Added in API level 9

Deque

interface Deque<E : Any!> : Queue<E>, SequencedCollection<E>

java.util.Deque

Known Direct Subclasses

ArrayDeque, BlockingDeque, ConcurrentLinkedDeque, LinkedList

ArrayDeque	Resizable-array implementation of the `Deque` interface.
BlockingDeque	A `Deque` that additionally supports blocking operations that wait for the deque to become non-empty when retrieving an element, and wait for space to become available in the deque when storing an element.
ConcurrentLinkedDeque	An unbounded concurrent deque based on linked nodes.
LinkedList	Doubly-linked list implementation of the `List` and `Deque` interfaces.

Known Indirect Subclasses

LinkedBlockingDeque

An optionally-bounded blocking deque based on linked nodes.

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.

Summary

Public methods
abstract Boolean	`add(element: 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.
abstract Boolean	`addAll(elements: Collection<E>)` Adds all of the elements in the specified collection at the end of this deque, as if by calling `addLast` on each one, in the order that they are returned by the collection's iterator.
abstract Unit	`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.
abstract Unit	`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.
abstract Boolean	`contains(element: E?)` Returns `true` if this deque contains the specified element.
abstract MutableIterator<E>	`descendingIterator()` Returns an iterator over the elements in this deque in reverse sequential order.
abstract 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).
abstract E	`getFirst()` Retrieves, but does not remove, the first element of this deque.
abstract E	`getLast()` Retrieves, but does not remove, the last element of this deque.
abstract MutableIterator<E>	`iterator()` Returns an iterator over the elements in this deque in proper sequence.
abstract 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.
abstract Boolean	`offerFirst(e: E)` Inserts the specified element at the front of this deque unless it would violate capacity restrictions.
abstract Boolean	`offerLast(e: E)` Inserts the specified element at the end of this deque unless it would violate capacity restrictions.
abstract 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.
abstract E?	`peekFirst()` Retrieves, but does not remove, the first element of this deque, or returns `null` if this deque is empty.
abstract E?	`peekLast()` Retrieves, but does not remove, the last element of this deque, or returns `null` if this deque is empty.
abstract 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.
abstract E?	`pollFirst()` Retrieves and removes the first element of this deque, or returns `null` if this deque is empty.
abstract E?	`pollLast()` Retrieves and removes the last element of this deque, or returns `null` if this deque is empty.
abstract E	`pop()` Pops an element from the stack represented by this deque.
abstract Unit	`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.
abstract E	`remove()` Retrieves and removes the head of the queue represented by this deque (in other words, the first element of this deque).
abstract Boolean	`remove(element: E?)` Removes the first occurrence of the specified element from this deque.
abstract E	`removeFirst()` Retrieves and removes the first element of this deque.
abstract Boolean	`removeFirstOccurrence(o: Any?)` Removes the first occurrence of the specified element from this deque.
abstract E	`removeLast()` Retrieves and removes the last element of this deque.
abstract Boolean	`removeLastOccurrence(o: Any?)` Removes the last occurrence of the specified element from this deque.
open Deque<E>	`reversed()` Returns a reverse-ordered view of this collection.

Properties
abstract Int	`size` Returns the number of elements in this deque.

Public methods

add

Added in API level 9

abstract fun add(element: E): Boolean

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.

Parameters
`e`	the element to add

Return
`Boolean`	`true` (as specified by `Collection.add`)

Exceptions
`java.lang.UnsupportedOperationException`	if the `add` operation is not supported by this collection
`java.lang.ClassCastException`	if the class of the specified element prevents it from being added to this deque
`java.lang.NullPointerException`	if the specified element is null and this deque does not permit null elements
`java.lang.IllegalArgumentException`	if some property of the specified element prevents it from being added to this deque
`java.lang.IllegalStateException`	if the element cannot be added at this time due to capacity restrictions

addAll

Added in API level 9

abstract fun addAll(elements: Collection<E>): Boolean

Adds all of the elements in the specified collection at the end of this deque, as if by calling addLast 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.

Parameters
`c`	the elements to be inserted into this deque

Return
`Boolean`	`true` if this deque changed as a result of the call

Exceptions
`java.lang.UnsupportedOperationException`	if the `addAll` operation is not supported by this collection
`java.lang.ClassCastException`	if the class of an element of the specified collection prevents it from being added to this deque
`java.lang.NullPointerException`	if the specified collection contains a null element and this deque does not permit null elements, or if the specified collection is null
`java.lang.IllegalArgumentException`	if some property of an element of the specified collection prevents it from being added to this deque
`java.lang.IllegalStateException`	if not all the elements can be added at this time due to insertion restrictions

addFirst

Added in API level 9

abstract fun addFirst(e: E): Unit

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.

Parameters
`e`	E: the element to add

Exceptions
`java.lang.NullPointerException`	if the specified element is null and this deque does not permit null elements
`java.lang.UnsupportedOperationException`	if this collection implementation does not support this operation
`java.lang.IllegalStateException`	if the element cannot be added at this time due to capacity restrictions
`java.lang.ClassCastException`	if the class of the specified element prevents it from being added to this deque
`java.lang.IllegalArgumentException`	if some property of the specified element prevents it from being added to this deque

addLast

Added in API level 9

abstract fun addLast(e: E): Unit

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.

This method is equivalent to add.

Parameters
`e`	E: the element to add

Exceptions
`java.lang.NullPointerException`	if the specified element is null and this deque does not permit null elements
`java.lang.UnsupportedOperationException`	if this collection implementation does not support this operation
`java.lang.IllegalStateException`	if the element cannot be added at this time due to capacity restrictions
`java.lang.ClassCastException`	if the class of the specified element prevents it from being added to this deque
`java.lang.IllegalArgumentException`	if some property of the specified element prevents it from being added to this deque

contains

Added in API level 9

abstract fun contains(element: E?): Boolean

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).

Parameters
`o`	element whose presence in this deque is to be tested

Return
`Boolean`	`true` if this deque contains the specified element

Exceptions
`java.lang.ClassCastException`	if the class of the specified element is incompatible with this deque (java.util.Collection#)
`java.lang.NullPointerException`	if the specified element is null and this deque does not permit null elements (java.util.Collection#)

descendingIterator

Added in API level 9

abstract fun descendingIterator(): MutableIterator<E>

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
`MutableIterator<E>`	an iterator over the elements in this deque in reverse sequence

element

Added in API level 9

abstract fun element(): E

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().

Return
`E`	the head of the queue represented by this deque

Exceptions
`java.util.NoSuchElementException`	if this deque is empty

getFirst

Added in API level 9

abstract fun getFirst(): E

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.

Return
`E`	the head of this deque

Exceptions
`java.util.NoSuchElementException`	if this deque is empty

getLast

Added in API level 9

abstract fun getLast(): E

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.

Return
`E`	the tail of this deque

Exceptions
`java.util.NoSuchElementException`	if this deque is empty

iterator

Added in API level 9

abstract fun iterator(): MutableIterator<E>

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
`MutableIterator<E>`	an iterator over the elements in this deque in proper sequence

offer

Added in API level 9

abstract fun offer(e: E): Boolean

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 method, which can fail to insert an element only by throwing an exception.

This method is equivalent to offerLast.

Parameters
`e`	E: the element to add

Return
`Boolean`	`true` if the element was added to this deque, else `false`

Exceptions
`java.lang.ClassCastException`	if the class of the specified element prevents it from being added to this deque
`java.lang.NullPointerException`	if the specified element is null and this deque does not permit null elements
`java.lang.IllegalArgumentException`	if some property of the specified element prevents it from being added to this deque

offerFirst

Added in API level 9

abstract fun offerFirst(e: E): Boolean

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 method, which can fail to insert an element only by throwing an exception.

Parameters
`e`	E: the element to add

Return
`Boolean`	`true` if the element was added to this deque, else `false`

Exceptions
`java.lang.ClassCastException`	if the class of the specified element prevents it from being added to this deque
`java.lang.NullPointerException`	if the specified element is null and this deque does not permit null elements
`java.lang.IllegalArgumentException`	if some property of the specified element prevents it from being added to this deque

offerLast

Added in API level 9

abstract fun offerLast(e: E): Boolean

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 method, which can fail to insert an element only by throwing an exception.

Parameters
`e`	E: the element to add

Return
`Boolean`	`true` if the element was added to this deque, else `false`

Exceptions
`java.lang.ClassCastException`	if the class of the specified element prevents it from being added to this deque
`java.lang.NullPointerException`	if the specified element is null and this deque does not permit null elements
`java.lang.IllegalArgumentException`	if some property of the specified element prevents it from being added to this deque

peek

Added in API level 9

abstract fun peek(): E?

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().

Return
`E?`	the head of the queue represented by this deque, or `null` if this deque is empty

peekFirst

Added in API level 9

abstract fun peekFirst(): E?

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

Return
`E?`	the head of this deque, or `null` if this deque is empty

peekLast

Added in API level 9

abstract fun peekLast(): E?

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

Return
`E?`	the tail of this deque, or `null` if this deque is empty

poll

Added in API level 9

abstract fun poll(): E?

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().

Return
`E?`	the first element of this deque, or `null` if this deque is empty

pollFirst

Added in API level 9

abstract fun pollFirst(): E?

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

Return
`E?`	the head of this deque, or `null` if this deque is empty

pollLast

Added in API level 9

abstract fun pollLast(): E?

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

Return
`E?`	the tail of this deque, or `null` if this deque is empty

pop

Added in API level 9

abstract fun pop(): 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 removeFirst().

Return
`E`	the element at the front of this deque (which is the top of the stack represented by this deque)

Exceptions
`java.util.NoSuchElementException`	if this deque is empty

push

Added in API level 9

abstract fun push(e: E): Unit

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.

Parameters
`e`	E: the element to push

Exceptions
`java.lang.IllegalStateException`	if the element cannot be added at this time due to capacity restrictions
`java.lang.ClassCastException`	if the class of the specified element prevents it from being added to this deque
`java.lang.NullPointerException`	if the specified element is null and this deque does not permit null elements
`java.lang.IllegalArgumentException`	if some property of the specified element prevents it from being added to this deque

remove

Added in API level 9

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

This method is equivalent to removeFirst().

Return
`E`	the head of the queue represented by this deque

Exceptions
`java.util.NoSuchElementException`	if this deque is empty

remove

Added in API level 9

abstract fun remove(element: E?): Boolean

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).

This method is equivalent to removeFirstOccurrence(java.lang.Object).

Parameters
`o`	element to be removed from this deque, if present

Return
`Boolean`	`true` if an element was removed as a result of this call

Exceptions
`java.lang.ClassCastException`	if the class of the specified element is incompatible with this deque (java.util.Collection#)
`java.lang.NullPointerException`	if the specified element is null and this deque does not permit null elements (java.util.Collection#)
`java.lang.UnsupportedOperationException`	if the `remove` operation is not supported by this collection

removeFirst

Added in API level 9

abstract fun removeFirst(): E

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.

Return
`E`	the head of this deque

Exceptions
`java.util.NoSuchElementException`	if this deque is empty
`java.lang.UnsupportedOperationException`	if this collection implementation does not support this operation

removeFirstOccurrence

Added in API level 9

abstract fun removeFirstOccurrence(o: Any?): Boolean

Parameters
`o`	Any?: element to be removed from this deque, if present

Return
`Boolean`	`true` if an element was removed as a result of this call

Exceptions
`java.lang.ClassCastException`	if the class of the specified element is incompatible with this deque (java.util.Collection#)
`java.lang.NullPointerException`	if the specified element is null and this deque does not permit null elements (java.util.Collection#)

removeLast

Added in API level 9

abstract fun removeLast(): E

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.

Return
`E`	the tail of this deque

Exceptions
`java.util.NoSuchElementException`	if this deque is empty
`java.lang.UnsupportedOperationException`	if this collection implementation does not support this operation

removeLastOccurrence

Added in API level 9

abstract fun removeLastOccurrence(o: Any?): Boolean

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`	Any?: element to be removed from this deque, if present

Return
`Boolean`	`true` if an element was removed as a result of this call

Exceptions
`java.lang.ClassCastException`	if the class of the specified element is incompatible with this deque (java.util.Collection#)
`java.lang.NullPointerException`	if the specified element is null and this deque does not permit null elements (java.util.Collection#)

reversed

Added in API level 35

open fun reversed(): Deque<E>

Returns a reverse-ordered view of this collection. The encounter order of elements in the returned view is the inverse of the encounter order of elements in this collection. The reverse ordering affects all order-sensitive operations, including those on the view collections of the returned view. If the collection implementation permits modifications to this view, the modifications "write through" to the underlying collection. Changes to the underlying collection might or might not be visible in this reversed view, depending upon the implementation.

Return
`Deque<E>`	a reverse-ordered view of this collection, as a `Deque`

Properties

size

Added in API level 9

abstract val size: Int

Returns the number of elements in this deque.

Return
`Int`	the number of elements in this deque