Deque

Deque

 

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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>  *  <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 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>  *  </tr>  *  <tr>  *    <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>  *  </tr>  *  <tr>  *    <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>  *  </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 {@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>  *  <tr>  *    <td ALIGN=CENTER> <b>{@code Queue} Method</b></td>  *    <td ALIGN=CENTER> <b>Equivalent {@code Deque} 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  * {@code Deque} methods as indicated in the table below:  *  * <table BORDER CELLPADDING=3 CELLSPACING=1>  * <caption>Comparison of Stack and Deque methods</caption>  *  <tr>  *    <td ALIGN=CENTER> <b>Stack Method</b></td>  *    <td ALIGN=CENTER> <b>Equivalent {@code Deque} 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 {@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      */     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 ? e==null : 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 ? e==null : 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 ? e==null : 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 ? e==null : 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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