java.util.concurrent
class ConcurrentLinkedQueue

java.lang.Object java.util.AbstractCollection java.util.AbstractQueue java.util.concurrent.ConcurrentLinkedQueue

All Implemented Interfaces:

Serializable, Collection, Queue

Most common way to construct:

ConcurrentLinkedQueue q = new ConcurrentLinkedQueue();

Based on 149 examples

public class ConcurrentLinkedQueue
extends AbstractQueue
implements Queue, 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. 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 {@link Collection} and {@link Iterator} interfaces.

Memory consistency effects: As with other concurrent collections, actions in a thread prior to placing an object into a {@code ConcurrentLinkedQueue} happen-before actions subsequent to the access or removal of that element from the {@code ConcurrentLinkedQueue} in another thread.

This class is a member of the Java Collections Framework.

ConcurrentLinkedQueue

public ConcurrentLinkedQueue()

Creates a ConcurrentLinkedQueue that is initially empty.

ConcurrentLinkedQueue

public ConcurrentLinkedQueue(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

add

public boolean add(Object e)

Inserts the specified element at the tail of this queue.

Overrides:

add in class AbstractQueue

Parameters:

e

Returns:

true (as specified by {@link Collection#add})

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).

Overrides:

contains in class AbstractCollection

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.

Overrides:

isEmpty in class AbstractCollection

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 {@link 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.

Overrides:

iterator in class AbstractCollection

Returns:

an iterator over the elements in this queue in proper sequence

offer

public boolean offer(Object e)

Inserts the specified element at the tail of this queue.

Parameters:

e

Returns:

true (as specified by {@link Queue#offer})

peek

public Object peek()

poll

public Object poll()

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).

Overrides:

remove in class AbstractCollection

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.

Overrides:

size in class AbstractCollection

Returns:

the number of elements in this queue

toArray

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.

Overrides:

toArray in class AbstractCollection

Returns:

an array containing all of the elements in this queue

toArray

public Object[] toArray(Object[] 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 {@link #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().

Overrides:

toArray in class AbstractCollection

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