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Android APIs
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. Like most other concurrent collection implementations, this class does not permit the use of 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.

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.

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.

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 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.
<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
From class java.lang.Object
From interface java.lang.Iterable
From interface java.util.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. As the queue is unbounded, this method will never throw IllegalStateException or return false.

Parameters
e the element to add
Returns
Throws
NullPointerException if the specified element is null

public boolean addAll (Collection<? extends E> c)

Since: API Level 1

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)

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 elements will be returned in order from first (head) to last (tail).

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. As the queue is unbounded, this method will never return false.

Parameters
e the element to add
Returns
Throws
NullPointerException if the specified element is null

public E peek ()

Since: API Level 1

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 ()

Since: API Level 1

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)

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. 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 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