/* * 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 jsr166x; // XXX This belongs in java.util!!! XXX import java.util.*; // XXX This import goes away XXX /** * A linear collection that supports element insertion and removal at * both ends. The name deque 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. * *
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. * *
The twelve methods described above are summarized in the * following table:
* *
* | First Element (Head) | *Last Element (Tail) | *||
* | Throws exception | *Returns special value | *Throws exception | *Returns special value | *
Insert | *{@link #addFirst addFirst(e)} | *{@link #offerFirst offerFirst(e)} | *{@link #addLast addLast(e)} | *{@link #offerLast offerLast(e)} | *
Remove | *{@link #removeFirst removeFirst()} | *{@link #pollFirst pollFirst()} | *{@link #removeLast removeLast()} | *{@link #pollLast pollLast()} | *
Examine | *{@link #getFirst getFirst()} | *{@link #peekFirst peekFirst()} | *{@link #getLast getLast()} | *{@link #peekLast peekLast()} | *
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 to 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:
* *
{@code Queue} Method | *Equivalent {@code Deque} Method | *
{@link java.util.Queue#offer offer(e)} | *{@link #offerLast offerLast(e)} | *
{@link java.util.Queue#add add(e)} | *{@link #addLast addLast(e)} | *
{@link java.util.Queue#poll poll()} | *{@link #pollFirst pollFirst()} | *
{@link java.util.Queue#remove remove()} | *{@link #removeFirst removeFirst()} | *
{@link java.util.Queue#peek peek()} | *{@link #peek peekFirst()} | *
{@link java.util.Queue#element element()} | *{@link #getFirst getFirst()} | *
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 dequeue 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:
* *
Stack Method | *Equivalent {@code Deque} Method | *
{@link #push push(e)} | *{@link #addFirst addFirst(e)} | *
{@link #pop pop()} | *{@link #removeFirst removeFirst()} | *
{@link #peek peek()} | *{@link #peekFirst peekFirst()} | *
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. * *
This interface provides two methods to remove interior * elements, {@link #removeFirstOccurrence removeFirstOccurrence} and * {@link #removeLastOccurrence removeLastOccurrence}. Unlike the * {@link List} interface, this interface does not provide support for * indexed access to elements. * *
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 not 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. * *
{@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}. * *
This interface is a member of the Java Collections
* Framework.
*
* @author Doug Lea
* @author Josh Bloch
* @since 1.6
* @param This method is equivalent to {@link #offerLast}.
*
* @param e the element to insert
* @return {@code true} if it was possible to insert the element,
* else {@code false}
* @throws NullPointerException if {@code e} is null and this
* deque does not permit null elements
*/
boolean offer(E e);
/**
* Inserts the specified element into the queue represented by this
* deque unless it would violate capacity restrictions. In other words,
* inserts the specified element as the last element of this deque.
*
* This method is equivalent to {@link #addLast}.
*
* @param e the element to insert
* @return {@code true} (as per the spec for {@link Collection#add})
* @throws IllegalStateException if it was not possible to insert
* the element due to capacity restrictions
* @throws NullPointerException if {@code e} is null and this
* deque does not permit null elements
*/
boolean add(E e);
/**
* Retrieves and removes the head of the queue represented by
* this deque, or {@code null} if this deque is empty. In other words,
* retrieves and removes the first element of this deque, or {@code null}
* if this deque is empty.
*
* 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 and removes the head of the queue represented by this deque.
* This method differs from the {@code poll} method only in that it
* throws an exception if this deque is empty.
*
* 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, but does not remove, the head of the queue represented by
* this deque, returning {@code null} if this deque is empty.
*
* 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();
/**
* Retrieves, but does not remove, the head of the queue represented by
* this deque. This method differs from the {@code peek} method only in
* that it throws an exception if this deque is empty.
*
* 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();
// *** Stack methods ***
/**
* Pushes an element onto the stack represented by this deque. In other
* words, inserts the element to the front this deque unless it would
* violate capacity restrictions.
*
* This method is equivalent to {@link #addFirst}.
*
* @throws IllegalStateException if it was not possible to insert
* the element due to capacity restrictions
* @throws NullPointerException if {@code e} is null and this
* deque does not permit null elements
*/
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.
*
* 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 Method ***
/**
* Returns an iterator over the elements in this deque. The elements
* will be ordered from first (head) to last (tail).
*
* @return an {@code Iterator} over the elements in this deque
*/
Iterator