Android
java.util.concurrent
public class

java.util.concurrent.ConcurrentLinkedQueue<E>

java.lang.Object
java.util.AbstractCollection<E> Collection<E>
java.util.AbstractQueue<E> Queue<E>
java.util.concurrent.ConcurrentLinkedQueue<E> Serializable Queue<E>

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 implements all of the optional methods of the Collection and Iterator interfaces.

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 o)
Adds the specified element to the tail of this queue.
          boolean  contains(Object o)
Searches this Collection for the specified object.
          boolean  isEmpty()
Returns true if the collection has no element, otherwise false.
          Iterator<E>  iterator()
Returns an iterator over the elements in this queue in proper sequence.
          boolean  offer(E o)
Inserts the specified element to the tail of this queue.
          peek()
Gets but not removes the element in the head of the queue, or throws exception if there is no element in the queue.
          poll()
Gets and removes the element in the head of the queue, or returns null if there is no element in the queue.
          boolean  remove(Object o)
Removes the first occurrence of the specified object from this Collection.
          int  size()
Returns the number of elements in this queue.
        <T>  T[]  toArray(T[] a)
Returns an array containing all elements contained in this Collection.
          Object[]  toArray()
Returns a new array containing all elements contained in this Collection.
Methods inherited from class java.util.AbstractQueue
Methods inherited from class java.util.AbstractCollection
Methods inherited from class java.lang.Object
Methods inherited from interface java.lang.Iterable
Methods inherited from interface java.util.Collection
Methods inherited from interface java.util.Queue

Details

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 c or any element within it is null

Public Methods

public boolean add(E o)

Adds the specified element to the tail of this queue.

Parameters

o the element to add.

Returns

  • true (as per the general contract of Collection.add).

Throws

NullPointerException if the specified element is null

public boolean contains(Object o)

Searches this Collection for the specified object.

Parameters

o the object to search for

Returns

  • true if object is an element of this Collection, false otherwise

public boolean isEmpty()

Returns true if the collection has no element, otherwise false.

Returns

  • true if the collection has no element.

public Iterator<E> 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 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 o)

Inserts the specified element to the tail of this queue.

Parameters

o the element to add.

Returns

  • true (as per the general contract of Queue.offer).

Throws

NullPointerException if the specified element is null

public E peek()

Gets but not removes the element in the head of the queue, or throws exception if there is no element in the queue.

public E poll()

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

public boolean remove(Object o)

Removes the first occurrence of the specified object from this Collection. This operation traverses over the collection, looking for the specified object. Once the object is found, the object will be removed from the collection using the iterator's remove method. This collection will throw an UnsupportedOperationException if the iterator returned does not implement remove method, and the specified object is in this collection.

Parameters

o the object to remove

Returns

  • true if this Collection is modified, false otherwise

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.

Returns

  • the number of elements in this queue.

public T[] toArray(T[] a)

Returns an array containing all elements contained in this Collection. If the specified array is large enough to hold the elements, the specified array is used, otherwise an array of the same type is created. If the specified array is used and is larger than this Collection, the array element following the collection elements is set to null.

Parameters

a the array

Returns

  • an array of the elements from this Collection

public Object[] toArray()

Returns a new array containing all elements contained in this Collection. All the elements in the array will not be referenced by the collection. The elements in the returned array will be sorted to the same order as those returned by the iterator of this collection itself if the collection guarantees the order.

Returns

  • an array of the elements from this Collection
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