- Type Parameters:
- E- the type of elements held in this queue
- All Implemented Interfaces:
- Serializable,- Iterable<E>,- Collection<E>,- Queue<E>
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.
 
Bulk operations that add, remove, or examine multiple elements,
 such as addAll(java.util.Collection<? extends E>), removeIf(java.util.function.Predicate<? super E>) or forEach(java.util.function.Consumer<? super E>),
 are not guaranteed to be performed atomically.
 For example, a forEach traversal concurrent with an 
 addAll operation might observe 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.
- Since:
- 1.5
- See Also:
- 
Constructor SummaryConstructorsConstructorDescriptionCreates aConcurrentLinkedQueuethat is initially empty.ConcurrentLinkedQueue(Collection<? extends E> c) Creates aConcurrentLinkedQueueinitially containing the elements of the given collection, added in traversal order of the collection's iterator.
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Method SummaryModifier and TypeMethodDescriptionbooleanInserts the specified element at the tail of this queue.booleanaddAll(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.voidclear()Removes all of the elements from this queue.booleanReturnstrueif this queue contains the specified element.voidPerforms the given action for each element of theIterableuntil all elements have been processed or the action throws an exception.booleanisEmpty()Returnstrueif this queue contains no elements.iterator()Returns an iterator over the elements in this queue in proper sequence.booleanInserts the specified element at the tail of this queue.peek()Retrieves, but does not remove, the head of this queue, or returnsnullif this queue is empty.poll()Retrieves and removes the head of this queue, or returnsnullif this queue is empty.booleanRemoves a single instance of the specified element from this queue, if it is present.booleanremoveAll(Collection<?> c) Removes all of this collection's elements that are also contained in the specified collection (optional operation).booleanRemoves all of the elements of this collection that satisfy the given predicate.booleanretainAll(Collection<?> c) Retains only the elements in this collection that are contained in the specified collection (optional operation).intsize()Returns the number of elements in this queue.Returns aSpliteratorover the 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.Methods declared in class java.util.AbstractQueueclear, element, removeMethods declared in class java.util.AbstractCollectioncontainsAll, toStringMethods declared in class java.lang.Objectclone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, waitMethods declared in interface java.util.CollectioncontainsAll, equals, hashCode, parallelStream, stream, toArray
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Constructor Details- 
ConcurrentLinkedQueuepublic ConcurrentLinkedQueue()Creates aConcurrentLinkedQueuethat is initially empty.
- 
ConcurrentLinkedQueueCreates aConcurrentLinkedQueueinitially 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
- Throws:
- NullPointerException- if the specified collection or any of its elements are null
 
 
- 
- 
Method Details- 
addInserts the specified element at the tail of this queue. As the queue is unbounded, this method will never throwIllegalStateExceptionor returnfalse.- Specified by:
- addin interface- Collection<E>
- Specified by:
- addin interface- Queue<E>
- Overrides:
- addin class- AbstractQueue<E>
- Parameters:
- e- the element to add
- Returns:
- true(as specified by- Collection.add(E))
- Throws:
- NullPointerException- if the specified element is null
 
- 
offerInserts the specified element at the tail of this queue. As the queue is unbounded, this method will never returnfalse.- Specified by:
- offerin interface- Queue<E>
- Parameters:
- e- the element to add
- Returns:
- true(as specified by- Queue.offer(E))
- Throws:
- NullPointerException- if the specified element is null
 
- 
pollDescription copied from interface:QueueRetrieves and removes the head of this queue, or returnsnullif this queue is empty.
- 
peekDescription copied from interface:QueueRetrieves, but does not remove, the head of this queue, or returnsnullif this queue is empty.
- 
isEmptypublic boolean isEmpty()Returnstrueif this queue contains no elements.- Specified by:
- isEmptyin interface- Collection<E>
- Overrides:
- isEmptyin class- AbstractCollection<E>
- Returns:
- trueif this queue contains no elements
 
- 
sizepublic int size()Returns the number of elements in this queue. If this queue contains more thanInteger.MAX_VALUEelements, returnsInteger.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. - Specified by:
- sizein interface- Collection<E>
- Returns:
- the number of elements in this queue
 
- 
containsReturnstrueif this queue contains the specified element. More formally, returnstrueif and only if this queue contains at least one elementesuch thato.equals(e).- Specified by:
- containsin interface- Collection<E>
- Overrides:
- containsin class- AbstractCollection<E>
- Parameters:
- o- object to be checked for containment in this queue
- Returns:
- trueif this queue contains the specified element
 
- 
removeRemoves a single instance of the specified element from this queue, if it is present. More formally, removes an elementesuch thato.equals(e), if this queue contains one or more such elements. Returnstrueif this queue contained the specified element (or equivalently, if this queue changed as a result of the call).- Specified by:
- removein interface- Collection<E>
- Overrides:
- removein class- AbstractCollection<E>
- Parameters:
- o- element to be removed from this queue, if present
- Returns:
- trueif this queue changed as a result of the call
 
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addAllAppends 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 toaddAllof a queue to itself result inIllegalArgumentException.- Specified by:
- addAllin interface- Collection<E>
- Overrides:
- addAllin class- AbstractQueue<E>
- Parameters:
- c- the elements to be inserted into this queue
- Returns:
- trueif this queue changed as a result of the call
- Throws:
- NullPointerException- if the specified collection or any of its elements are null
- IllegalArgumentException- if the collection is this queue
- See Also:
 
- 
toArrayReturns 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. - Specified by:
- toArrayin interface- Collection<E>
- Overrides:
- toArrayin class- AbstractCollection<E>
- Returns:
- an array containing all of the elements in this queue
 
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toArraypublic <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. 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 xis a queue known to contain only strings. The following code can be used to dump the queue into a newly allocated array ofString:
 Note thatString[] y = x.toArray(new String[0]);toArray(new Object[0])is identical in function totoArray().- Specified by:
- toArrayin interface- Collection<E>
- Overrides:
- toArrayin class- AbstractCollection<E>
- Type Parameters:
- T- the component type of the array to contain the collection
- 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
- Throws:
- 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 null
 
- 
iteratorReturns 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 weakly consistent. - Specified by:
- iteratorin interface- Collection<E>
- Specified by:
- iteratorin interface- Iterable<E>
- Specified by:
- iteratorin class- AbstractCollection<E>
- Returns:
- an iterator over the elements in this queue in proper sequence
 
- 
spliteratorReturns aSpliteratorover the elements in this queue.The returned spliterator is weakly consistent. The SpliteratorreportsSpliterator.CONCURRENT,Spliterator.ORDERED, andSpliterator.NONNULL.- Specified by:
- spliteratorin interface- Collection<E>
- Specified by:
- spliteratorin interface- Iterable<E>
- Implementation Note:
- The SpliteratorimplementstrySplitto permit limited parallelism.
- Returns:
- a Spliteratorover the elements in this queue
- Since:
- 1.8
 
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removeIfDescription copied from interface:CollectionRemoves all of the elements of this collection that satisfy the given predicate. Errors or runtime exceptions thrown during iteration or by the predicate are relayed to the caller.- Specified by:
- removeIfin interface- Collection<E>
- Parameters:
- filter- a predicate which returns- truefor elements to be removed
- Returns:
- trueif any elements were removed
- Throws:
- NullPointerException- if the specified filter is null
 
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removeAllDescription copied from class:AbstractCollectionRemoves all of this collection's elements that are also contained in the specified collection (optional operation). After this call returns, this collection will contain no elements in common with the specified collection.- Specified by:
- removeAllin interface- Collection<E>
- Overrides:
- removeAllin class- AbstractCollection<E>
- Parameters:
- c- collection containing elements to be removed from this collection
- Returns:
- trueif this collection changed as a result of the call
- Throws:
- NullPointerException- if this collection contains one or more null elements and the specified collection does not support null elements (optional) or if the specified collection is null
- See Also:
 
- 
retainAllDescription copied from class:AbstractCollectionRetains only the elements in this collection that are contained in the specified collection (optional operation). In other words, removes from this collection all of its elements that are not contained in the specified collection.- Specified by:
- retainAllin interface- Collection<E>
- Overrides:
- retainAllin class- AbstractCollection<E>
- Parameters:
- c- collection containing elements to be retained in this collection
- Returns:
- trueif this collection changed as a result of the call
- Throws:
- NullPointerException- if this collection contains one or more null elements and the specified collection does not permit null elements (optional) or if the specified collection is null
- See Also:
 
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clearpublic void clear()Description copied from class:AbstractQueueRemoves all of the elements from this queue. The queue will be empty after this call returns.This implementation repeatedly invokes polluntil it returnsnull.- Specified by:
- clearin interface- Collection<E>
- Overrides:
- clearin class- AbstractQueue<E>
 
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forEachDescription copied from interface:IterablePerforms the given action for each element of theIterableuntil all elements have been processed or the action throws an exception. Actions are performed in the order of iteration, if that order is specified. Exceptions thrown by the action are relayed to the caller.The behavior of this method is unspecified if the action performs side-effects that modify the underlying source of elements, unless an overriding class has specified a concurrent modification policy. - Specified by:
- forEachin interface- Iterable<E>
- Parameters:
- action- The action to be performed for each element
- Throws:
- NullPointerException- if the specified action is null
 
 
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