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| java.lang.Object | |||
| ↳ | java.util.AbstractCollection<E> | ||
| ↳ | java.util.AbstractQueue<E> | ||
| ↳ | java.util.concurrent.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.
Like most other concurrent collection implementations, this class
does not permit the use of null elements.
This implementation employs an efficient non-blocking 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.
| Public Constructors | |||||||||||
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Creates a
ConcurrentLinkedQueue that is initially empty.
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Creates a
ConcurrentLinkedQueue
initially containing the elements of the given collection,
added in traversal order of the collection's iterator.
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| Public Methods | |||||||||||
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Inserts the specified element at the tail of this queue.
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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.
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Returns
true if this queue contains the specified element.
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Returns
true if this queue contains no elements.
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Returns an iterator over the elements in this queue in proper sequence.
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Inserts the specified element at the tail of this queue.
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Retrieves, but does not remove, the head of this queue,
or returns null if this queue is empty.
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Retrieves and removes the head of this queue,
or returns null if this queue is empty.
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Removes a single instance of the specified element from this queue,
if it is present.
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Returns the number of elements in this queue.
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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.
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Returns an array containing all of the elements in this queue, in
proper sequence.
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[Expand]
Inherited Methods | |||||||||||
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From class
java.util.AbstractQueue
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From class
java.util.AbstractCollection
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From class
java.lang.Object
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From interface
java.lang.Iterable
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From interface
java.util.Collection
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From interface
java.util.Queue
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Creates a ConcurrentLinkedQueue that is initially empty.
Creates a ConcurrentLinkedQueue
initially containing the elements of the given collection,
added in traversal order of the collection's iterator.
| c | the collection of elements to initially contain |
|---|
| NullPointerException | if the specified collection or any of its elements are null |
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Inserts the specified element at the tail of this queue.
As the queue is unbounded, this method will never throw
IllegalStateException or return false.
| e | the element to add |
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true (as specified by add(E))| NullPointerException | if the specified element is null |
|---|
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.
| c | the elements to be inserted into this queue |
|---|
true if this queue changed as a result of the call| NullPointerException | if the specified collection or any of its elements are null |
|---|---|
| IllegalArgumentException | if the collection is this queue |
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).
| o | object to be checked for containment in this queue |
|---|
true if this queue contains the specified element
Returns true if this queue contains no elements.
true if this queue contains no elements
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.
Inserts the specified element at the tail of this queue.
As the queue is unbounded, this method will never return false.
| e | the element to add |
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true (as specified by offer(E))| NullPointerException | if the specified element is null |
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Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty.
Retrieves and removes the head of this queue, or returns null if this queue is empty.
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).
| o | element to be removed from this queue, if present |
|---|
true if this queue changed as a result of the call
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 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().| 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 |
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| ArrayStoreException | if the runtime type of the specified array is not a supertype of the runtime type of every element in this queue |
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| NullPointerException | if the specified array is null |
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.