Interface Deque<E>

Type Parameters:: E - the type of elements held in this deque

All Superinterfaces:: Collection<E>, Iterable<E>, Queue<E>

All Known Subinterfaces:: BlockingDeque<E>

All Known Implementing Classes:: ArrayDeque, ConcurrentLinkedDeque, LinkedBlockingDeque, LinkedList

public interface Deque<E>
extends Queue<E>

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 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 null or false, depending on the operation). The latter form of the insert operation is designed specifically for use with capacity-restricted Deque implementations; in most implementations, insert operations cannot fail.

The twelve methods described above are summarized in the following table:

Summary of Deque methods
	Throws exception	Special value	Throws exception	Special value
	First Element (Head)		Last Element (Tail)
Insert	`addFirst(e)`	`offerFirst(e)`	`addLast(e)`	`offerLast(e)`
Remove	`removeFirst()`	`pollFirst()`	`removeLast()`	`pollLast()`
Examine	`getFirst()`	`peekFirst()`	`getLast()`	`peekLast()`

This interface extends the 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 Queue interface are precisely equivalent to Deque methods as indicated in the following table:

Comparison of Queue and Deque methods
`Queue` Method	Equivalent `Deque` Method
`add(e)`	`addLast(e)`
`offer(e)`	`offerLast(e)`
`remove()`	`removeFirst()`
`poll()`	`pollFirst()`
`element()`	`getFirst()`
`peek()`	`peekFirst()`

Deques can also be used as LIFO (Last-In-First-Out) stacks. This interface should be used in preference to the legacy Stack class. When a deque is used as a stack, elements are pushed and popped from the beginning of the deque. Stack methods are equivalent to Deque methods as indicated in the table below:

Comparison of Stack and Deque methods
Stack Method	Equivalent `Deque` Method
`push(e)`	`addFirst(e)`
`pop()`	`removeFirst()`
`peek()`	`getFirst()`

Note that the 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, removeFirstOccurrence and removeLastOccurrence.

Unlike the List interface, this interface does not provide support for indexed access to elements.

While Deque implementations are not strictly required to prohibit the insertion of null elements, they are strongly encouraged to do so. Users of any Deque implementations that do allow null elements are strongly encouraged not to take advantage of the ability to insert nulls. This is so because null is used as a special return value by various methods to indicate that the deque is empty.

Deque implementations generally do not define element-based versions of the equals and hashCode methods, but instead inherit the identity-based versions from class Object.

This interface is a member of the Java Collections Framework.

Since:: 1.6

Method Summary

All Methods Instance Methods Abstract Methods
Modifier and Type	Method	Description
`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 `true` upon success and throwing an `IllegalStateException` if no space is currently available.
`boolean`	`addAll(Collection<? extends E> c)`	Adds all of the elements in the specified collection at the end of this deque, as if by calling `addLast(E)` on each one, in the order that they are returned by the collection's iterator.
`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 `IllegalStateException` if no space is currently available.
`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 `IllegalStateException` if no space is currently available.
`boolean`	`contains(Object o)`	Returns `true` if this deque contains the specified element.
`Iterator<E>`	`descendingIterator()`	Returns an iterator over the elements in this deque in reverse sequential order.
`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).
`E`	`getFirst()`	Retrieves, but does not remove, the first element of this deque.
`E`	`getLast()`	Retrieves, but does not remove, the last element of this deque.
`Iterator<E>`	`iterator()`	Returns an iterator over the elements in this deque in proper sequence.
`boolean`	`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 `true` upon success and `false` if no space is currently available.
`boolean`	`offerFirst(E e)`	Inserts the specified element at the front of this deque unless it would violate capacity restrictions.
`boolean`	`offerLast(E e)`	Inserts the specified element at the end of this deque unless it would violate capacity restrictions.
`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 `null` if this deque is empty.
`E`	`peekFirst()`	Retrieves, but does not remove, the first element of this deque, or returns `null` if this deque is empty.
`E`	`peekLast()`	Retrieves, but does not remove, the last element of this deque, or returns `null` if this deque is empty.
`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 `null` if this deque is empty.
`E`	`pollFirst()`	Retrieves and removes the first element of this deque, or returns `null` if this deque is empty.
`E`	`pollLast()`	Retrieves and removes the last element of this deque, or returns `null` if this deque is empty.
`E`	`pop()`	Pops an element from the stack represented by this deque.
`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 `IllegalStateException` if no space is currently available.
`E`	`remove()`	Retrieves and removes the head of the queue represented by this deque (in other words, the first element of this deque).
`boolean`	`remove(Object o)`	Removes the first occurrence of the specified element from this deque.
`E`	`removeFirst()`	Retrieves and removes the first element of this deque.
`boolean`	`removeFirstOccurrence(Object o)`	Removes the first occurrence of the specified element from this deque.
`E`	`removeLast()`	Retrieves and removes the last element of this deque.
`boolean`	`removeLastOccurrence(Object o)`	Removes the last occurrence of the specified element from this deque.
`int`	`size()`	Returns the number of elements in this deque.

Methods declared in interface java.util.Collection

clear, containsAll, equals, hashCode, isEmpty, parallelStream, removeAll, removeIf, retainAll, spliterator, stream, toArray, toArray, toArray

Methods declared in interface java.lang.Iterable

forEach

Method Detail

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 IllegalStateException if no space is currently available. When using a capacity-restricted deque, it is generally preferable to use method offerFirst(E).

Parameters:: e - the element to add
Throws:: IllegalStateException - if the element cannot be added at this time due to capacity restrictions; ClassCastException - if the class of the specified element prevents it from being added to this deque; NullPointerException - if the specified element is null and this deque does not permit null elements; IllegalArgumentException - if some property of the specified element prevents it from being added to this deque

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 IllegalStateException if no space is currently available. When using a capacity-restricted deque, it is generally preferable to use method offerLast(E).

This method is equivalent to add(E).

Parameters:: e - the element to add
Throws:: IllegalStateException - if the element cannot be added at this time due to capacity restrictions; ClassCastException - if the class of the specified element prevents it from being added to this deque; NullPointerException - if the specified element is null and this deque does not permit null elements; IllegalArgumentException - if some property of the specified element prevents it from being added to this deque

offerFirst

boolean 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 addFirst(E) method, which can fail to insert an element only by throwing an exception.

Parameters:: e - the element to add
Returns:: true if the element was added to this deque, else false
Throws:: ClassCastException - if the class of the specified element prevents it from being added to this deque; NullPointerException - if the specified element is null and this deque does not permit null elements; IllegalArgumentException - if some property of the specified element prevents it from being added to this deque

offerLast

boolean 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 addLast(E) method, which can fail to insert an element only by throwing an exception.

Parameters:: e - the element to add
Returns:: true if the element was added to this deque, else false
Throws:: ClassCastException - if the class of the specified element prevents it from being added to this deque; NullPointerException - if the specified element is null and this deque does not permit null elements; IllegalArgumentException - if some property of the specified element prevents it from being added to this deque

removeFirst

E removeFirst()

Retrieves and removes the first element of this deque. This method differs from pollFirst only in that it throws an exception if this deque is empty.

Returns:: the head of this deque
Throws:: NoSuchElementException - if this deque is empty

removeLast

E removeLast()

Retrieves and removes the last element of this deque. This method differs from pollLast only in that it throws an exception if this deque is empty.

Returns:: the tail of this deque
Throws:: NoSuchElementException - if this deque is empty

pollFirst

E pollFirst()

Retrieves and removes the first element of this deque, or returns null if this deque is empty.

Returns:: the head of this deque, or null if this deque is empty

pollLast

E pollLast()

Retrieves and removes the last element of this deque, or returns null if this deque is empty.

Returns:: the tail of this deque, or null if this deque is empty

getFirst

E getFirst()

Retrieves, but does not remove, the first element of this deque. This method differs from peekFirst only in that it throws an exception if this deque is empty.

Returns:: the head of this deque
Throws:: NoSuchElementException - if this deque is empty

getLast

E getLast()

Retrieves, but does not remove, the last element of this deque. This method differs from peekLast only in that it throws an exception if this deque is empty.

Returns:: the tail of this deque
Throws:: NoSuchElementException - if this deque is empty

peekFirst

E peekFirst()

Retrieves, but does not remove, the first element of this deque, or returns null if this deque is empty.

Returns:: the head of this deque, or null if this deque is empty

peekLast

E peekLast()

Retrieves, but does not remove, the last element of this deque, or returns null if this deque is empty.

Returns:: the tail of this deque, or null if this deque is empty

removeFirstOccurrence

boolean removeFirstOccurrence(Object 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 e such that Objects.equals(o, e) (if such an element exists). Returns true if this deque contained the specified element (or equivalently, if this deque changed as a result of the call).

Parameters:: o - element to be removed from this deque, if present
Returns:: 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 (optional); NullPointerException - if the specified element is null and this deque does not permit null elements (optional)

removeLastOccurrence

boolean removeLastOccurrence(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 e such that Objects.equals(o, e) (if such an element exists). Returns true if this deque contained the specified element (or equivalently, if this deque changed as a result of the call).

Parameters:: o - element to be removed from this deque, if present
Returns:: 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 (optional); NullPointerException - if the specified element is null and this deque does not permit null elements (optional)

add

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 true upon success and throwing an IllegalStateException if no space is currently available. When using a capacity-restricted deque, it is generally preferable to use offer.

This method is equivalent to addLast(E).

Specified by:: add in interface Collection<E>
Specified by:: add in interface Queue<E>
Parameters:: e - the element to add
Returns:: true (as specified by Collection.add(E))
Throws:: IllegalStateException - if the element cannot be added at this time due to capacity restrictions; ClassCastException - if the class of the specified element prevents it from being added to this deque; NullPointerException - if the specified element is null and this deque does not permit null elements; IllegalArgumentException - if some property of the specified element prevents it from being added to this deque

offer

boolean 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 true upon success and false if no space is currently available. When using a capacity-restricted deque, this method is generally preferable to the add(E) method, which can fail to insert an element only by throwing an exception.

This method is equivalent to offerLast(E).

Specified by:: offer in interface Queue<E>
Parameters:: e - the element to add
Returns:: true if the element was added to this deque, else false
Throws:: ClassCastException - if the class of the specified element prevents it from being added to this deque; NullPointerException - if the specified element is null and this deque does not permit null elements; IllegalArgumentException - if some property of the specified element prevents it from being added to this deque

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 poll() only in that it throws an exception if this deque is empty.

This method is equivalent to removeFirst().

Specified by:: remove in interface Queue<E>
Returns:: the head of the queue represented by this deque
Throws:: NoSuchElementException - 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 null if this deque is empty.

This method is equivalent to pollFirst().

Specified by:: poll in interface Queue<E>
Returns:: the first element of this deque, or null if this deque is empty

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 peek only in that it throws an exception if this deque is empty.

This method is equivalent to getFirst().

Specified by:: element in interface Queue<E>
Returns:: the head of the queue represented by this deque
Throws:: NoSuchElementException - if this deque is empty

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 null if this deque is empty.

This method is equivalent to peekFirst().

Specified by:: peek in interface Queue<E>
Returns:: the head of the queue represented by this deque, or null if this deque is empty

addAll

boolean addAll(Collection<? extends E> c)

Adds all of the elements in the specified collection at the end of this deque, as if by calling addLast(E) on each one, in the order that they are returned by the collection's iterator.

When using a capacity-restricted deque, it is generally preferable to call offer separately on each element.

An exception encountered while trying to add an element may result in only some of the elements having been successfully added when the associated exception is thrown.

Specified by:: addAll in interface Collection<E>
Parameters:: c - the elements to be inserted into this deque
Returns:: true if this deque changed as a result of the call
Throws:: IllegalStateException - if not all the elements can be added at this time due to insertion restrictions; ClassCastException - if the class of an element of the specified collection prevents it from being added to this deque; NullPointerException - if the specified collection contains a null element and this deque does not permit null elements, or if the specified collection is null; IllegalArgumentException - if some property of an element of the specified collection prevents it from being added to this deque
See Also:: Collection.add(Object)

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 IllegalStateException if no space is currently available.

This method is equivalent to addFirst(E).

Parameters:: e - the element to push
Throws:: IllegalStateException - if the element cannot be added at this time due to capacity restrictions; ClassCastException - if the class of the specified element prevents it from being added to this deque; NullPointerException - if the specified element is null and this deque does not permit null elements; IllegalArgumentException - if some property of the specified element prevents it from being added to this deque

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.

This method is equivalent to removeFirst().

Returns:: 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

remove

boolean remove(Object o)

This method is equivalent to removeFirstOccurrence(Object).

Specified by:: remove in interface Collection<E>
Parameters:: o - element to be removed from this deque, if present
Returns:: 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 (optional); NullPointerException - if the specified element is null and this deque does not permit null elements (optional)

contains

boolean contains(Object o)

Returns true if this deque contains the specified element. More formally, returns true if and only if this deque contains at least one element e such that Objects.equals(o, e).

Specified by:: contains in interface Collection<E>
Parameters:: o - element whose presence in this deque is to be tested
Returns:: true if this deque contains the specified element
Throws:: ClassCastException - if the class of the specified element is incompatible with this deque (optional); NullPointerException - if the specified element is null and this deque does not permit null elements (optional)

size

int size()

Returns the number of elements in this deque.

Specified by:: size in interface Collection<E>
Returns:: the number of elements in this deque

iterator

Iterator<E> iterator()

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).

Specified by:: iterator in interface Collection<E>
Specified by:: iterator in interface Iterable<E>
Returns:: an iterator over the elements in this deque in proper sequence

descendingIterator

Iterator<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).

Returns:: an iterator over the elements in this deque in reverse sequence