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/* |
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* Written by Doug Lea and Josh Bloch with assistance from members of |
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* JCP JSR-166 Expert Group and released to the public domain, as explained |
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* at http://creativecommons.org/licenses/publicdomain |
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*/ |
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|
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package java.util; |
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|
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/** |
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* A linear collection that supports element insertion and removal at |
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* both ends. The name <i>deque</i> is short for "double ended queue" |
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* and is usually pronounced "deck". Most <tt>Deque</tt> |
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* implementations place no fixed limits on the number of elements |
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* they may contain, but this interface supports capacity-restricted |
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* deques as well as those with no fixed size limit. |
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* |
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* <p>This interface defines methods to access the elements at both |
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* ends of the deque. Methods are provided to insert, remove, and |
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* examine the element. Each of these methods exists in two forms: |
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* one throws an exception if the operation fails, the other returns a |
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* special value (either <tt>null</tt> or <tt>false</tt>, depending on |
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* the operation). The latter form of the insert operation is |
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* designed specifically for use with capacity-restricted |
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* <tt>Deque</tt> implementations; in most implementations, insert |
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* operations cannot fail. |
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* |
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* <p>The twelve methods described above are summarized in the |
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* following table:<p> |
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* |
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* <table BORDER CELLPADDING=3 CELLSPACING=1> |
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* <tr> |
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* <td></td> |
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* <td ALIGN=CENTER COLSPAN = 2> <b>First Element (Head)</b></td> |
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* <td ALIGN=CENTER COLSPAN = 2> <b>Last Element (Tail)</b></td> |
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* </tr> |
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* <tr> |
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* <td></td> |
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* <td ALIGN=CENTER><em>Throws exception</em></td> |
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* <td ALIGN=CENTER><em>Returns special value</em></td> |
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* <td ALIGN=CENTER><em>Throws exception</em></td> |
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* <td ALIGN=CENTER><em>Returns special value</em></td> |
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* </tr> |
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* <tr> |
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* <td><b>Insert</b></td> |
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* <td>{@link #addFirst addFirst(e)}</td> |
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* <td>{@link #offerFirst offerFirst(e)}</td> |
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* <td>{@link #addLast addLast(e)}</td> |
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* <td>{@link #offerLast offerLast(e)}</td> |
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* </tr> |
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* <tr> |
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* <td><b>Remove</b></td> |
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* <td>{@link #removeFirst removeFirst()}</td> |
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* <td>{@link #pollFirst pollFirst()}</td> |
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* <td>{@link #removeLast removeLast()}</td> |
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* <td>{@link #pollLast pollLast()}</td> |
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* </tr> |
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* <tr> |
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* <td><b>Examine</b></td> |
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* <td>{@link #getFirst getFirst()}</td> |
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* <td>{@link #peekFirst peekFirst()}</td> |
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* <td>{@link #getLast getLast()}</td> |
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* <td>{@link #peekLast peekLast()}</td> |
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* </tr> |
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* </table> |
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* |
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* <p>This interface extends the {@link Queue} interface. When a deque is |
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* used as a queue, FIFO (First-In-First-Out) behavior results. Elements are |
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* added at the end of the deque and removed from the beginning. The methods |
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* inherited from the <tt>Queue</tt> interface are precisely equivalent to |
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* <tt>Deque</tt> methods as indicated in the following table:<p> |
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* |
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* <table BORDER CELLPADDING=3 CELLSPACING=1> |
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* <tr> |
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* <td ALIGN=CENTER> <b><tt>Queue</tt> Method</b></td> |
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* <td ALIGN=CENTER> <b>Equivalent <tt>Deque</tt> Method</b></td> |
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* </tr> |
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* <tr> |
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* <tr> |
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* <td>{@link java.util.Queue#offer offer(e)}</td> |
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* <td>{@link #offerLast offerLast(e)}</td> |
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* </tr> |
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* <tr> |
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* <td>{@link java.util.Queue#add add(e)}</td> |
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* <td>{@link #addLast addLast(e)}</td> |
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* </tr> |
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* <tr> |
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* <td>{@link java.util.Queue#poll poll()}</td> |
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* <td>{@link #pollFirst pollFirst()}</td> |
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* </tr> |
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* <tr> |
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* <td>{@link java.util.Queue#remove remove()}</td> |
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* <td>{@link #removeFirst removeFirst()}</td> |
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* </tr> |
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* <tr> |
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* <td>{@link java.util.Queue#peek peek()}</td> |
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* <td>{@link #peek peekFirst()}</td> |
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* </tr> |
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* <tr> |
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* <td>{@link java.util.Queue#element element()}</td> |
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* <td>{@link #getFirst getFirst()}</td> |
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* </tr> |
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* </table> |
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* |
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* <p>Deques can also be used as LIFO (Last-In-First-Out) stacks. This |
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* interface should be used in preference to the legacy {@link Stack} class. |
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* When a deque is used as a stack, elements are pushed and popped from the |
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* beginning of the deque. Stack methods are precisely equivalent to |
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* <tt>Deque</tt> methods as indicated in the table below:<p> |
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* |
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* <table BORDER CELLPADDING=3 CELLSPACING=1> |
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* <tr> |
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* <td ALIGN=CENTER> <b>Stack Method</b></td> |
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* <td ALIGN=CENTER> <b>Equivalent <tt>Deque</tt> Method</b></td> |
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* </tr> |
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* <tr> |
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* <tr> |
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* <td>{@link #push push(e)}</td> |
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* <td>{@link #addFirst addFirst(e)}</td> |
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* </tr> |
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* <tr> |
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* <td>{@link #pop pop()}</td> |
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* <td>{@link #removeFirst removeFirst()}</td> |
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* </tr> |
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* <tr> |
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* <td>{@link #peek peek()}</td> |
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* <td>{@link #peekFirst peekFirst()}</td> |
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* </tr> |
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* </table> |
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* |
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* <p>Note that the {@link #peek peek} method works equally well when |
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* a deque is used as a queue or a stack; in either case, elements are |
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* drawn from the beginning of the deque. |
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* |
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* <p>This interface provides two methods to remove interior |
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* elements, {@link #removeFirstOccurrence removeFirstOccurrence} and |
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* {@link #removeLastOccurrence removeLastOccurrence}. Unlike the |
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* {@link List} interface, this interface does not provide support for |
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* indexed access to elements. |
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* |
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* <p>While <tt>Deque</tt> implementations are not strictly required |
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* to prohibit the insertion of null elements, they are strongly |
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* encouraged to do so. Users of any <tt>Deque</tt> implementations |
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* that do allow null elements are strongly encouraged <i>not</i> to |
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* take advantage of the ability to insert nulls. This is so because |
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* <tt>null</tt> is used as a special return value by various methods |
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* to indicated that the deque is empty. |
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* |
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* <p><tt>Deque</tt> implementations generally do not define |
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* element-based versions of the <tt>equals</tt> and <tt>hashCode</tt> |
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* methods, but instead inherit the identity-based versions from class |
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* <tt>Object</tt>. |
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* |
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* <p>This interface is a member of the <a |
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* href="{@docRoot}/../guide/collections/index.html"> Java Collections |
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* Framework</a>. |
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* |
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* @author Doug Lea |
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* @author Josh Bloch |
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* @since 1.6 |
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* @param <E> the type of elements held in this collection |
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*/ |
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|
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public interface Deque<E> extends Queue<E> { |
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/** |
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* Inserts the specified element at the front of this deque unless it would |
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* violate capacity restrictions. When using a capacity-restricted deque, |
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* this method is generally preferable to method <tt>addFirst</tt>, which |
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* can fail to insert an element only by throwing an exception. |
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* |
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* @param e the element to insert |
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* @return <tt>true</tt> if it was possible to insert the element, |
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* else <tt>false</tt> |
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* @throws NullPointerException if the specified element is null and this |
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* deque does not permit null elements |
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*/ |
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boolean offerFirst(E e); |
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|
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/** |
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* Inserts the specified element at the end of this deque unless it would |
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* violate capacity restrictions. When using a capacity-restricted deque, |
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* this method is generally preferable to method <tt>addLast</tt> which |
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* can fail to insert an element only by throwing an exception. |
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* |
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* @param e the element to insert |
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* @return <tt>true</tt> if it was possible to insert the element, |
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* else <tt>false</tt> |
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* @throws NullPointerException if the specified element is null and this |
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* deque does not permit null elements |
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*/ |
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boolean offerLast(E e); |
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|
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/** |
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* Inserts the specified element at the front of this deque unless it |
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* would violate capacity restrictions. |
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* |
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* @param e the element to insert |
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* @throws IllegalStateException if it was not possible to insert |
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* the element due to capacity restrictions |
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* @throws NullPointerException if the specified element is null and this |
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* deque does not permit null elements |
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*/ |
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void addFirst(E e); |
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|
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/** |
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* Inserts the specified element at the end of this deque unless it would |
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* violate capacity restrictions. |
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* |
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* @param e the element to insert |
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* @throws IllegalStateException if it was not possible to insert |
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* the element due to capacity restrictions |
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* @throws NullPointerException if the specified element is null and this |
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* deque does not permit null elements |
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*/ |
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void addLast(E e); |
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|
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/** |
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* Retrieves and removes the first element of this deque, or |
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* <tt>null</tt> if this deque is empty. |
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* |
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* @return the first element of this deque, or <tt>null</tt> if |
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* this deque is empty |
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*/ |
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E pollFirst(); |
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|
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/** |
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* Retrieves and removes the last element of this deque, or |
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* <tt>null</tt> if this deque is empty. |
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* |
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* @return the last element of this deque, or <tt>null</tt> if |
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* this deque is empty |
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*/ |
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E pollLast(); |
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|
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/** |
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* Removes and returns the first element of this deque. This method |
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* differs from the <tt>pollFirst</tt> method only in that it throws an |
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* exception if this deque is empty. |
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* |
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* @return the first element of this deque |
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* @throws NoSuchElementException if this deque is empty |
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*/ |
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E removeFirst(); |
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|
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/** |
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* Retrieves and removes the last element of this deque. This method |
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* differs from the <tt>pollLast</tt> method only in that it throws an |
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* exception if this deque is empty. |
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* |
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* @return the last element of this deque |
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* @throws NoSuchElementException if this deque is empty |
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*/ |
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E removeLast(); |
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|
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/** |
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* Retrieves, but does not remove, the first element of this deque, |
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* returning <tt>null</tt> if this deque is empty. |
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* |
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* @return the first element of this deque, or <tt>null</tt> if |
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* this deque is empty |
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*/ |
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E peekFirst(); |
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|
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/** |
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* Retrieves, but does not remove, the last element of this deque, |
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* returning <tt>null</tt> if this deque is empty. |
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* |
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* @return the last element of this deque, or <tt>null</tt> if this deque |
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* is empty |
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*/ |
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E peekLast(); |
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|
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/** |
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* Retrieves, but does not remove, the first element of this |
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* deque. This method differs from the <tt>peekFirst</tt> method only |
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* in that it throws an exception if this deque is empty. |
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* |
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* @return the first element of this deque |
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* @throws NoSuchElementException if this deque is empty |
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*/ |
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E getFirst(); |
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|
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/** |
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* Retrieves, but does not remove, the last element of this |
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* deque. This method differs from the <tt>peekLast</tt> method only |
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* in that it throws an exception if this deque is empty. |
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* |
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* @return the last element of this deque |
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* @throws NoSuchElementException if this deque is empty |
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*/ |
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E getLast(); |
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|
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/** |
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* Removes the first occurrence of the specified element in this |
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* deque. If the deque does not contain the element, it is |
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* unchanged. More formally, removes the first element <tt>e</tt> |
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* such that <tt>(o==null ? e==null : o.equals(e))</tt> (if |
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* such an element exists). |
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* |
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* @param o element to be removed from this deque, if present |
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* @return <tt>true</tt> if the deque contained the specified element |
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* @throws NullPointerException if the specified element is null and this |
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* deque does not permit null elements |
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*/ |
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boolean removeFirstOccurrence(Object o); |
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|
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/** |
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* Removes the last occurrence of the specified element in this |
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* deque. If the deque does not contain the element, it is |
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* unchanged. More formally, removes the last element <tt>e</tt> |
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* such that <tt>(o==null ? e==null : o.equals(e))</tt> (if |
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* such an element exists). |
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* |
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* @param o element to be removed from this deque, if present |
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* @return <tt>true</tt> if the deque contained the specified element |
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* @throws NullPointerException if the specified element is null and this |
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* deque does not permit null elements |
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*/ |
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boolean removeLastOccurrence(Object o); |
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|
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|
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// *** Queue methods *** |
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|
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/** |
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* Inserts the specified element into the queue represented by this deque |
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* unless it would violate capacity restrictions. In other words, inserts |
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* the specified element at the end of this deque. When using a |
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* capacity-restricted deque, this method is generally preferable to the |
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* {@link #add} method, which can fail to insert an element only by |
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* throwing an exception. |
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* |
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* <p>This method is equivalent to {@link #offerLast}. |
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* |
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* @param e the element to insert |
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* @return <tt>true</tt> if it was possible to insert the element, |
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* else <tt>false</tt> |
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* @throws NullPointerException if the specified element is null and this |
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* deque does not permit null elements |
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*/ |
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boolean offer(E e); |
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|
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/** |
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* Inserts the specified element into the queue represented by this |
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* deque unless it would violate capacity restrictions. In other words, |
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* inserts the specified element as the last element of this deque. |
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* |
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* <p>This method is equivalent to {@link #addLast}. |
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* |
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* @param e the element to insert |
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* @return <tt>true</tt> (as per the spec for {@link Collection#add}) |
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* @throws IllegalStateException if it was not possible to insert |
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* the element due to capacity restrictions |
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* @throws NullPointerException if the specified element is null and this |
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* deque does not permit null elements |
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*/ |
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boolean add(E e); |
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|
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/** |
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* Retrieves and removes the head of the queue represented by |
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* this deque, or <tt>null</tt> if this deque is empty. In other words, |
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* retrieves and removes the first element of this deque, or <tt>null</tt> |
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* if this deque is empty. |
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* |
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* <p>This method is equivalent to {@link #pollFirst()}. |
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* |
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* @return the first element of this deque, or <tt>null</tt> if |
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* this deque is empty |
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*/ |
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E poll(); |
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|
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/** |
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* Retrieves and removes the head of the queue represented by this deque. |
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* This method differs from the <tt>poll</tt> method only in that it |
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* throws an exception if this deque is empty. |
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* |
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* <p>This method is equivalent to {@link #removeFirst()}. |
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* |
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* @return the head of the queue represented by this deque |
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* @throws NoSuchElementException if this deque is empty |
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*/ |
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E remove(); |
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|
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/** |
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* Retrieves, but does not remove, the head of the queue represented by |
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* this deque, returning <tt>null</tt> if this deque is empty. |
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* |
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* <p>This method is equivalent to {@link #peekFirst()}. |
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* |
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* @return the head of the queue represented by this deque, or |
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* <tt>null</tt> if this deque is empty |
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*/ |
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E peek(); |
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|
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/** |
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* Retrieves, but does not remove, the head of the queue represented by |
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* this deque. This method differs from the <tt>peek</tt> method only in |
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* that it throws an exception if this deque is empty. |
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* |
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* <p>This method is equivalent to {@link #getFirst()}. |
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* |
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* @return the head of the queue represented by this deque |
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* @throws NoSuchElementException if this deque is empty |
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*/ |
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E element(); |
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|
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|
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// *** Stack methods *** |
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|
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/** |
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* Pushes an element onto the stack represented by this deque. In other |
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* words, inserts the element at the front of this deque unless it would |
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* violate capacity restrictions. |
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* |
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* <p>This method is equivalent to {@link #addFirst}. |
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* |
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* @param e the element to push |
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* @throws IllegalStateException if it was not possible to insert |
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* the element due to capacity restrictions |
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* @throws NullPointerException if the specified element is null and this |
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* deque does not permit null elements |
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*/ |
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void push(E e); |
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|
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/** |
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* Pops an element from the stack represented by this deque. In other |
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* words, removes and returns the first element of this deque. |
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* |
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* <p>This method is equivalent to {@link #removeFirst()}. |
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* |
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* @return the element at the front of this deque (which is the top |
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* of the stack represented by this deque) |
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* @throws NoSuchElementException if this deque is empty |
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*/ |
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E pop(); |
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|
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|
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// *** Collection Method *** |
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|
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/** |
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* Returns an iterator over the elements in this deque. The elements |
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* will be ordered from first (head) to last (tail). |
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* |
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* @return an <tt>Iterator</tt> over the elements in this deque |
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*/ |
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Iterator<E> iterator(); |
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} |