java.lang.Object java.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
implements
all of the optional methods of the
Collection
and
Iterator
interfaces.
This class is a member of the Java Collections Framework .
Constructor Summary | |
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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. |
Method Summary | ||
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boolean |
add
(
E
o) Adds the specified element to the tail of this queue. |
|
boolean |
contains
(
Object
o) Returns true if this collection contains the specified element. |
|
boolean |
isEmpty
() Returns true if this collection contains no elements. |
|
Iterator < E > |
iterator
() Returns an iterator over the elements in this queue in proper sequence. |
|
boolean |
offer
(
E
o) Inserts the specified element to the tail of this queue. |
|
E |
peek
() Retrieves, but does not remove, the head of this queue, returning null if this queue is empty. |
|
E |
poll
() Retrieves and removes the head of this queue, or null if this queue is empty. |
|
boolean |
remove
(
Object
o) Removes a single instance of the specified element from this collection, if it is present (optional operation). |
|
int |
size
() Returns the number of elements in this queue. |
|
Object [] |
toArray
() Returns an array containing all of the elements in this collection. |
|
|
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. |
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(E o)
public boolean offer(E 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()
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 the collection fits in the specified array with room to spare (i.e., the array has more elements than the collection), the element in the array immediately following the end of the 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.)
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.
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 null is stored in the first location after the end of the collection.
public Iterator<E> iterator()