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*
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* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
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* 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).
*
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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 {@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 indicate 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 deque
*/
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
* {@code Objects.equals(o, e)} (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="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified element is null and this
* deque does not permit null elements
* (<a href="{@docRoot}/java/util/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
* {@code Objects.equals(o, e)} (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="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified element is null and this
* deque does not permit null elements
* (<a href="{@docRoot}/java/util/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
* {@code Objects.equals(o, e)} (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="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified element is null and this
* deque does not permit null elements
* (<a href="{@docRoot}/java/util/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 {@code Objects.equals(o, e)}.
*
* @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 class of the specified element
* is incompatible with this deque
* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified element is null and this
* deque does not permit null elements
* (<a href="{@docRoot}/java/util/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
*/
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();
}