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java.util.concurrent

Class ConcurrentLinkedQueue<E>

Implemented Interfaces:
Collection<E>, Iterable<E>, Queue<E>, Serializable
public 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. This queue does not permit null elements.

This implementation employs an efficient "wait-free" 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.

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.

This class and its iterator implement all of the optional methods of the Collection 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 ../../../../technotes/guides/collections/index.html"> Java Collections Framework.

Since:
1.5
See Also:
Serialized Form

Constructor Summary

ConcurrentLinkedQueue()
Creates a ConcurrentLinkedQueue that is initially empty.
ConcurrentLinkedQueue(E> c)
Creates a ConcurrentLinkedQueue initially containing the elements of the given collection, added in traversal order of the collection's iterator.

Method Summary

boolean
add(E e)
Inserts the specified element at the tail of this queue.
boolean
contains(Object o)
Returns true if this queue contains the specified element.
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(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.

Methods inherited from class java.util.AbstractQueue<E>

add, addAll, clear, element, remove

Methods inherited from class java.util.AbstractCollection<E>

T[] toArray, add, addAll, clear, contains, containsAll, isEmpty, iterator, remove, removeAll, retainAll, size, toArray, toString

Methods inherited from class java.lang.Object

clone, equals, extends Object> getClass, finalize, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Details

ConcurrentLinkedQueue

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

ConcurrentLinkedQueue

public ConcurrentLinkedQueue(E> c)
Creates a ConcurrentLinkedQueue 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

public boolean add(E e)
Inserts the specified element at the tail of this queue.
Specified by:
add in interface Queue<E>
add in interface Collection<E>
Overrides:
add in interface AbstractQueue<E>
Returns:
true (as specified by Collection.add(E))
Throws:
NullPointerException - if the specified element is null

contains

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).
Specified by:
contains in interface Collection<E>
Overrides:
contains in interface AbstractCollection<E>
Parameters:
o - object to be checked for containment in this queue
Returns:
true if this queue contains the specified element

isEmpty

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

iterator

public Iterator iterator()
Returns an iterator over the elements in this queue in proper sequence. The returned iterator is a "weakly consistent" iterator that will never throw ConcurrentModificationException, and guarantees to traverse elements as they existed upon construction of the iterator, and may (but is not guaranteed to) reflect any modifications subsequent to construction.
Specified by:
iterator in interface Collection<E>
iterator in interface Iterable<E>
Overrides:
iterator in interface AbstractCollection<E>
Returns:
an iterator over the elements in this queue in proper sequence

offer

public boolean offer(E e)
Inserts the specified element at the tail of this queue.
Specified by:
offer in interface Queue<E>
Returns:
true (as specified by Queue.offer(E))
Throws:
NullPointerException - if the specified element is null

peek

public E peek()
Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty.
Specified by:
peek in interface Queue<E>
Returns:
the head of this queue, or null if this queue is empty

poll

public E poll()
Retrieves and removes the head of this queue, or returns null if this queue is empty.
Specified by:
poll in interface Queue<E>
Returns:
the head of this queue, or null if this queue is empty

remove

public 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).
Specified by:
remove in interface Collection<E>
Overrides:
remove in interface AbstractCollection<E>
Parameters:
o - element to be removed from this queue, if present
Returns:
true if this queue changed as a result of the call

size

public 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.

Specified by:
size in interface Collection<E>
Overrides:
size in interface AbstractCollection<E>
Returns:
the number of elements in this queue

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