Class Overview
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.
Summary
Public Constructors |
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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.
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Public Methods |
boolean
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add(E e)
Inserts the specified element at the tail of this queue.
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boolean
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contains(Object o)
Returns true if this queue contains the specified element.
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boolean
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isEmpty()
Returns true if this queue contains no elements.
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Iterator<E>
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iterator()
Returns an iterator over the elements in this queue in proper sequence.
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boolean
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offer(E e)
Inserts the specified element at the tail of this queue.
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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
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remove(Object o)
Removes a single instance of the specified element from this queue,
if it is present.
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int
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size()
Returns the number of elements in this queue.
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<T>
T[]
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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.
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Object[]
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toArray()
Returns an array containing all of the elements in this queue, in
proper sequence.
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[Expand]
Inherited Methods |
From class
java.util.AbstractQueue
boolean
|
add(E e)
Inserts the specified element into this queue if it is possible to do so
immediately without violating capacity restrictions, returning
true upon success and throwing an IllegalStateException
if no space is currently available.
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boolean
|
addAll(Collection<? extends E> c)
Adds all of the elements in the specified collection to this
queue.
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void
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clear()
Removes all of the elements from this queue.
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E
|
element()
Retrieves, but does not remove, the head of this queue.
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E
|
remove()
Retrieves and removes the head of this queue.
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From class
java.util.AbstractCollection
boolean
|
add(E e)
Ensures that this collection contains the specified element (optional
operation).
This implementation always throws an
UnsupportedOperationException.
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boolean
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addAll(Collection<? extends E> c)
Adds all of the elements in the specified collection to this collection
(optional operation).
This implementation iterates over the specified collection, and adds
each object returned by the iterator to this collection, in turn.
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void
|
clear()
Removes all of the elements from this collection (optional operation).
This implementation iterates over this collection, removing each
element using the Iterator.remove operation.
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boolean
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contains(Object o)
Returns true if this collection contains the specified element.
This implementation iterates over the elements in the collection,
checking each element in turn for equality with the specified element.
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boolean
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containsAll(Collection<?> c)
Returns true if this collection contains all of the elements
in the specified collection.
This implementation iterates over the specified collection,
checking each element returned by the iterator in turn to see
if it's contained in this collection.
|
boolean
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isEmpty()
Returns true if this collection contains no elements.
This implementation returns size() == 0.
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abstract
Iterator<E>
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iterator()
Returns an iterator over the elements contained in this collection.
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boolean
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remove(Object o)
Removes a single instance of the specified element from this
collection, if it is present (optional operation).
This implementation iterates over the collection looking for the
specified element.
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boolean
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removeAll(Collection<?> c)
Removes all of this collection's elements that are also contained in the
specified collection (optional operation).
This implementation iterates over this collection, checking each
element returned by the iterator in turn to see if it's contained
in the specified collection.
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boolean
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retainAll(Collection<?> c)
Retains only the elements in this collection that are contained in the
specified collection (optional operation).
This implementation iterates over this collection, checking each
element returned by the iterator in turn to see if it's contained
in the specified collection.
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abstract
int
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size()
Returns the number of elements in this collection.
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<T>
T[]
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toArray(T[] a)
Returns an array containing all of the elements in this collection;
the runtime type of the returned array is that of the specified array.
This implementation returns an array containing all the elements
returned by this collection's iterator in the same order, stored in
consecutive elements of the array, starting with index 0 .
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Object[]
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toArray()
Returns an array containing all of the elements in this collection.
This implementation returns an array containing all the elements
returned by this collection's iterator, in the same order, stored in
consecutive elements of the array, starting with index 0 .
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String
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toString()
Returns a string representation of this collection.
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From class
java.lang.Object
Object
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clone()
Creates and returns a copy of this object.
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boolean
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equals(Object obj)
Indicates whether some other object is "equal to" this one.
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void
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finalize()
Called by the garbage collector on an object when garbage collection
determines that there are no more references to the object.
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final
Class<?>
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getClass()
Returns the runtime class of this Object .
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int
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hashCode()
Returns a hash code value for the object.
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final
void
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notify()
Wakes up a single thread that is waiting on this object's
monitor.
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final
void
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notifyAll()
Wakes up all threads that are waiting on this object's monitor.
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String
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toString()
Returns a string representation of the object.
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final
void
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wait()
Causes the current thread to wait until another thread invokes the
notify() method or the
notifyAll() method for this object.
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final
void
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wait(long timeout, int nanos)
Causes the current thread to wait until another thread invokes the
notify() method or the
notifyAll() method for this object, or
some other thread interrupts the current thread, or a certain
amount of real time has elapsed.
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final
void
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wait(long timeout)
Causes the current thread to wait until either another thread invokes the
notify() method or the
notifyAll() method for this object, or a
specified amount of time has elapsed.
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From interface
java.lang.Iterable
abstract
Iterator<T>
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iterator()
Returns an iterator over a set of elements of type T.
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From interface
java.util.Collection
abstract
boolean
|
add(E e)
Ensures that this collection contains the specified element (optional
operation).
|
abstract
boolean
|
addAll(Collection<? extends E> c)
Adds all of the elements in the specified collection to this collection
(optional operation).
|
abstract
void
|
clear()
Removes all of the elements from this collection (optional operation).
|
abstract
boolean
|
contains(Object o)
Returns true if this collection contains the specified element.
|
abstract
boolean
|
containsAll(Collection<?> c)
Returns true if this collection contains all of the elements
in the specified collection.
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abstract
boolean
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equals(Object o)
Compares the specified object with this collection for equality.
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abstract
int
|
hashCode()
Returns the hash code value for this collection.
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abstract
boolean
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isEmpty()
Returns true if this collection contains no elements.
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abstract
Iterator<E>
|
iterator()
Returns an iterator over the elements in this collection.
|
abstract
boolean
|
remove(Object o)
Removes a single instance of the specified element from this
collection, if it is present (optional operation).
|
abstract
boolean
|
removeAll(Collection<?> c)
Removes all of this collection's elements that are also contained in the
specified collection (optional operation).
|
abstract
boolean
|
retainAll(Collection<?> c)
Retains only the elements in this collection that are contained in the
specified collection (optional operation).
|
abstract
int
|
size()
Returns the number of elements in this collection.
|
abstract
<T>
T[]
|
toArray(T[] a)
Returns an array containing all of the elements in this collection;
the runtime type of the returned array is that of the specified array.
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abstract
Object[]
|
toArray()
Returns an array containing all of the elements in this collection.
|
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From interface
java.util.Queue
abstract
boolean
|
add(E e)
Inserts the specified element into this queue if it is possible to do so
immediately without violating capacity restrictions, returning
true upon success and throwing an IllegalStateException
if no space is currently available.
|
abstract
E
|
element()
Retrieves, but does not remove, the head of this queue.
|
abstract
boolean
|
offer(E e)
Inserts the specified element into this queue if it is possible to do
so immediately without violating capacity restrictions.
|
abstract
E
|
peek()
Retrieves, but does not remove, the head of this queue,
or returns null if this queue is empty.
|
abstract
E
|
poll()
Retrieves and removes the head of this queue,
or returns null if this queue is empty.
|
abstract
E
|
remove()
Retrieves and removes the head of this queue.
|
|
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 |
Public Methods
public
boolean
add
(E e)
Inserts the specified element at the tail of this queue.
public
boolean
contains
(Object o)
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).
Parameters
o
| object to be checked for containment in this queue |
Returns
- true if this queue contains the specified element
public
boolean
isEmpty
()
Returns true if this queue contains no elements.
Returns
- true if this queue contains no elements
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 e)
Inserts the specified element at the tail of this queue.
public
E
peek
()
Retrieves, but does not remove, the head of this queue,
or returns null if this queue is empty.
Returns
- the head of this queue, or null if this queue is empty
public
E
poll
()
Retrieves and removes the head of this queue,
or returns null if this queue is empty.
Returns
- the head of this queue, or null if this queue is empty
public
boolean
remove
(Object o)
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).
Parameters
o
| element to be removed from this queue, if present |
Returns
- true if this queue changed as a result of the call
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 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().
Parameters
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 |
Returns
- an array containing all of the elements in this queue
public
Object[]
toArray
()
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.
Returns
- an array containing all of the elements in this queue