E
- the type of elements held in this queueSerializable
, Iterable<E>
, Collection<E>
, Queue<E>
public class ConcurrentLinkedQueue<E> extends AbstractQueue<E> implements Queue<E>, Serializable
ConcurrentLinkedQueue
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. This implementation employs an efficient non-blocking algorithm based on one described in Simple, Fast, and Practical Non-Blocking and Blocking Concurrent Queue Algorithms by Maged M. Michael and Michael L. Scott.
Iterators are weakly consistent, returning elements reflecting the state of the queue at some point at or since the creation of the iterator. They do not throw ConcurrentModificationException
, and may proceed concurrently with other operations. Elements contained in the queue since the creation of the iterator will be returned exactly once.
Beware that, unlike in most collections, the size
method is NOT a constant-time operation. Because of the asynchronous nature of these queues, 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 Queue
and Iterator
interfaces.
Memory consistency effects: As with other concurrent collections, actions in a thread prior to placing an object into a ConcurrentLinkedQueue
happen-before actions subsequent to the access or removal of that element from the ConcurrentLinkedQueue
in another thread.
This class is a member of the Java Collections Framework.
Constructor | Description |
---|---|
ConcurrentLinkedQueue() |
Creates a ConcurrentLinkedQueue that is initially empty. |
ConcurrentLinkedQueue |
Creates a ConcurrentLinkedQueue initially containing the elements of the given collection, added in traversal order of the collection's iterator. |
Modifier and Type | Method | Description |
---|---|---|
boolean |
add |
Inserts the specified element at the tail of this queue. |
boolean |
addAll |
Appends all of the elements in the specified collection to the end of this queue, in the order that they are returned by the specified collection's iterator. |
void |
clear() |
Removes all of the elements from this queue. |
boolean |
contains |
Returns true if this queue contains the specified element. |
void |
forEach |
Performs the given action for each element of the Iterable until all elements have been processed or the action throws an exception. |
boolean |
isEmpty() |
Returns true if this queue contains no elements. |
Iterator |
iterator() |
Returns an iterator over the elements in this queue in proper sequence. |
boolean |
offer |
Inserts the specified element at the tail of this queue. |
E |
peek() |
Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty. |
E |
poll() |
Retrieves and removes the head of this queue, or returns null if this queue is empty. |
boolean |
remove |
Removes a single instance of the specified element from this queue, if it is present. |
boolean |
removeAll |
Removes all of this collection's elements that are also contained in the specified collection (optional operation). |
boolean |
removeIf |
Removes all of the elements of this collection that satisfy the given predicate. |
boolean |
retainAll |
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 queue. |
Spliterator |
spliterator() |
Returns a Spliterator over the elements in this queue. |
Object[] |
toArray() |
Returns an array containing all of the elements in this queue, in proper sequence. |
<T> T[] |
toArray |
Returns an array containing all of the elements in this queue, in proper sequence; the runtime type of the returned array is that of the specified array. |
clear, element, remove
containsAll, toString
clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait
containsAll, equals, hashCode, parallelStream, stream, toArray
public ConcurrentLinkedQueue()
ConcurrentLinkedQueue
that is initially empty.public ConcurrentLinkedQueue(Collection<? extends E> c)
ConcurrentLinkedQueue
initially containing the elements of the given collection, added in traversal order of the collection's iterator.c
- the collection of elements to initially containNullPointerException
- if the specified collection or any of its elements are nullpublic boolean add(E e)
IllegalStateException
or return false
.add
in interface Collection<E>
add
in interface Queue<E>
add
in class AbstractQueue<E>
e
- the element to addtrue
(as specified by Collection.add(E)
)NullPointerException
- if the specified element is nullpublic boolean offer(E e)
false
.offer
in interface Queue<E>
e
- the element to addtrue
(as specified by Queue.offer(E)
)NullPointerException
- if the specified element is nullpublic E poll()
Queue
null
if this queue is empty.public E peek()
Queue
null
if this queue is empty.public boolean isEmpty()
true
if this queue contains no elements.isEmpty
in interface Collection<E>
isEmpty
in class AbstractCollection<E>
true
if this queue contains no elementspublic int size()
Integer.MAX_VALUE
elements, 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 queues, determining the current number of elements requires an O(n) traversal. Additionally, if elements are added or removed during execution of this method, the returned result may be inaccurate. Thus, this method is typically not very useful in concurrent applications.
size
in interface Collection<E>
public boolean contains(Object o)
true
if this queue contains the specified element. More formally, returns true
if and only if this queue contains at least one element e
such that o.equals(e)
.contains
in interface Collection<E>
contains
in class AbstractCollection<E>
o
- object to be checked for containment in this queuetrue
if this queue contains the specified elementpublic boolean remove(Object o)
e
such that o.equals(e)
, if this queue contains one or more such elements. Returns true
if this queue contained the specified element (or equivalently, if this queue changed as a result of the call).remove
in interface Collection<E>
remove
in class AbstractCollection<E>
o
- element to be removed from this queue, if presenttrue
if this queue changed as a result of the callpublic boolean addAll(Collection<? extends E> c)
addAll
of a queue to itself result in IllegalArgumentException
.addAll
in interface Collection<E>
addAll
in class AbstractQueue<E>
c
- the elements to be inserted into this queuetrue
if this queue changed as a result of the callNullPointerException
- if the specified collection or any of its elements are nullIllegalArgumentException
- if the collection is this queuepublic Object[] toArray()
The returned array will be "safe" in that no references to it are maintained by this queue. (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.
toArray
in interface Collection<E>
toArray
in class AbstractCollection<E>
public <T> T[] toArray(T[] a)
If this queue fits in the specified array with room to spare (i.e., the array has more elements than this queue), the element in the array immediately following the end of the queue 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 queue known to contain only strings. The following code can be used to dump the queue 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()
.toArray
in interface Collection<E>
toArray
in class AbstractCollection<E>
T
- the component type of the array to contain the collectiona
- the array into which the elements of the queue are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purposeArrayStoreException
- if the runtime type of the specified array is not a supertype of the runtime type of every element in this queueNullPointerException
- if the specified array is nullpublic Iterator<E> iterator()
The returned iterator is weakly consistent.
iterator
in interface Collection<E>
iterator
in interface Iterable<E>
iterator
in class AbstractCollection<E>
public Spliterator<E> spliterator()
Spliterator
over the elements in this queue. The returned spliterator is weakly consistent.
The Spliterator
reports Spliterator.CONCURRENT
, Spliterator.ORDERED
, and Spliterator.NONNULL
.
spliterator
in interface Collection<E>
spliterator
in interface Iterable<E>
Spliterator
implements trySplit
to permit limited parallelism.Spliterator
over the elements in this queuepublic boolean removeIf(Predicate<? super E> filter)
Collection
removeIf
in interface Collection<E>
filter
- a predicate which returns true
for elements to be removedtrue
if any elements were removedNullPointerException
- if the specified filter is nullpublic boolean removeAll(Collection<?> c)
AbstractCollection
removeAll
in interface Collection<E>
removeAll
in class AbstractCollection<E>
c
- collection containing elements to be removed from this collectiontrue
if this collection changed as a result of the callNullPointerException
- 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 nullpublic boolean retainAll(Collection<?> c)
AbstractCollection
retainAll
in interface Collection<E>
retainAll
in class AbstractCollection<E>
c
- collection containing elements to be retained in this collectiontrue
if this collection changed as a result of the callNullPointerException
- 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 nullpublic void clear()
AbstractQueue
This implementation repeatedly invokes poll
until it returns null
.
clear
in interface Collection<E>
clear
in class AbstractQueue<E>
public void forEach(Consumer<? super E> action)
Iterable
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.
forEach
in interface Iterable<E>
action
- The action to be performed for each elementNullPointerException
- 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/ConcurrentLinkedQueue.html