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

From class java.util.AbstractQueue

boolean	add(E e) Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning true upon success and throwing an IllegalStateException if no space is currently available.
boolean	addAll(Collection<? extends E> c) Adds all of the elements in the specified collection to this queue.
void	clear() Removes all of the elements from this queue.
E	element() Retrieves, but does not remove, the head of this queue.
E	remove() Retrieves and removes the head of this queue.

From class java.util.AbstractCollection

boolean	add(E e) Ensures that this collection contains the specified element (optional operation). This implementation always throws an UnsupportedOperationException.
boolean	addAll(Collection<? extends E> c) Adds all of the elements in the specified collection to this collection (optional operation). This implementation iterates over the specified collection, and adds each object returned by the iterator to this collection, in turn.
void	clear() Removes all of the elements from this collection (optional operation). This implementation iterates over this collection, removing each element using the Iterator.remove operation.
boolean	contains(Object o) Returns true if this collection contains the specified element. This implementation iterates over the elements in the collection, checking each element in turn for equality with the specified element.
boolean	containsAll(Collection<?> c) Returns true if this collection contains all of the elements in the specified collection. This implementation iterates over the specified collection, checking each element returned by the iterator in turn to see if it's contained in this collection.
boolean	isEmpty() Returns true if this collection contains no elements. This implementation returns size() == 0.
abstract Iterator<E>	iterator() Returns an iterator over the elements contained in this collection.
boolean	remove(Object o) Removes a single instance of the specified element from this collection, if it is present (optional operation). This implementation iterates over the collection looking for the specified element.
boolean	removeAll(Collection<?> c) Removes all of this collection's elements that are also contained in the specified collection (optional operation). This implementation iterates over this collection, checking each element returned by the iterator in turn to see if it's contained in the specified collection.
boolean	retainAll(Collection<?> c) Retains only the elements in this collection that are contained in the specified collection (optional operation). This implementation iterates over this collection, checking each element returned by the iterator in turn to see if it's contained in the specified collection.
abstract int	size() Returns the number of elements in this collection.
<T> T[]	toArray(T[] contents) Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array. This implementation returns an array containing all the elements returned by this collection's iterator in the same order, stored in consecutive elements of the array, starting with index 0.
Object[]	toArray() Returns an array containing all of the elements in this collection. This implementation returns an array containing all the elements returned by this collection's iterator, in the same order, stored in consecutive elements of the array, starting with index 0.
String	toString() Returns a string representation of this collection.

From class java.lang.Object

Object	clone() Creates and returns a copy of this Object.
boolean	equals(Object obj) Compares this instance with the specified object and indicates if they are equal.
void	finalize() Invoked when the garbage collector has detected that this instance is no longer reachable.
final Class<?>	getClass() Returns the unique instance of Class that represents this object's class.
int	hashCode() Returns an integer hash code for this object.
final void	notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
final void	notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
String	toString() Returns a string containing a concise, human-readable description of this object.
final void	wait(long timeout, int nanos) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait(long timeout) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait() Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object.

From interface java.util.Queue

abstract boolean	add(E e) Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning true upon success and throwing an IllegalStateException if no space is currently available.
abstract E	element() Retrieves, but does not remove, the head of this queue.
abstract boolean	offer(E e) Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions.
abstract E	peek() Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty.
abstract E	poll() Retrieves and removes the head of this queue, or returns null if this queue is empty.
abstract E	remove() Retrieves and removes the head of this queue.

From interface java.util.Collection

abstract boolean	add(E e) Ensures that this collection contains the specified element (optional operation).
abstract boolean	addAll(Collection<? extends E> c) Adds all of the elements in the specified collection to this collection (optional operation).
abstract void	clear() Removes all of the elements from this collection (optional operation).
abstract boolean	contains(Object o) Returns true if this collection contains the specified element.
abstract boolean	containsAll(Collection<?> c) Returns true if this collection contains all of the elements in the specified collection.
abstract boolean	equals(Object o) Compares the specified object with this collection for equality.
abstract int	hashCode() Returns the hash code value for this collection.
abstract boolean	isEmpty() Returns true if this collection contains no elements.
abstract Iterator<E>	iterator() Returns an iterator over the elements in this collection.
Stream<E>	parallelStream() Returns a possibly parallel Stream with this collection as its source.
abstract boolean	remove(Object o) Removes a single instance of the specified element from this collection, if it is present (optional operation).
abstract boolean	removeAll(Collection<?> c) Removes all of this collection's elements that are also contained in the specified collection (optional operation).
boolean	removeIf(Predicate<? super E> filter) Removes all of the elements of this collection that satisfy the given predicate.
abstract boolean	retainAll(Collection<?> c) Retains only the elements in this collection that are contained in the specified collection (optional operation).
abstract int	size() Returns the number of elements in this collection.
Spliterator<E>	spliterator() Creates a Spliterator over the elements in this collection.
Stream<E>	stream() Returns a sequential Stream with this collection as its source.
abstract <T> T[]	toArray(T[] a) Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array.
abstract Object[]	toArray() Returns an array containing all of the elements in this collection.

From interface java.lang.Iterable

void	forEach(Consumer<? super T> action) Performs the given action for each element of the Iterable until all elements have been processed or the action throws an exception.
abstract Iterator<E>	iterator() Returns an iterator over elements of type T.
Spliterator<E>	spliterator() Creates a Spliterator over the elements described by this Iterable.

Public Constructors

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.

Parameters

c	the collection of elements to initially contain

Throws

NullPointerException	if the specified collection or any of its elements are null

Public Methods

public 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.

Parameters

e	the element to add

Returns

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

Throws

NullPointerException	if the specified element is null

public 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.

Parameters

c	the elements to be inserted into this queue

Returns

• true if this queue changed as a result of the call

Throws

NullPointerException	if the specified collection or any of its elements are null
IllegalArgumentException	if the collection is this queue

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).

Parameters

o	object to be checked for containment in this queue

Returns

• true if this queue contains the specified element

public boolean isEmpty ()

Returns true if this queue contains no elements.

Returns

• true if this queue contains no elements

public 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.

Returns

• an iterator over the elements in this queue in proper sequence

public 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.

Parameters

e	the element to add

Returns

• true (as specified by Queue.offer(E))

Throws

NullPointerException	if the specified element is null

public E peek ()

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

Returns

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

public E poll ()

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

Returns

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

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).

Parameters

o	element to be removed from this queue, if present

Returns

• true if this queue changed as a result of the call

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. 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.

Returns

• the number of elements in this queue

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.

Returns

• a Spliterator over the elements in this queue

public 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().

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

Throws

ArrayStoreException	if the runtime type of the specified array is not a supertype of the runtime type of every element in this queue
NullPointerException	if the specified array is null

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.

Returns

• an array containing all of the elements in this queue

public String toString ()

Returns a string representation of this collection. The string representation consists of a list of the collection's elements in the order they are returned by its iterator, enclosed in square brackets ("[]"). Adjacent elements are separated by the characters ", " (comma and space). Elements are converted to strings as by String.valueOf(Object).

Returns

• a string representation of this collection