Added in API level 9

Summary: Methods | Inherited Methods

Deque

public interface Deque
implements Queue<E>, SequencedCollection<E>

java.util.Deque<E>

Known indirect subclasses

ArrayDeque<E>, BlockingDeque<E>, ConcurrentLinkedDeque<E>, LinkedBlockingDeque<E>, LinkedList<E>

ArrayDeque<E>	Resizable-array implementation of the `Deque` interface.
BlockingDeque<E>	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<E>	An unbounded concurrent deque based on linked nodes.
LinkedBlockingDeque<E>	An optionally-bounded blocking deque based on linked nodes.
LinkedList<E>	Doubly-linked list implementation of the `List` and `Deque` interfaces.

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(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.
`abstract 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.
`abstract 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.
`abstract 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.
`abstract boolean`	`contains(Object o)` Returns `true` if this deque contains the specified element.
`abstract Iterator<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 Iterator<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 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.
`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(Object o)` 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(Object o)` 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(Object o)` Removes the last occurrence of the specified element from this deque.
`default Deque<E>`	`reversed()` Returns a reverse-ordered view of this collection.
`abstract int`	`size()` Returns the number of elements in this deque.

Inherited methods

From interface


        
          java.util.Queue

`abstract boolean`	`add(E e)` Inserts the specified element into this queue 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 E`	`element()` Retrieves, but does not remove, the head of this queue.
`abstract boolean`	`offer(E e)` Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions.
`abstract E`	`peek()` Retrieves, but does not remove, the head of this queue, or returns `null` if this queue is empty.
`abstract E`	`poll()` Retrieves and removes the head of this queue, or returns `null` if this queue is empty.
`abstract E`	`remove()` Retrieves and removes the head of this queue.

From interface


        
          java.util.SequencedCollection

`default void`	`addFirst(E e)` Adds an element as the first element of this collection (optional operation).
`default void`	`addLast(E e)` Adds an element as the last element of this collection (optional operation).
`default E`	`getFirst()` Gets the first element of this collection.
`default E`	`getLast()` Gets the last element of this collection.
`default E`	`removeFirst()` Removes and returns the first element of this collection (optional operation).
`default E`	`removeLast()` Removes and returns the last element of this collection (optional operation).
`abstract SequencedCollection<E>`	`reversed()` Returns a reverse-ordered view of this collection.

From interface


        
          java.util.Collection

`abstract boolean`	`add(E e)` Ensures that this collection contains the specified element (optional operation).
`abstract boolean`	`addAll(Collection<? extends E> c)` Adds all of the elements in the specified collection to this collection (optional operation).
`abstract void`	`clear()` Removes all of the elements from this collection (optional operation).
`abstract boolean`	`contains(Object o)` Returns `true` if this collection contains the specified element.
`abstract boolean`	`containsAll(Collection<?> c)` Returns `true` if this collection contains all of the elements in the specified collection.
`abstract boolean`	`equals(Object o)` Compares the specified object with this collection for equality.
`abstract int`	`hashCode()` Returns the hash code value for this collection.
`abstract boolean`	`isEmpty()` Returns `true` if this collection contains no elements.
`abstract Iterator<E>`	`iterator()` Returns an iterator over the elements in this collection.
`default Stream<E>`	`parallelStream()` Returns a possibly parallel `Stream` with this collection as its source.
`abstract boolean`	`remove(Object o)` Removes a single instance of the specified element from this collection, if it is present (optional operation).
`abstract boolean`	`removeAll(Collection<?> c)` Removes all of this collection's elements that are also contained in the specified collection (optional operation).
`default boolean`	`removeIf(Predicate<? super E> filter)` Removes all of the elements of this collection that satisfy the given predicate.
`abstract boolean`	`retainAll(Collection<?> c)` Retains only the elements in this collection that are contained in the specified collection (optional operation).
`abstract int`	`size()` Returns the number of elements in this collection.
`default Spliterator<E>`	`spliterator()` Creates a `Spliterator` over the elements in this collection.
`default Stream<E>`	`stream()` Returns a sequential `Stream` with this collection as its source.
`abstract <T> T[]`	`toArray(T[] a)` Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array.
`abstract Object[]`	`toArray()` Returns an array containing all of the elements in this collection.
`default <T> T[]`	`toArray(IntFunction<T[]> generator)` Returns an array containing all of the elements in this collection, using the provided `generator` function to allocate the returned array.

From interface


        
          java.lang.Iterable

`default void`	`forEach(Consumer<? super T> action)` Performs the given action for each element of the `Iterable` until all elements have been processed or the action throws an exception.
`abstract Iterator<E>`	`iterator()` Returns an iterator over elements of type `T`.
`default Spliterator<E>`	`spliterator()` Creates a `Spliterator` over the elements described by this `Iterable`.

Public methods

add

Added in API level 9

public abstract 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).

Parameters
`e`	`E`: the element to add

Returns
`boolean`	`true` (as specified by `Collection.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

addAll

Added in API level 9

public abstract 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.

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

Returns
`boolean`	`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

addFirst

Added in API level 9

public abstract 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`	`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

Added in API level 9

public abstract 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`	`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

contains

Added in API level 9

public abstract 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).

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

Returns
`boolean`	`true` if this deque contains the specified element

Throws
`ClassCastException`	if the class of the specified element is incompatible with this deque (ERROR(-restrictions optional/java.util.Collection# -restrictions optional))
`NullPointerException`	if the specified element is null and this deque does not permit null elements (ERROR(-restrictions optional/java.util.Collection# -restrictions optional))

descendingIterator

Added in API level 9

public abstract 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
`Iterator<E>`	an iterator over the elements in this deque in reverse sequence

element

Added in API level 9

public 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). This method differs from peek only in that it throws an exception if this deque is empty.

This method is equivalent to getFirst().

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

Throws
`NoSuchElementException`	if this deque is empty

getFirst

Added in API level 9

public abstract 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
`E`	the head of this deque

Throws
`NoSuchElementException`	if this deque is empty

getLast

Added in API level 9

public abstract 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
`E`	the tail of this deque

Throws
`NoSuchElementException`	if this deque is empty

iterator

Added in API level 9

public abstract 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).

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

offer

Added in API level 9

public 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. 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).

Parameters
`e`	`E`: the element to add

Returns
`boolean`	`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

offerFirst

Added in API level 9

public abstract 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`	`E`: the element to add

Returns
`boolean`	`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

Added in API level 9

public abstract 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`	`E`: the element to add

Returns
`boolean`	`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

peek

Added in API level 9

public 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.

This method is equivalent to peekFirst().

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

peekFirst

Added in API level 9

public abstract E peekFirst ()

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

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

peekLast

Added in API level 9

public abstract E peekLast ()

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

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

poll

Added in API level 9

public 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.

This method is equivalent to pollFirst().

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

pollFirst

Added in API level 9

public abstract E pollFirst ()

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

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

pollLast

Added in API level 9

public abstract E pollLast ()

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

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

pop

Added in API level 9

public abstract 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
`E`	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

push

Added in API level 9

public abstract 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`	`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

remove

Added in API level 9

public abstract 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().

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

Throws
`NoSuchElementException`	if this deque is empty

remove

Added in API level 9

public abstract boolean remove (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).

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

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

Returns
`boolean`	`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 (ERROR(-restrictions optional/java.util.Collection# -restrictions optional))
`NullPointerException`	if the specified element is null and this deque does not permit null elements (ERROR(-restrictions optional/java.util.Collection# -restrictions optional))

removeFirst

Added in API level 9

public abstract 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
`E`	the head of this deque

Throws
`NoSuchElementException`	if this deque is empty

removeFirstOccurrence

Added in API level 9

public abstract boolean removeFirstOccurrence (Object o)

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

Returns
`boolean`	`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 (ERROR(-restrictions optional/java.util.Collection# -restrictions optional))
`NullPointerException`	if the specified element is null and this deque does not permit null elements (ERROR(-restrictions optional/java.util.Collection# -restrictions optional))

removeLast

Added in API level 9

public abstract 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
`E`	the tail of this deque

Throws
`NoSuchElementException`	if this deque is empty

removeLastOccurrence

Added in API level 9

public abstract 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`	`Object`: element to be removed from this deque, if present

Returns
`boolean`	`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 (ERROR(-restrictions optional/java.util.Collection# -restrictions optional))
`NullPointerException`	if the specified element is null and this deque does not permit null elements (ERROR(-restrictions optional/java.util.Collection# -restrictions optional))

reversed

Added in API level 35

public Deque<E> reversed ()

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.

Implementation Requirements:

The implementation in this interface returns a reverse-ordered Deque view. The reversed() method of the view returns a reference to this Deque. Other operations on the view are implemented via calls to public methods on this Deque. The exact relationship between calls on the view and calls on this Deque is unspecified. However, order-sensitive operations generally delegate to the appropriate method with the opposite orientation. For example, calling getFirst on the view results in a call to getLast on this Deque.

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

size

Added in API level 9

public abstract int size ()

Returns the number of elements in this deque.

Returns
`int`	the number of elements in this deque