edu.emory.mathcs.backport.java.util.concurrent
Class ConcurrentLinkedQueue

java.lang.Object
  java.util.AbstractCollection
      edu.emory.mathcs.backport.java.util.AbstractCollection
          edu.emory.mathcs.backport.java.util.AbstractQueue
              edu.emory.mathcs.backport.java.util.concurrent.ConcurrentLinkedQueue

All Implemented Interfaces:

Queue, java.io.Serializable, java.lang.Iterable, java.util.Collection

public class ConcurrentLinkedQueue
extends AbstractQueue
implements Queue, java.io.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 Java Collections Framework.

Since:

1.5

Author:

Doug Lea

See Also:

Serialized Form

Constructor Summary

ConcurrentLinkedQueue()
Creates a ConcurrentLinkedQueue that is initially empty.

ConcurrentLinkedQueue(java.util.Collection 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(java.lang.Object e) Inserts the specified element at the tail of this queue.
boolean	contains(java.lang.Object o) Returns true if this queue contains the specified element.
boolean	isEmpty() Returns true if this queue contains no elements.
java.util.Iterator	iterator() Returns an iterator over the elements in this queue in proper sequence.
boolean	offer(java.lang.Object e) Inserts the specified element at the tail of this queue.
java.lang.Object	peek() Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty.
java.lang.Object	poll() Retrieves and removes the head of this queue, or returns null if this queue is empty.
boolean	remove(java.lang.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 edu.emory.mathcs.backport.java.util.AbstractQueue

addAll, clear, element, remove

Methods inherited from class edu.emory.mathcs.backport.java.util.AbstractCollection

toArray, toArray

Methods inherited from class java.util.AbstractCollection

containsAll, removeAll, retainAll, toString

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Methods inherited from interface edu.emory.mathcs.backport.java.util.Queue

element, remove

Methods inherited from interface java.util.Collection

addAll, clear, containsAll, equals, hashCode, removeAll, retainAll, toArray, toArray

Constructor Detail

ConcurrentLinkedQueue

public ConcurrentLinkedQueue()

Creates a ConcurrentLinkedQueue that is initially empty.

ConcurrentLinkedQueue

public ConcurrentLinkedQueue(java.util.Collection 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:

java.lang.NullPointerException - if the specified collection or any of its elements are null

Method Detail

add

public boolean add(java.lang.Object e)

Inserts the specified element at the tail of this queue.

Specified by:

add in interface Queue

Specified by:

add in interface java.util.Collection

Overrides:

add in class AbstractQueue

Parameters:

e - the element to add

Returns:

true (as specified by Collection.add(E))

Throws:

java.lang.NullPointerException - if the specified element is null

offer

public boolean offer(java.lang.Object e)

Inserts the specified element at the tail of this queue.

Specified by:

offer in interface Queue

Parameters:

e - the element to add

Returns:

true (as specified by Queue.offer(java.lang.Object))

Throws:

java.lang.NullPointerException - if the specified element is null

poll

public java.lang.Object poll()

Description copied from interface: Queue

Retrieves and removes the head of this queue, or returns null if this queue is empty.

Specified by:

poll in interface Queue

Returns:

the head of this queue, or null if this queue is empty

peek

public java.lang.Object peek()

Description copied from interface: Queue

Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty.

Specified by:

peek in interface Queue

Returns:

the head of this queue, or null if this queue is empty

isEmpty

public boolean isEmpty()

Returns true if this queue contains no elements.

Specified by:

isEmpty in interface java.util.Collection

Overrides:

isEmpty in class java.util.AbstractCollection

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 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 java.util.Collection

Specified by:

size in class java.util.AbstractCollection

Returns:

the number of elements in this queue

contains

public boolean contains(java.lang.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 java.util.Collection

Overrides:

contains in class java.util.AbstractCollection

Parameters:

o - object to be checked for containment in this queue

Returns:

true if this queue contains the specified element

remove

public boolean remove(java.lang.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 java.util.Collection

Overrides:

remove in class java.util.AbstractCollection

Parameters:

o - element to be removed from this queue, if present

Returns:

true if this queue changed as a result of the call

iterator

public java.util.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 java.lang.Iterable

Specified by:

iterator in interface java.util.Collection

Specified by:

iterator in class java.util.AbstractCollection

Returns:

an iterator over the elements in this queue in proper sequence