Summary: Ctors | Methods | Inherited Methods | [Expand All]

public class

ConcurrentLinkedQueue

extends AbstractQueue<E>
implements Serializable Queue<E>

java.lang.Object
↳	java.util.AbstractCollection<E>
	↳	java.util.AbstractQueue<E>
		↳	java.util.concurrent.ConcurrentLinkedQueue<E>

Class Overview

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.

Summary

Public Constructors
	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 Methods
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<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() Gets but does not remove the element at the head of the queue.
E	poll() Gets and removes the element at the head of the queue, or returns `null` if there is no element in the queue.
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.
<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.

[Expand]

Inherited Methods

From class java.util.AbstractQueue

From class java.util.AbstractCollection

boolean	add(E object) Attempts to add `object` to the contents of this `Collection` (optional).
boolean	addAll(Collection<? extends E> collection) Attempts to add all of the objects contained in `collection` to the contents of this `Collection` (optional).
void	clear() Removes all elements from this `Collection`, leaving it empty (optional).
boolean	contains(Object object) Tests whether this `Collection` contains the specified object.
boolean	containsAll(Collection<?> collection) Tests whether this `Collection` contains all objects contained in the specified `Collection`.
boolean	isEmpty() Returns if this `Collection` contains no elements.
abstract Iterator<E>	iterator() Returns an instance of `Iterator` that may be used to access the objects contained by this `Collection`.
boolean	remove(Object object) Removes one instance of the specified object from this `Collection` if one is contained (optional).
boolean	removeAll(Collection<?> collection) Removes all occurrences in this `Collection` of each object in the specified `Collection` (optional).
boolean	retainAll(Collection<?> collection) Removes all objects from this `Collection` that are not also found in the `Collection` passed (optional).
abstract int	size() Returns a count of how many objects this `Collection` contains.
<T> T[]	toArray(T[] contents) Returns an array containing all elements contained in this `Collection`.
Object[]	toArray() Returns a new array containing all elements contained in this `Collection`.
String	toString() Returns the string representation of this `Collection`.

From class java.lang.Object

Object	clone() Creates and returns a copy of this `Object`.
boolean	equals(Object o) Compares this instance with the specified object and indicates if they are equal.
void	finalize() Called before the object's memory is reclaimed by the VM.
final Class<? extends Object>	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() Causes the calling thread to wait until another thread calls the `notify()` or `notifyAll()` method of this object.
final void	wait(long millis, 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 millis) Causes the calling thread to wait until another thread calls the `notify()` or `notifyAll()` method of this object or until the specified timeout expires.

From interface java.lang.Iterable

From interface java.util.Collection

abstract boolean	add(E object) Attempts to add `object` to the contents of this `Collection` (optional).
abstract boolean	addAll(Collection<? extends E> collection) Attempts to add all of the objects contained in `Collection` to the contents of this `Collection` (optional).
abstract void	clear() Removes all elements from this `Collection`, leaving it empty (optional).
abstract boolean	contains(Object object) Tests whether this `Collection` contains the specified object.
abstract boolean	containsAll(Collection<?> collection) Tests whether this `Collection` contains all objects contained in the specified `Collection`.
abstract boolean	equals(Object object) Compares the argument to the receiver, and returns true if they represent the same object using a class specific comparison.
abstract int	hashCode() Returns an integer hash code for the receiver.
abstract boolean	isEmpty() Returns if this `Collection` contains no elements.
abstract Iterator<E>	iterator() Returns an instance of `Iterator` that may be used to access the objects contained by this `Collection`.
abstract boolean	remove(Object object) Removes one instance of the specified object from this `Collection` if one is contained (optional).
abstract boolean	removeAll(Collection<?> collection) Removes all occurrences in this `Collection` of each object in the specified `Collection` (optional).
abstract boolean	retainAll(Collection<?> collection) Removes all objects from this `Collection` that are not also found in the `Collection` passed (optional).
abstract int	size() Returns a count of how many objects this `Collection` contains.
abstract <T> T[]	toArray(T[] array) Returns an array containing all elements contained in this `Collection`.
abstract Object[]	toArray() Returns a new array containing all elements contained in this `Collection`.

From interface java.util.Queue

Public Constructors

public ConcurrentLinkedQueue ()

Since: API Level 1

Creates a ConcurrentLinkedQueue that is initially empty.

public ConcurrentLinkedQueue (Collection<? extends E> c)

Since: API Level 1

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)

Since: API Level 1

Inserts the specified element at the tail of this queue.

Parameters

e	the element to be added to the queue.

Returns

true (as specified by add(E))

Throws

NullPointerException	if the specified element is null

public boolean contains (Object o)

Since: API Level 1

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 ()

Since: API Level 1

Returns true if this queue contains no elements.

Returns

true if this queue contains no elements

public Iterator<E> iterator ()

Since: API Level 1

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.

Returns

an iterator over the elements in this queue in proper sequence

public boolean offer (E e)

Since: API Level 1

Inserts the specified element at the tail of this queue.

Parameters

e	the specified element to insert into the queue.

Returns

true (as specified by offer(E))

Throws

NullPointerException	if the specified element is null

public E peek ()

Since: API Level 1

Gets but does not remove the element at the head of the queue.

Returns

the element at the head of the queue or null if there is no element in the queue.

public E poll ()

Since: API Level 1

Gets and removes the element at the head of the queue, or returns null if there is no element in the queue.

Returns

the element at the head of the queue or null if there is no element in the queue.

public boolean remove (Object o)

Since: API Level 1

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 ()

Since: API Level 1

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.

Returns

the number of elements in this queue

public T[] toArray (T[] a)

Since: API Level 1

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 ()

Since: API Level 1

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

Interfaces

Classes

Enums

Exceptions

ConcurrentLinkedQueue

Class Overview

Summary

Public Constructors

public ConcurrentLinkedQueue ()

public ConcurrentLinkedQueue (Collection<? extends E> c)

Parameters

Throws

Public Methods

public boolean add (E e)

Parameters

Returns

Throws

public boolean contains (Object o)

Parameters

Returns

public boolean isEmpty ()

Returns

public Iterator<E> iterator ()

Returns

public boolean offer (E e)

Parameters

Returns

Throws

public E peek ()

Returns

public E poll ()

Returns

public boolean remove (Object o)

Parameters

Returns

public int size ()

Returns

public T[] toArray (T[] a)

Parameters

Returns

Throws

public Object[] toArray ()

Returns