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/OpenJDK 21

Class ConcurrentLinkedDeque<E>

java.lang.Object
java.util.AbstractCollection<E>
java.util.concurrent.ConcurrentLinkedDeque<E>
Type Parameters:
E - the type of elements held in this deque
All Implemented Interfaces:
Serializable, Iterable<E>, Collection<E>, Deque<E>, Queue<E>, SequencedCollection<E> 
public class ConcurrentLinkedDeque<E> extends AbstractCollection<E> implements Deque<E>, Serializable
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(java.util.Collection<? extends E>), removeIf(java.util.function.Predicate<? super E>) or forEach(java.util.function.Consumer<? super E>), 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.

Since:
1.7
See Also:

Constructor Summary

Constructor Description
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.

Method Summary

Modifier and Type Method Description
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)
Performs the given action for each element of the Iterable until all elements have been processed or the action throws an exception.
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.

Methods declared in class java.util.AbstractCollection

containsAll, toString

Methods declared in class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Methods declared in interface java.util.Collection

containsAll, equals, hashCode, parallelStream, stream, toArray

Methods declared in interface java.util.Deque

reversed

Constructor Details

ConcurrentLinkedDeque

public ConcurrentLinkedDeque()
Constructs an empty deque.

ConcurrentLinkedDeque

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 - the collection of elements to initially contain
Throws:
NullPointerException - if the specified collection or any of its elements are null

Method Details

addFirst

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.
Specified by:
addFirst in interface Deque<E>
Specified by:
addFirst in interface SequencedCollection<E>
Parameters:
e - the element to add
Throws:
NullPointerException - if the specified element is null

addLast

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).

Specified by:
addLast in interface Deque<E>
Specified by:
addLast in interface SequencedCollection<E>
Parameters:
e - the element to add
Throws:
NullPointerException - if the specified element is null

offerFirst

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.
Specified by:
offerFirst in interface Deque<E>
Parameters:
e - the element to add
Returns:
true (as specified by Deque.offerFirst(E))
Throws:
NullPointerException - if the specified element is null

offerLast

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).

Specified by:
offerLast in interface Deque<E>
Parameters:
e - the element to add
Returns:
true (as specified by Deque.offerLast(E))
Throws:
NullPointerException - if the specified element is null

peekFirst

public E peekFirst()
Description copied from interface: Deque
Retrieves, but does not remove, the first element of this deque, or returns null if this deque is empty.
Specified by:
peekFirst in interface Deque<E>
Returns:
the head of this deque, or null if this deque is empty

peekLast

public E peekLast()
Description copied from interface: Deque
Retrieves, but does not remove, the last element of this deque, or returns null if this deque is empty.
Specified by:
peekLast in interface Deque<E>
Returns:
the tail of this deque, or null if this deque is empty

getFirst

public E getFirst()
Description copied from interface: Deque
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.
Specified by:
getFirst in interface Deque<E>
Specified by:
getFirst in interface SequencedCollection<E>
Returns:
the head of this deque
Throws:
NoSuchElementException - if this deque is empty

getLast

public E getLast()
Description copied from interface: Deque
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.
Specified by:
getLast in interface Deque<E>
Specified by:
getLast in interface SequencedCollection<E>
Returns:
the tail of this deque
Throws:
NoSuchElementException - if this deque is empty

pollFirst

public E pollFirst()
Description copied from interface: Deque
Retrieves and removes the first element of this deque, or returns null if this deque is empty.
Specified by:
pollFirst in interface Deque<E>
Returns:
the head of this deque, or null if this deque is empty

pollLast

public E pollLast()
Description copied from interface: Deque
Retrieves and removes the last element of this deque, or returns null if this deque is empty.
Specified by:
pollLast in interface Deque<E>
Returns:
the tail of this deque, or null if this deque is empty

removeFirst

public E removeFirst()
Description copied from interface: Deque
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.
Specified by:
removeFirst in interface Deque<E>
Specified by:
removeFirst in interface SequencedCollection<E>
Returns:
the head of this deque
Throws:
NoSuchElementException - if this deque is empty

removeLast

public E removeLast()
Description copied from interface: Deque
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.
Specified by:
removeLast in interface Deque<E>
Specified by:
removeLast in interface SequencedCollection<E>
Returns:
the tail of this deque
Throws:
NoSuchElementException - if this deque is empty

offer

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.
Specified by:
offer in interface Deque<E>
Specified by:
offer in interface Queue<E>
Parameters:
e - the element to add
Returns:
true (as specified by Queue.offer(E))
Throws:
NullPointerException - if the specified element is null

add

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.
Specified by:
add in interface Collection<E>
Specified by:
add in interface Deque<E>
Specified by:
add in interface Queue<E>
Overrides:
add in class AbstractCollection<E>
Parameters:
e - element whose presence in this collection is to be ensured
Returns:
true (as specified by Collection.add(E))
Throws:
NullPointerException - if the specified element is null

poll

public E poll()
Description copied from interface: Deque
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 Deque.pollFirst().

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

peek

public E peek()
Description copied from interface: Deque
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 Deque.peekFirst().

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

remove

public E remove()
Description copied from interface: Deque
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 Deque.removeFirst().

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

pop

public E pop()
Description copied from interface: Deque
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 Deque.removeFirst().

Specified by:
pop in interface Deque<E>
Returns:
the element at the front of this deque (which is the top of the stack represented by this deque)
Throws:
NoSuchElementException - if this deque is empty

element

public E element()
Description copied from interface: Deque
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 Deque.getFirst().

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

push

public void push(E e)
Description copied from interface: Deque
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 Deque.addFirst(E).

Specified by:
push in interface Deque<E>
Parameters:
e - the element to push
Throws:
NullPointerException - if the specified element is null and this deque does not permit null elements

removeFirstOccurrence

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).
Specified by:
removeFirstOccurrence in interface Deque<E>
Parameters:
o - element to be removed from this deque, if present
Returns:
true if the deque contained the specified element
Throws:
NullPointerException - if the specified element is null

removeLastOccurrence

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).
Specified by:
removeLastOccurrence in interface Deque<E>
Parameters:
o - element to be removed from this deque, if present
Returns:
true if the deque contained the specified element
Throws:
NullPointerException - if the specified element is null

contains

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).
Specified by:
contains in interface Collection<E>
Specified by:
contains in interface Deque<E>
Overrides:
contains in class AbstractCollection<E>
Parameters:
o - element whose presence in this deque is to be tested
Returns:
true if this deque contains the specified element

isEmpty

public boolean isEmpty()
Returns true if this collection contains no elements.
Specified by:
isEmpty in interface Collection<E>
Overrides:
isEmpty in class AbstractCollection<E>
Returns:
true if this collection contains no elements

size

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.

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

remove

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).

Specified by:
remove in interface Collection<E>
Specified by:
remove in interface Deque<E>
Overrides:
remove in class AbstractCollection<E>
Parameters:
o - element to be removed from this deque, if present
Returns:
true if the deque contained the specified element
Throws:
NullPointerException - if the specified element is null

addAll

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.
Specified by:
addAll in interface Collection<E>
Specified by:
addAll in interface Deque<E>
Overrides:
addAll in class AbstractCollection<E>
Parameters:
c - the elements to be inserted into this deque
Returns:
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
See Also:

clear

public void clear()
Removes all of the elements from this deque.
Specified by:
clear in interface Collection<E>
Overrides:
clear in class AbstractCollection<E>

toArray

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.

Specified by:
toArray in interface Collection<E>
Overrides:
toArray in class AbstractCollection<E>
Returns:
an array containing all of the elements in this deque

toArray

public <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. 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().
Specified by:
toArray in interface Collection<E>
Overrides:
toArray in class AbstractCollection<E>
Type Parameters:
T - the component type of the array to contain the collection
Parameters:
a - 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:
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

iterator

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.

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

descendingIterator

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.

Specified by:
descendingIterator in interface Deque<E>
Returns:
an iterator over the elements in this deque in reverse order

spliterator

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.

Specified by:
spliterator in interface Collection<E>
Specified by:
spliterator in interface Iterable<E>
Implementation Note:
The Spliterator implements trySplit to permit limited parallelism.
Returns:
a Spliterator over the elements in this deque
Since:
1.8

removeIf

public boolean removeIf(Predicate<? super E> filter)
Description copied from interface: Collection
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.
Specified by:
removeIf in interface Collection<E>
Parameters:
filter - a predicate which returns true for elements to be removed
Returns:
true if any elements were removed
Throws:
NullPointerException - if the specified filter is null

removeAll

public boolean removeAll(Collection<?> c)
Description copied from class: AbstractCollection
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.
Specified by:
removeAll in interface Collection<E>
Overrides:
removeAll in class AbstractCollection<E>
Parameters:
c - collection containing elements to be removed from this collection
Returns:
true if this collection changed as a result of the call
Throws:
NullPointerException - if this collection contains one or more null elements and the specified collection does not support null elements (optional) or if the specified collection is null
See Also:

retainAll

public boolean retainAll(Collection<?> c)
Description copied from class: AbstractCollection
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.
Specified by:
retainAll in interface Collection<E>
Overrides:
retainAll in class AbstractCollection<E>
Parameters:
c - collection containing elements to be retained in this collection
Returns:
true if this collection changed as a result of the call
Throws:
NullPointerException - if this collection contains one or more null elements and the specified collection does not permit null elements (optional) or if the specified collection is null
See Also:

forEach

public void forEach(Consumer<? super E> action)
Description copied from interface: Iterable
Performs the given action for each element of the Iterable until all elements have been processed or the action throws an exception. Actions are performed in the order of iteration, if that order is specified. Exceptions thrown by the action are relayed to the caller.

The behavior of this method is unspecified if the action performs side-effects that modify the underlying source of elements, unless an overriding class has specified a concurrent modification policy.

Specified by:
forEach in interface Iterable<E>
Parameters:
action - The action to be performed for each element
Throws:
NullPointerException - if the specified action is null

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https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/concurrent/ConcurrentLinkedDeque.html