Deque

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:

<table BORDER CELLPADDING=3 CELLSPACING=1> <caption>Summary of Deque methods</caption> <tr> <td></td> <td ALIGN=CENTER COLSPAN = 2> First Element (Head)</td> <td ALIGN=CENTER COLSPAN = 2> Last Element (Tail)</td> </tr> <tr> <td></td> <td ALIGN=CENTER>Throws exception</td> <td ALIGN=CENTER>Special value</td> <td ALIGN=CENTER>Throws exception</td> <td ALIGN=CENTER>Special value</td> </tr> <tr> <td>Insert</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>Remove</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>Examine</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>

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> {@code Queue} Method</td> <td ALIGN=CENTER> Equivalent {@code Deque} Method</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>

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> Stack Method</td> <td ALIGN=CENTER> Equivalent {@code Deque} Method</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>

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 <a href="{@docRoot}/../technotes/guides/collections/index.html"> Java Collections Framework</a>.

@author Doug Lea @author Josh Bloch @param !E the type of elements held in this collection

Members

Functions

add bool 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 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}.
addFirst void addFirst(E 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}.
addLast void addLast(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}.
contains bool contains(E 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>.
descendingIterator InputRange!E descendingIterator(): 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).
element 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). This method differs from {@link #peek peek} only in that it throws an exception if this deque is empty.
getFirst E getFirst(): Retrieves, but does not remove, the first element of this deque.
getLast E getLast(): 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.
offer bool offer(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.
offerFirst bool offerFirst(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.
offerLast bool offerLast(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.
peek E peek(): 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.
peekFirst E peekFirst(): Retrieves, but does not remove, the first element of this deque, or returns {@code null} if this deque is empty.
peekLast E peekLast(): Retrieves, but does not remove, the last element of this deque, or returns {@code null} if this deque is empty.
poll E poll(): 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.
pollFirst E pollFirst(): Retrieves and removes the first element of this deque, or returns {@code null} if this deque is empty.
pollLast E pollLast(): Retrieves and removes the last element of this deque, or returns {@code null} if this deque is empty.
pop E pop(): Pops an element from the stack represented by this deque. In other words, removes and returns the first element of this deque.
push void push(E e): 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.
remove E remove(): 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.
remove bool remove(E o): 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).
removeFirst E removeFirst(): 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.
removeFirstOccurrence bool removeFirstOccurrence(E o): 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).
removeLast E removeLast(): 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.
size int size(): Returns the number of elements in this deque.

Deque

Members

Functions

Meta

Source