ConcurrentLinkedQueue

public class ConcurrentLinkedQueue extends AbstractQueue<E>
implements Queue<E> Serializable

An unbounded thread-safe {@linkplain Queue 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.

Public Constructor Summary

ConcurrentLinkedQueue()
Creates a ConcurrentLinkedQueue that is initially empty.
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.

Public Method Summary

boolean
add(E e)
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.
boolean
contains(Object o)
Returns true if this queue contains the specified element.
boolean
isEmpty()
Returns true if this queue contains no elements.
Iterator<E>
iterator()
Returns an iterator over the elements in this queue in proper sequence.
boolean
offer(E e)
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(Object o)
Removes a single instance of the specified element from this queue, if it is present.
int
size()
Returns the number of elements in this queue.
Spliterator<E>
spliterator()
Returns a Spliterator over the elements in this queue.
<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.
Object[]
toArray()
Returns an array containing all of the elements in this queue, in proper sequence.
String
toString()
Returns a string representation of this collection.

Inherited Method Summary