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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. Additionally, the bulk operations addAll, removeAll, retainAll, containsAll, equals, and toArray are not guaranteed to be performed atomically. For example, an iterator operating concurrently with an addAll operation might view 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.

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).

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.

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.

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

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(E))

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

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(E))

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(E))

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(E))

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(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

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)

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).

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

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

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