Package java.util.concurrent

Class ConcurrentLinkedQueue<E>

java.lang.Object
java.util.AbstractCollection<E>
java.util.AbstractQueue<E>
java.util.concurrent.ConcurrentLinkedQueue<E>
Type Parameters:
E - the type of elements held in this collection
All Implemented Interfaces:
Serializable, Iterable<E>, Collection<E>, Queue<E>
public class ConcurrentLinkedQueue<E>
extends AbstractQueue<E>
implements Queue<E>, 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. 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, 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.

Since:
1.5
Author:
Doug Lea
See Also:
Serialized Form

  • Constructor Summary

    Constructors
    Constructor Description
    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.

  • Method Summary

    Modifier and Type Method Description
    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.
    Object[] toArray()
    Returns an array containing all of the elements in this queue, in proper sequence.
    <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.

    Methods inherited from class java.util.AbstractQueue

    clear, element, remove

    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 java.util.Collection

    clear, containsAll, equals, hashCode, removeAll, retainAll

    Methods inherited from interface java.util.Queue

    element, remove

  • Constructor Details

    • ConcurrentLinkedQueue

      public ConcurrentLinkedQueue()
      Creates a ConcurrentLinkedQueue that is initially empty.

    • ConcurrentLinkedQueue

      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

  • Method Details

    • add

      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.
      Specified by:
      add in interface Collection<E>
      Specified by:
      add in interface Queue<E>
      Overrides:
      add in class AbstractQueue<E>
      Parameters:
      e - the element to add
      Returns:
      true (as specified by Collection.add(E))
      Throws:
      NullPointerException - if the specified element is null

    • offer

      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.
      Specified by:
      offer in interface Queue<E>
      Parameters:
      e - the element to add
      Returns:
      true (as specified by Queue.offer(E))
      Throws:
      NullPointerException - if the specified element is null

    • poll

      public E 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<E>
      Returns:
      the head of this queue, or null if this queue is empty

    • peek

      public E 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<E>
      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 Collection<E>
      Overrides:
      isEmpty in class AbstractCollection<E>
      Returns:
      true if this queue contains no elements
      See Also:
      AbstractCollection.size()

    • 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. 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.

      Specified by:
      size in interface Collection<E>
      Specified by:
      size in class AbstractCollection<E>
      Returns:
      the number of elements in this queue

    • 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).
      Specified by:
      contains in interface Collection<E>
      Overrides:
      contains in class AbstractCollection<E>
      Parameters:
      o - object to be checked for containment in this queue
      Returns:
      true if this queue contains the specified element

    • 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).
      Specified by:
      remove in interface Collection<E>
      Overrides:
      remove in class AbstractCollection<E>
      Parameters:
      o - element to be removed from this queue, if present
      Returns:
      true if this queue changed as a result of the call

    • addAll

      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.
      Specified by:
      addAll in interface Collection<E>
      Overrides:
      addAll in class AbstractQueue<E>
      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
      See Also:
      AbstractQueue.add(Object)

    • 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.

      Specified by:
      toArray in interface Collection<E>
      Overrides:
      toArray in class AbstractCollection<E>
      Returns:
      an array containing all of the elements in this queue

    • toArray

      public <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. 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().
      Specified by:
      toArray in interface Collection<E>
      Overrides:
      toArray in class AbstractCollection<E>
      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

    • iterator

      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 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 Collection<E>
      Specified by:
      iterator in interface Iterable<E>
      Specified by:
      iterator in class AbstractCollection<E>
      Returns:
      an iterator over the elements in this queue in proper sequence