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Java™ Platform Standard Ed. 6 |
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java.lang.Object java.util.AbstractCollection<E> java.util.AbstractQueue<E> java.util.concurrent.ConcurrentLinkedQueue<E>
E
- the type of elements held in this collectionpublic 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. |
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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 | ||
---|---|---|
boolean |
add(E e)
Inserts the specified element at the tail of this queue. |
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boolean |
contains(Object o)
Returns true if this queue contains the specified element. |
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boolean |
isEmpty()
Returns true if this queue contains no elements. |
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Iterator<E> |
iterator()
Returns an iterator over the elements in this queue in proper sequence. |
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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. |
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E |
poll()
Retrieves and removes the head of this queue, or returns null if this queue is empty. |
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boolean |
remove(Object o)
Removes a single instance of the specified element from this queue, if it is present. |
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int |
size()
Returns the number of elements in this queue. |
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Object[] |
toArray()
Returns an array containing all of the elements in this queue, in proper sequence. |
|
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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 |
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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 |
---|
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()
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 contain
NullPointerException
- if the specified collection or any
of its elements are nullMethod Detail |
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public boolean add(E e)
add
in interface Collection<E>
add
in interface Queue<E>
add
in class AbstractQueue<E>
e
- the element to add
true
(as specified by Collection.add(E)
)
NullPointerException
- if the specified element is nullpublic boolean offer(E e)
offer
in interface Queue<E>
e
- the element to add
true
(as specified by Queue.offer(E)
)
NullPointerException
- if the specified element is nullpublic E poll()
Queue
poll
in interface Queue<E>
public E peek()
Queue
peek
in interface Queue<E>
public 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.
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 queue
true
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 present
true
if this queue changed as a result of the callpublic 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 purpose
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 nullpublic Iterator<E> iterator()
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>
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Java™ Platform Standard Ed. 6 |
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