E - the type of elements held in this collectionpublic class ConcurrentLinkedQueue<E> extends AbstractQueue<E> implements Queue<E>, Serializable
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.
This class is a member of the Java Collections Framework.
| Constructor and 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. |
| Modifier and Type | Method and 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.
|
clear, element, removecontainsAll, removeAll, retainAll, toStringclone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, waitclear, containsAll, equals, hashCode, removeAll, retainAllpublic ConcurrentLinkedQueue()
ConcurrentLinkedQueue that is initially empty.public ConcurrentLinkedQueue(Collection<? extends E> c)
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 containNullPointerException - if the specified collection or any
of its elements are nullpublic boolean add(E e)
IllegalStateException or return false.add in interface Collection<E>add in interface Queue<E>add in class AbstractQueue<E>e - the element to addtrue (as specified by Collection.add(E))NullPointerException - if the specified element is nullpublic boolean offer(E e)
false.offer in interface Queue<E>e - the element to addtrue (as specified by Queue.offer(E))NullPointerException - if the specified element is nullpublic E poll()
Queuepublic E peek()
Queuepublic boolean isEmpty()
true if this queue contains no elements.isEmpty in interface Collection<E>isEmpty in class AbstractCollection<E>true if this queue contains no elementspublic int size()
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.
size in interface Collection<E>size in class AbstractCollection<E>public boolean contains(Object o)
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).contains in interface Collection<E>contains in class AbstractCollection<E>o - object to be checked for containment in this queuetrue if this queue contains the specified elementpublic boolean remove(Object o)
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).remove in interface Collection<E>remove in class AbstractCollection<E>o - element to be removed from this queue, if presenttrue if this queue changed as a result of the callpublic boolean addAll(Collection<? extends E> c)
addAll of a queue to
itself result in IllegalArgumentException.addAll in interface Collection<E>addAll in class AbstractQueue<E>c - the elements to be inserted into this queuetrue if this queue changed as a result of the callNullPointerException - if the specified collection or any
of its elements are nullIllegalArgumentException - if the collection is this queueAbstractQueue.add(Object)public Object[] toArray()
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.
toArray in interface Collection<E>toArray in class AbstractCollection<E>public <T> T[] toArray(T[] a)
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().toArray in interface Collection<E>toArray in class AbstractCollection<E>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 purposeArrayStoreException - if the runtime type of the specified array
is not a supertype of the runtime type of every element in
this queueNullPointerException - if the specified array is nullpublic Iterator<E> iterator()
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.
iterator in interface Iterable<E>iterator in interface Collection<E>iterator in class AbstractCollection<E>