFO的堆栈。
 * 该接口优先于旧的{@link Stack}使用。
 * 当deque用作stack使用时，元素从deque的开头压入和弹出。
 * Stack方法账号相当于下表中列出的{@code Deque} 方法：
 * <table BORDER CELLPADDING=3 CELLSPACING=1>
 * <caption>Comparison of Stack and Deque methods</caption>
 *    <td ALIGN=CENTER> <b>Stack Method</b></td>
 *    <td ALIGN=CENTER> <b>Equivalent {@code Deque} Method</b></td>
 *    <td>{@link #push push(e)}</td>
 *    <td>{@link #addFirst addFirst(e)}</td>
 *    <td>{@link #pop pop()}</td>
 *    <td>{@link #removeFirst removeFirst()}</td>
 *    <td>{@link #peek peek()}</td>
 *    <td>{@link #peekFirst peekFirst()}</td>
 * </table>
 * <p>注意{@link #peek peek}方法在deque用作queue或队列时作用相同。
 * 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 {@code Deque} implementations are not strictly required
 * to prohibit the insertion of null elements, they are strongly
 * encouraged to do so.  Users of any {@code Deque} 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
 * {@code null} is used as a special return value by various methods
 * to indicated that the deque is empty.
 * <p>{@code Deque} implementations generally do not define
 * element-based versions of the {@code equals} and {@code hashCode}
 * methods, but instead inherit the identity-based versions from class
 * {@code Object}.
 * <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,
     * throwing an {@code IllegalStateException} if no space is currently
     * available.  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
     * Inserts the specified element at the front of this deque if it is
     * possible to do so immediately without violating capacity restrictions,
     * throwing an {@code IllegalStateException} if no space is currently
     * available.  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,
     * throwing an {@code IllegalStateException} if no space is currently
     * available.  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 {@code true} if the element was added to this deque, else
     *         {@code false}
     * @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 {@code true} if the element was added to this deque, else
     *         {@code false}
     * @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 {@code null} if this deque is empty.
     * @return the head of this deque, or {@code null} if this deque is empty
    E pollFirst();
     * Retrieves and removes the last element of this deque,
     * or returns {@code null} if this deque is empty.
     * @return the tail of this deque, or {@code null} 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 {@code null} if this deque is empty.
     * @return the head of this deque, or {@code null} if this deque is empty
    E peekFirst();
     * Retrieves, but does not remove, the last element of this deque,
     * or returns {@code null} if this deque is empty.
     * @return the tail of this deque, or {@code null} 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 {@code e} such that
     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>
     * (if such an element exists).
     * Returns {@code true} 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 {@code true} 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 {@code e} such that
     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>
     * (if such an element exists).
     * Returns {@code true} 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 {@code true} 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
     * {@code true} upon success and throwing an
     * {@code IllegalStateException} 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 {@code true} (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
     * {@code true} upon success and {@code false} 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 {@code true} if the element was added to this deque, else
     *         {@code false}
     * @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
     * {@code null} if this deque is empty.
     * <p>This method is equivalent to {@link #pollFirst()}.
     * @return the first element of this deque, or {@code null} 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 {@code null} if this deque is empty.
     * <p>This method is equivalent to {@link #peekFirst()}.
     * @return the head of the queue represented by this deque, or
     *         {@code null} 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, throwing an
     * {@code IllegalStateException} 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 {@code e} such that
     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>
     * (if such an element exists).
     * Returns {@code true} 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(Object)}.
     * @param o element to be removed from this deque, if present
     * @return {@code true} 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 {@code true} if this deque contains the specified element.
     * More formally, returns {@code true} if and only if this deque contains
     * at least one element {@code e} 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 {@code true} 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();
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