java.lang.Objectjava.util.AbstractCollection<E>
java.util.AbstractQueue<E>
java.util.concurrent.ConcurrentLinkedQueue<E>
public class ConcurrentLinkedQueue<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 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.
This class is a member of the Java Collections Framework .
Constructor Summary | |
---|---|
ConcurrentLinkedQueue
() Creates a ConcurrentLinkedQueue that is initially empty. |
|
ConcurrentLinkedQueue
(
Collection
<? extends
E
Creates a ConcurrentLinkedQueue initially containing the elements of the given collection, added in traversal order of the collection's iterator. |
Method Summary | ||
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boolean |
add
(
E
![]() ![]() ![]() ![]() |
|
boolean |
contains
(
Object
Returns true if this ![]() ![]() |
|
boolean |
isEmpty
() Returns true if this ![]() ![]() |
|
Iterator < E > |
iterator
() Returns an iterator over the elements in this queue in proper sequence. |
|
boolean |
offer
(
E
Inserts the specified element ![]() ![]() |
|
E |
peek
() Retrieves, but does not remove, the head of this queue, ![]() ![]() |
|
E |
poll
() Retrieves and removes the head of this queue, or ![]() |
|
boolean |
remove
(
Object
Removes a single instance of the specified element from this ![]() ![]() ![]() ![]() |
|
int |
size
() Returns the number of elements in this queue. |
|
Object |
toArray
() Returns an array containing all of the elements in this ![]() ![]() |
|
|
toArray
(T[] a) Returns an array containing all of the elements in this ![]() ![]() |
Methods inherited from class java.util. AbstractQueue |
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addAll , clear , element , remove |
Methods inherited from class java.util. AbstractCollection |
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containsAll , removeAll , retainAll , toString |
Methods inherited from class java.lang. Object |
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clone , equals , finalize , getClass , hashCode , notify , notifyAll , wait , wait , wait |
Methods inherited from interface java.util. Queue |
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element , remove |
Methods inherited from interface java.util. Collection |
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addAll , clear , containsAll , equals , hashCode , removeAll , retainAll |
Constructor Detail |
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public ConcurrentLinkedQueue()
public ConcurrentLinkedQueue(Collection<? extends E> c)
Method Detail |
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public boolean add(Ee)
o)
public boolean offer(Ee)
o)
public E poll()
public E peek()
public boolean isEmpty()
This implementation returns
size() == 0
.
public int size()
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.
public boolean contains(Object o)
This implementation iterates over the elements in the collection, checking each element in turn for equality with the specified element.
public boolean remove(Object o)
This implementation iterates over the collection looking for the specified element. If it finds the element, it removes the element from the collection using the iterator's remove method.
Note that this implementation throws an
UnsupportedOperationException
if the iterator returned by this collection's iterator method does not implement the
remove
method and this collection contains the specified object.
public Object[] toArray()
the same order.
The returned array will be "safe" in that no references to it are maintained by
this queue.
the collection.
(In other words, this method must allocate a new
array).
array even if the collection is backed by an Array).
The caller is thus free to modify the returned array.
This method acts as bridge between array-based and collection-based APIs.
This implementation allocates the array to be returned, and iterates over the elements in the collection, storing each object reference in the next consecutive element of the array, starting with element 0.
public <T> T[] toArray(T[] a)
If
this queue
the collection
fits in the specified array with room to spare (i.e., the array has more elements than
this queue),
the
collection), the
element in the array immediately following the end of the
queue
collection
is set to
null
.
This is useful in determining the length of the collection
only
if the caller knows that the collection does not contain any
null
elements.)
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.
If this collection makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order.
Suppose
This implementation checks if the array is large enough to contain the collection; if not, it allocates a new array of the correct size and type (using reflection). Then, it iterates over the collection, storing each object reference in the next consecutive element of the array, starting with element 0. If the array is larger than the collection, a
x
null
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]);
public Iterator<E> iterator()