E
- the type of elements held in this collectionpublic class ConcurrentLinkedQueue<E> extends AbstractQueue<E> implements Queue<E>, Serializable
An unbounded thread-safe queue based on linked nodes. This queue orders elements FIFO (first-in-first-out). The head of the queue is that element that has been on the queue the longest time. The tail of the queue is that element that has been on the queue the shortest time. New elements are inserted at the tail of the queue, and the queue retrieval operations obtain elements at the head of the queue. A 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. 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 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.
public ConcurrentLinkedQueue()
Creates a ConcurrentLinkedQueue
that is initially empty.
public ConcurrentLinkedQueue(Collection<? extends E> c)
Creates a 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)
Inserts the specified element at the tail of this queue. As the queue is unbounded, this method will never throw 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)
Inserts the specified element at the tail of this queue. As the queue is unbounded, this method will never return 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()
Description copied from interface: Queue
Retrieves and removes the head of this queue, or returns null
if this queue is empty.
poll
in interface Queue<E>
null
if this queue is emptypublic E peek()
Description copied from interface: Queue
Retrieves, but does not remove, the head of this queue, or returns null
if this queue is empty.
peek
in interface Queue<E>
null
if this queue is emptypublic boolean isEmpty()
Returns 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()
Returns the number of elements in this queue. If this queue contains more than 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>
size
in class AbstractCollection<E>
public boolean contains(Object o)
Returns 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)
Removes a single instance of the specified element from this queue, if it is present. More formally, removes an element 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)
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 to 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 queueAbstractQueue.add(Object)
public Object[] 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.
toArray
in interface Collection<E>
toArray
in class AbstractCollection<E>
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 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 runtime 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()
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.
iterator
in interface Iterable<E>
iterator
in interface Collection<E>
iterator
in class AbstractCollection<E>
public Spliterator<E> spliterator()
Returns a 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 Iterable<E>
spliterator
in interface Collection<E>
Spliterator
implements trySplit
to permit limited parallelism.Spliterator
over the elements in this queue
© 1993–2017, Oracle and/or its affiliates. All rights reserved.
Documentation extracted from Debian's OpenJDK Development Kit package.
Licensed under the GNU General Public License, version 2, with the Classpath Exception.
Various third party code in OpenJDK is licensed under different licenses (see Debian package).
Java and OpenJDK are trademarks or registered trademarks of Oracle and/or its affiliates.