- Type Parameters:
E
- the type of elements held in this queue
- All Implemented Interfaces:
Serializable
,Iterable<E>
,Collection<E>
,Queue<E>
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.
- Since:
- 1.5
- See Also:
-
Constructor Summary
ConstructorDescriptionCreates aConcurrentLinkedQueue
that is initially empty.ConcurrentLinkedQueue
(Collection<? extends E> c) Creates aConcurrentLinkedQueue
initially containing the elements of the given collection, added in traversal order of the collection's iterator. -
Method Summary
Modifier and TypeMethodDescriptionboolean
Inserts the specified element at the tail of this queue.boolean
addAll
(Collection<? extends E> c) 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
Returnstrue
if this queue contains the specified element.void
Performs the given action for each element of theIterable
until all elements have been processed or the action throws an exception.boolean
isEmpty()
Returnstrue
if this queue contains no elements.iterator()
Returns an iterator over the elements in this queue in proper sequence.boolean
Inserts the specified element at the tail of this queue.peek()
Retrieves, but does not remove, the head of this queue, or returnsnull
if this queue is empty.poll()
Retrieves and removes the head of this queue, or returnsnull
if this queue is empty.boolean
Removes a single instance of the specified element from this queue, if it is present.boolean
removeAll
(Collection<?> c) Removes all of this collection's elements that are also contained in the specified collection (optional operation).boolean
Removes all of the elements of this collection that satisfy the given predicate (optional operation).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 queue.Returns aSpliterator
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
(T[] a) 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.Methods declared in class java.util.AbstractQueue
clear, element, remove
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
-
Constructor Details
-
ConcurrentLinkedQueue
public ConcurrentLinkedQueue()Creates aConcurrentLinkedQueue
that is initially empty. -
ConcurrentLinkedQueue
Creates aConcurrentLinkedQueue
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
-
add
Inserts the specified element at the tail of this queue. As the queue is unbounded, this method will never throwIllegalStateException
or returnfalse
.- Specified by:
add
in interfaceCollection<E>
- Specified by:
add
in interfaceQueue<E>
- Overrides:
add
in classAbstractQueue<E>
- Parameters:
e
- the element to add- Returns:
true
(as specified byCollection.add(E)
)- Throws:
NullPointerException
- if the specified element is null
-
offer
Inserts the specified element at the tail of this queue. As the queue is unbounded, this method will never returnfalse
.- Specified by:
offer
in interfaceQueue<E>
- Parameters:
e
- the element to add- Returns:
true
(as specified byQueue.offer(E)
)- Throws:
NullPointerException
- if the specified element is null
-
poll
-
peek
-
isEmpty
public boolean isEmpty()Returnstrue
if this queue contains no elements.- Specified by:
isEmpty
in interfaceCollection<E>
- Overrides:
isEmpty
in classAbstractCollection<E>
- Returns:
true
if this queue contains no elements
-
size
public int size()Returns the number of elements in this queue. If this queue contains more thanInteger.MAX_VALUE
elements, returnsInteger.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.
- Specified by:
size
in interfaceCollection<E>
- Returns:
- the number of elements in this queue
-
contains
Returnstrue
if this queue contains the specified element. More formally, returnstrue
if and only if this queue contains at least one elemente
such thato.equals(e)
.- Specified by:
contains
in interfaceCollection<E>
- Overrides:
contains
in classAbstractCollection<E>
- Parameters:
o
- object to be checked for containment in this queue- Returns:
true
if this queue contains the specified element
-
remove
Removes a single instance of the specified element from this queue, if it is present. More formally, removes an elemente
such thato.equals(e)
, if this queue contains one or more such elements. Returnstrue
if this queue contained the specified element (or equivalently, if this queue changed as a result of the call).- Specified by:
remove
in interfaceCollection<E>
- Overrides:
remove
in classAbstractCollection<E>
- Parameters:
o
- element to be removed from this queue, if present- Returns:
true
if this queue changed as a result of the call
-
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. Attempts toaddAll
of a queue to itself result inIllegalArgumentException
.- Specified by:
addAll
in interfaceCollection<E>
- Overrides:
addAll
in classAbstractQueue<E>
- Parameters:
c
- the elements to be inserted into this queue- Returns:
true
if this queue changed as a result of the call- Throws:
NullPointerException
- if the specified collection or any of its elements are nullIllegalArgumentException
- if the collection is this queue- See Also:
-
toArray
Returns an array containing all of the elements in this queue, in proper sequence.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.
- Specified by:
toArray
in interfaceCollection<E>
- Overrides:
toArray
in classAbstractCollection<E>
- Returns:
- an array containing all of the elements in this queue
-
toArray
public <T> T[] toArray(T[] a) 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. If the queue 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 queue.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 ofString
:String[] y = x.toArray(new String[0]);
toArray(new Object[0])
is identical in function totoArray()
.- Specified by:
toArray
in interfaceCollection<E>
- Overrides:
toArray
in classAbstractCollection<E>
- Type Parameters:
T
- the component type of the array to contain the collection- Parameters:
a
- 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 purpose- Returns:
- an array containing all of the elements in this queue
- Throws:
ArrayStoreException
- 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 null
-
iterator
Returns an iterator over the elements in this queue 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 interfaceCollection<E>
- Specified by:
iterator
in interfaceIterable<E>
- Specified by:
iterator
in classAbstractCollection<E>
- Returns:
- an iterator over the elements in this queue in proper sequence
-
spliterator
Returns aSpliterator
over the elements in this queue.The returned spliterator is weakly consistent.
The
Spliterator
reportsSpliterator.CONCURRENT
,Spliterator.ORDERED
, andSpliterator.NONNULL
.- Specified by:
spliterator
in interfaceCollection<E>
- Specified by:
spliterator
in interfaceIterable<E>
- Implementation Note:
- The
Spliterator
implementstrySplit
to permit limited parallelism. - Returns:
- a
Spliterator
over the elements in this queue - Since:
- 1.8
-
removeIf
Description copied from interface:Collection
Removes all of the elements of this collection that satisfy the given predicate (optional operation). Errors or runtime exceptions thrown during iteration or by the predicate are relayed to the caller.- Specified by:
removeIf
in interfaceCollection<E>
- Parameters:
filter
- a predicate which returnstrue
for elements to be removed- Returns:
true
if any elements were removed- Throws:
NullPointerException
- if the specified filter is null
-
removeAll
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 interfaceCollection<E>
- Overrides:
removeAll
in classAbstractCollection<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
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 interfaceCollection<E>
- Overrides:
retainAll
in classAbstractCollection<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:
-
clear
public void clear()Description copied from class:AbstractQueue
Removes all of the elements from this queue. The queue will be empty after this call returns.This implementation repeatedly invokes
poll
until it returnsnull
.- Specified by:
clear
in interfaceCollection<E>
- Overrides:
clear
in classAbstractQueue<E>
-
forEach
Description copied from interface:Iterable
Performs the given action for each element of theIterable
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 interfaceIterable<E>
- Parameters:
action
- The action to be performed for each element- Throws:
NullPointerException
- if the specified action is null
-