Added in API level 21

Summary: Ctors | Methods | Inherited Methods

ConcurrentLinkedDeque

public class ConcurrentLinkedDeque
extends AbstractCollection<E> implements Deque<E>, Serializable

java.lang.Object
↳	java.util.AbstractCollection<E>
	↳	java.util.concurrent.ConcurrentLinkedDeque<E>

An unbounded concurrent deque based on linked nodes. Concurrent insertion, removal, and access operations execute safely across multiple threads. A ConcurrentLinkedDeque is an appropriate choice when many threads will share access to a common collection. Like most other concurrent collection implementations, this class does not permit the use of null elements.

Iterators and spliterators are weakly consistent.

Beware that, unlike in most collections, the size method is NOT a constant-time operation. Because of the asynchronous nature of these deques, determining the current number of elements requires a traversal of the elements, and so may report inaccurate results if this collection is modified during traversal.

Bulk operations that add, remove, or examine multiple elements, such as addAll(Collection), removeIf(Predicate) or forEach(Consumer), are not guaranteed to be performed atomically. For example, a forEach traversal concurrent with an addAll operation might observe only some of the added elements.

This class and its iterator implement all of the optional methods of the Deque and Iterator interfaces.

Memory consistency effects: As with other concurrent collections, actions in a thread prior to placing an object into a ConcurrentLinkedDeque happen-before actions subsequent to the access or removal of that element from the ConcurrentLinkedDeque in another thread.

This class is a member of the Java Collections Framework.

Summary

Public constructors
`ConcurrentLinkedDeque()` Constructs an empty deque.
`ConcurrentLinkedDeque(Collection<? extends E> c)` Constructs a deque initially containing the elements of the given collection, added in traversal order of the collection's iterator.

Public methods
`boolean`	`add(E e)` Inserts the specified element at the tail of this deque.
`boolean`	`addAll(Collection<? extends E> c)` Appends all of the elements in the specified collection to the end of this deque, in the order that they are returned by the specified collection's iterator.
`void`	`addFirst(E e)` Inserts the specified element at the front of this deque.
`void`	`addLast(E e)` Inserts the specified element at the end of this deque.
`void`	`clear()` Removes all of the elements from this deque.
`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).
`void`	`forEach(Consumer<? super E> action)`
`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.
`boolean`	`isEmpty()` Returns `true` if this collection contains no elements.
`Iterator<E>`	`iterator()` Returns an iterator over the elements in this deque in proper sequence.
`boolean`	`offer(E e)` Inserts the specified element at the tail of this deque.
`boolean`	`offerFirst(E e)` Inserts the specified element at the front of this deque.
`boolean`	`offerLast(E e)` Inserts the specified element at the end of this deque.
`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.
`boolean`	`removeAll(Collection<?> c)` Removes all of this collection's elements that are also contained in the specified collection (optional operation).
`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.
`boolean`	`removeIf(Predicate<? super E> filter)` Removes all of the elements of this collection that satisfy the given predicate.
`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.
`boolean`	`retainAll(Collection<?> c)` Retains only the elements in this collection that are contained in the specified collection (optional operation).
`int`	`size()` Returns the number of elements in this deque.
`Spliterator<E>`	`spliterator()` Returns a `Spliterator` over the elements in this deque.
`Object[]`	`toArray()` Returns an array containing all of the elements in this deque, in proper sequence (from first to last element).
`<T> T[]`	`toArray(T[] a)` Returns an array containing all of the elements in this deque, in proper sequence (from first to last element); the runtime type of the returned array is that of the specified array.
`String`	`toString()` Returns a string representation of this collection.

Inherited methods

From class


        
          java.util.AbstractCollection

`boolean`	`add(E e)` Ensures that this collection contains the specified element (optional operation).
`boolean`	`addAll(Collection<? extends E> c)` Adds all of the elements in the specified collection to this collection (optional operation).
`void`	`clear()` Removes all of the elements from this collection (optional operation).
`boolean`	`contains(Object o)` Returns `true` if this collection contains the specified element.
`boolean`	`containsAll(Collection<?> c)` Returns `true` if this collection contains all of the elements in the specified collection.
`boolean`	`isEmpty()` Returns `true` if this collection contains no elements.
`abstract Iterator<E>`	`iterator()` Returns an iterator over the elements contained in this collection.
`boolean`	`remove(Object o)` Removes a single instance of the specified element from this collection, if it is present (optional operation).
`boolean`	`removeAll(Collection<?> c)` Removes all of this collection's elements that are also contained in the specified collection (optional operation).
`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.
`<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.
`Object[]`	`toArray()` Returns an array containing all of the elements in this collection.
`String`	`toString()` Returns a string representation of this collection.

From class


        
          java.lang.Object

`Object`	`clone()` Creates and returns a copy of this object.
`boolean`	`equals(Object obj)` Indicates whether some other object is "equal to" this one.
`void`	`finalize()` Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.
`final Class<?>`	`getClass()` Returns the runtime class of this `Object`.
`int`	`hashCode()` Returns a hash code value for the object.
`final void`	`notify()` Wakes up a single thread that is waiting on this object's monitor.
`final void`	`notifyAll()` Wakes up all threads that are waiting on this object's monitor.
`String`	`toString()` Returns a string representation of the object.
`final void`	`wait(long timeoutMillis, int nanos)` Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.
`final void`	`wait(long timeoutMillis)` Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.
`final void`	`wait()` Causes the current thread to wait until it is awakened, typically by being notified or interrupted.

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.util.Deque

`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.

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`.

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.

Public constructors

ConcurrentLinkedDeque

Added in API level 21

public ConcurrentLinkedDeque ()

Constructs an empty deque.

ConcurrentLinkedDeque

Added in API level 21

public ConcurrentLinkedDeque (Collection<? extends E> c)

Constructs a deque initially containing the elements of the given collection, added in traversal order of the collection's iterator.

Parameters
`c`	`Collection`: the collection of elements to initially contain

Throws
`NullPointerException`	if the specified collection or any of its elements are null

Public methods

add

Added in API level 21

public boolean add (E e)

Inserts the specified element at the tail of this deque. As the deque is unbounded, this method will never throw IllegalStateException or return false.

Parameters
`e`	`E`: element whose presence in this collection is to be ensured

Returns
`boolean`	`true` (as specified by `Collection#add`)

Throws
`NullPointerException`	if the specified element is null

addAll

Added in API level 21

public boolean addAll (Collection<? extends E> c)

Appends all of the elements in the specified collection to the end of this deque, in the order that they are returned by the specified collection's iterator. Attempts to addAll of a deque to itself result in IllegalArgumentException.

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
`NullPointerException`	if the specified collection or any of its elements are null
`IllegalArgumentException`	if the collection is this deque

addFirst

Added in API level 21

public void addFirst (E e)

Inserts the specified element at the front of this deque. As the deque is unbounded, this method will never throw IllegalStateException.

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

Throws
`NullPointerException`	if the specified element is null

addLast

Added in API level 21

public void addLast (E e)

Inserts the specified element at the end of this deque. As the deque is unbounded, this method will never throw IllegalStateException.

This method is equivalent to add(E).

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

Throws
`NullPointerException`	if the specified element is null

clear

Added in API level 21

public void clear ()

Removes all of the elements from this deque.

contains

Added in API level 21

public 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 o.equals(e).

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

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

descendingIterator

Added in API level 21

public 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).

The returned iterator is weakly consistent.

Returns
`Iterator<E>`	an iterator over the elements in this deque in reverse order

element

Added in API level 21

public 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`

forEach

Added in API level 24

public void forEach (Consumer<? super E> action)

Parameters
`action`	`Consumer`

Throws
`NullPointerException`

getFirst

Added in API level 21

public 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`

getLast

Added in API level 21

public 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`

isEmpty

Added in API level 21

public boolean isEmpty ()

Returns true if this collection contains no elements.

Returns
`boolean`	`true` if this collection contains no elements

iterator

Added in API level 21

public 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).

The returned iterator is weakly consistent.

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

offer

Added in API level 21

public boolean offer (E e)

Inserts the specified element at the tail of this deque. As the deque is unbounded, this method will never return false.

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

Returns
`boolean`	`true` (as specified by `Queue#offer`)

Throws
`NullPointerException`	if the specified element is null

offerFirst

Added in API level 21

public boolean offerFirst (E e)

Inserts the specified element at the front of this deque. As the deque is unbounded, this method will never return false.

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

Returns
`boolean`	`true` (as specified by `Deque#offerFirst`)

Throws
`NullPointerException`	if the specified element is null

offerLast

Added in API level 21

public boolean offerLast (E e)

Inserts the specified element at the end of this deque. As the deque is unbounded, this method will never return false.

This method is equivalent to add(E).

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

Returns
`boolean`	`true` (as specified by `Deque#offerLast`)

Throws
`NullPointerException`	if the specified element is null

peek

Added in API level 21

public 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 21

public 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 21

public 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 21

public 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 21

public 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 21

public 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 21

public 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`

push

Added in API level 21

public 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
`NullPointerException`

remove

Added in API level 21

public 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`

remove

Added in API level 21

public 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 o.equals(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 the deque contained the specified element

Throws
`NullPointerException`	if the specified element is null

removeAll

Added in API level 21

public boolean removeAll (Collection<?> c)

Removes all of this collection's elements that are also contained in the specified collection (optional operation). After this call returns, this collection will contain no elements in common with the specified collection.

Parameters
`c`	`Collection`: collection containing elements to be removed from this collection

Returns
`boolean`	`true` if this collection changed as a result of the call

Throws
`NullPointerException`

removeFirst

Added in API level 21

public 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`

removeFirstOccurrence

Added in API level 21

public boolean removeFirstOccurrence (Object o)

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

Returns
`boolean`	`true` if the deque contained the specified element

Throws
`NullPointerException`	if the specified element is null

removeIf

Added in API level 24

public boolean removeIf (Predicate<? super E> filter)

Removes all of the elements of this collection that satisfy the given predicate. Errors or runtime exceptions thrown during iteration or by the predicate are relayed to the caller.

Parameters
`filter`	`Predicate`: a predicate which returns `true` for elements to be removed

Returns
`boolean`	`true` if any elements were removed

Throws
`NullPointerException`

removeLast

Added in API level 21

public 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`

removeLastOccurrence

Added in API level 21

public 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 o.equals(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 the deque contained the specified element

Throws
`NullPointerException`	if the specified element is null

retainAll

Added in API level 21

public boolean retainAll (Collection<?> c)

Retains only the elements in this collection that are contained in the specified collection (optional operation). In other words, removes from this collection all of its elements that are not contained in the specified collection.

Parameters
`c`	`Collection`: collection containing elements to be retained in this collection

Returns
`boolean`	`true` if this collection changed as a result of the call

Throws
`NullPointerException`

size

Added in API level 21

public int size ()

Returns the number of elements in this deque. If this deque contains more than Integer.MAX_VALUE elements, it returns Integer.MAX_VALUE.

Beware that, unlike in most collections, this method is NOT a constant-time operation. Because of the asynchronous nature of these deques, determining the current number of elements requires traversing them all to count them. Additionally, it is possible for the size to change during execution of this method, in which case the returned result will be inaccurate. Thus, this method is typically not very useful in concurrent applications.

Returns
`int`	the number of elements in this deque

spliterator

Added in API level 24

public Spliterator<E> spliterator ()

Returns a Spliterator over the elements in this deque.

The returned spliterator is weakly consistent.

The Spliterator reports Spliterator#CONCURRENT, Spliterator#ORDERED, and Spliterator#NONNULL.

Implementation Note:

The Spliterator implements trySplit to permit limited parallelism.

Returns
`Spliterator<E>`	a `Spliterator` over the elements in this deque

toArray

Added in API level 21

public Object[] toArray ()

Returns an array containing all of the elements in this deque, in proper sequence (from first to last element).

The returned array will be "safe" in that no references to it are maintained by this deque. (In other words, this method must allocate a new array). The caller is thus free to modify the returned array.

This method acts as bridge between array-based and collection-based APIs.

Returns
`Object[]`	an array containing all of the elements in this deque

toArray

Added in API level 21

public T[] toArray (T[] a)

Returns an array containing all of the elements in this deque, in proper sequence (from first to last element); the runtime type of the returned array is that of the specified array. If the deque fits in the specified array, it is returned therein. Otherwise, a new array is allocated with the runtime type of the specified array and the size of this deque.

If this deque fits in the specified array with room to spare (i.e., the array has more elements than this deque), the element in the array immediately following the end of the deque is set to null.

Like the toArray() method, this method acts as bridge between array-based and collection-based APIs. Further, this method allows precise control over the runtime type of the output array, and may, under certain circumstances, be used to save allocation costs.

Suppose x is a deque known to contain only strings. The following code can be used to dump the deque into a newly allocated array of String:

 String[] y = x.toArray(new String[0]);

Note that toArray(new Object[0]) is identical in function to toArray().

Parameters
`a`	`T`: the array into which the elements of the deque are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purpose

Returns
`T[]`	an array containing all of the elements in this deque

Throws
`ArrayStoreException`	if the runtime type of the specified array is not a supertype of the runtime type of every element in this deque
`NullPointerException`	if the specified array is null

toString

Added in API level 21

public String toString ()

Returns a string representation of this collection. The string representation consists of a list of the collection's elements in the order they are returned by its iterator, enclosed in square brackets ("[]"). Adjacent elements are separated by the characters ", " (comma and space). Elements are converted to strings as by String#valueOf(Object).

Returns
`String`	a string representation of this collection