# ConcurrentLinkedQueue

public class **ConcurrentLinkedQueue** extends [AbstractQueue](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/AbstractQueue.html)\<E\>  
implements [Queue](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Queue.html)\<E\> [Serializable](https://developers.google.com/j2objc/javadoc/jre/reference/java/io/Serializable.html) An unbounded thread-safe {@linkplain Queue 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.
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](http://www.cs.rochester.edu/~scott/papers/1996_PODC_queues.pdf) 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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/ConcurrentModificationException.html`, 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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Queue.html` and `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Iterator.html` interfaces.

Memory consistency effects: As with other concurrent
collections, actions in a thread prior to placing an object into a
`ConcurrentLinkedQueue`
[*happen-before*](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/package-summary#MemoryVisibility)
actions subsequent to the access or removal of that element from
the `ConcurrentLinkedQueue` in another thread.

### Public Constructor Summary

|---|---|
|   | [ConcurrentLinkedQueue](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/ConcurrentLinkedQueue.html#ConcurrentLinkedQueue())() Creates a `ConcurrentLinkedQueue` that is initially empty. |
|   | [ConcurrentLinkedQueue](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/ConcurrentLinkedQueue.html#ConcurrentLinkedQueue(java.util.Collection<?%20extends%20E>))([Collection](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html)\<? extends E\> c) Creates a `ConcurrentLinkedQueue` initially containing the elements of the given collection, added in traversal order of the collection's iterator. |

### Public Method Summary

|---|---|
| boolean | [add](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/ConcurrentLinkedQueue.html#add(E))(E e) Inserts the specified element at the tail of this queue. |
| boolean | [addAll](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/ConcurrentLinkedQueue.html#addAll(java.util.Collection<?%20extends%20E>))([Collection](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html)\<? 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](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/ConcurrentLinkedQueue.html#contains(java.lang.Object))([Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) o) Returns `true` if this queue contains the specified element. |
| boolean | [isEmpty](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/ConcurrentLinkedQueue.html#isEmpty())() Returns `true` if this queue contains no elements. |
| [Iterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Iterator.html)\<E\> | [iterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/ConcurrentLinkedQueue.html#iterator())() Returns an iterator over the elements in this queue in proper sequence. |
| boolean | [offer](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/ConcurrentLinkedQueue.html#offer(E))(E e) Inserts the specified element at the tail of this queue. |
| E | [peek](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/ConcurrentLinkedQueue.html#peek())() Retrieves, but does not remove, the head of this queue, or returns `null` if this queue is empty. |
| E | [poll](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/ConcurrentLinkedQueue.html#poll())() Retrieves and removes the head of this queue, or returns `null` if this queue is empty. |
| boolean | [remove](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/ConcurrentLinkedQueue.html#remove(java.lang.Object))([Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) o) Removes a single instance of the specified element from this queue, if it is present. |
| int | [size](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/ConcurrentLinkedQueue.html#size())() Returns the number of elements in this queue. |
| [Spliterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Spliterator.html)\<E\> | [spliterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/ConcurrentLinkedQueue.html#spliterator())() Returns a `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Spliterator.html` over the elements in this queue. |
| \<T\> T\[\] | [toArray](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/ConcurrentLinkedQueue.html#toArray(T[]))(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. |
| [Object\[\]](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) | [toArray](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/ConcurrentLinkedQueue.html#toArray())() Returns an array containing all of the elements in this queue, in proper sequence. |
| [String](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/String.html) | [toString](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/ConcurrentLinkedQueue.html#toString())() Returns a string representation of this collection. |

### Inherited Method Summary

From class [java.util.AbstractQueue](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/AbstractQueue.html)

|---|---|
| boolean | [add](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/AbstractQueue.html#add(E))(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. |
| boolean | [addAll](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/AbstractQueue.html#addAll(java.util.Collection<?%20extends%20E>))([Collection](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html)\<? extends E\> c) Adds all of the elements in the specified collection to this queue. |
| void | [clear](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/AbstractQueue.html#clear())() Removes all of the elements from this queue. |
| E | [element](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/AbstractQueue.html#element())() Retrieves, but does not remove, the head of this queue. |
| E | [remove](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/AbstractQueue.html#remove())() Retrieves and removes the head of this queue. |

From class [java.util.AbstractCollection](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/AbstractCollection.html)

|---|---|
| boolean | [add](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/AbstractCollection.html#add(E))(E e) Ensures that this collection contains the specified element (optional operation). This implementation always throws an UnsupportedOperationException. |
| boolean | [addAll](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/AbstractCollection.html#addAll(java.util.Collection<?%20extends%20E>))([Collection](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html)\<? 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. |
| void | [clear](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/AbstractCollection.html#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. |
| boolean | [contains](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/AbstractCollection.html#contains(java.lang.Object))([Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) 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. |
| boolean | [containsAll](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/AbstractCollection.html#containsAll(java.util.Collection<?>))([Collection](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html)\<?\> 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 | [isEmpty](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/AbstractCollection.html#isEmpty())() Returns true if this collection contains no elements. This implementation returns size() == 0. |
| abstract [Iterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Iterator.html)\<E\> | [iterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/AbstractCollection.html#iterator())() Returns an iterator over the elements contained in this collection. |
| boolean | [remove](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/AbstractCollection.html#remove(java.lang.Object))([Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) 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. |
| boolean | [removeAll](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/AbstractCollection.html#removeAll(java.util.Collection<?>))([Collection](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html)\<?\> 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. |
| boolean | [retainAll](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/AbstractCollection.html#retainAll(java.util.Collection<?>))([Collection](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html)\<?\> 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. |
| abstract int | [size](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/AbstractCollection.html#size())() Returns the number of elements in this collection. |
| \<T\> T\[\] | [toArray](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/AbstractCollection.html#toArray(T[]))(T\[\] contents) 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`. |
| [Object\[\]](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) | [toArray](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/AbstractCollection.html#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`. |
| [String](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/String.html) | [toString](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/AbstractCollection.html#toString())() Returns a string representation of this collection. |

From class [java.lang.Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html)

|---|---|
| [Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) | [clone](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#clone())() Creates and returns a copy of this `Object`. |
| boolean | [equals](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#equals(java.lang.Object))([Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) obj) Compares this instance with the specified object and indicates if they are equal. |
| void | [finalize](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#finalize())() Invoked when the garbage collector has detected that this instance is no longer reachable. |
| final [Class](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Class.html)\<?\> | [getClass](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#getClass())() Returns the unique instance of `https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Class.html` that represents this object's class. |
| int | [hashCode](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#hashCode())() Returns an integer hash code for this object. |
| final void | [notify](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#notify())() Causes a thread which is waiting on this object's monitor (by means of calling one of the `wait()` methods) to be woken up. |
| final void | [notifyAll](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#notifyAll())() Causes all threads which are waiting on this object's monitor (by means of calling one of the `wait()` methods) to be woken up. |
| [String](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/String.html) | [toString](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#toString())() Returns a string containing a concise, human-readable description of this object. |
| final void | [wait](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#wait(long,%20int))(long timeout, int nanos) Causes the calling thread to wait until another thread calls the `notify()` or `notifyAll()` method of this object or until the specified timeout expires. |
| final void | [wait](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#wait(long))(long timeout) Causes the calling thread to wait until another thread calls the `notify()` or `notifyAll()` method of this object or until the specified timeout expires. |
| final void | [wait](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#wait())() Causes the calling thread to wait until another thread calls the `notify()` or `notifyAll()` method of this object. |

From interface [java.util.Queue](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Queue.html)

|---|---|
| abstract boolean | [add](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Queue.html#add(E))(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](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Queue.html#element())() Retrieves, but does not remove, the head of this queue. |
| abstract boolean | [offer](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Queue.html#offer(E))(E e) Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions. |
| abstract E | [peek](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Queue.html#peek())() Retrieves, but does not remove, the head of this queue, or returns `null` if this queue is empty. |
| abstract E | [poll](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Queue.html#poll())() Retrieves and removes the head of this queue, or returns `null` if this queue is empty. |
| abstract E | [remove](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Queue.html#remove())() Retrieves and removes the head of this queue. |

From interface [java.util.Collection](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html)

|---|---|
| abstract boolean | [add](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#add(E))(E e) Ensures that this collection contains the specified element (optional operation). |
| abstract boolean | [addAll](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#addAll(java.util.Collection<?%20extends%20E>))([Collection](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html)\<? extends E\> c) Adds all of the elements in the specified collection to this collection (optional operation). |
| abstract void | [clear](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#clear())() Removes all of the elements from this collection (optional operation). |
| abstract boolean | [contains](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#contains(java.lang.Object))([Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) o) Returns true if this collection contains the specified element. |
| abstract boolean | [containsAll](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#containsAll(java.util.Collection<?>))([Collection](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html)\<?\> c) Returns true if this collection contains all of the elements in the specified collection. |
| abstract boolean | [equals](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#equals(java.lang.Object))([Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) o) Compares the specified object with this collection for equality. |
| abstract int | [hashCode](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#hashCode())() Returns the hash code value for this collection. |
| abstract boolean | [isEmpty](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#isEmpty())() Returns true if this collection contains no elements. |
| abstract [Iterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Iterator.html)\<E\> | [iterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#iterator())() Returns an iterator over the elements in this collection. |
| [Stream](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/stream/Stream.html)\<E\> | [parallelStream](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#parallelStream())() Returns a possibly parallel `Stream` with this collection as its source. |
| abstract boolean | [remove](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#remove(java.lang.Object))([Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) o) Removes a single instance of the specified element from this collection, if it is present (optional operation). |
| abstract boolean | [removeAll](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#removeAll(java.util.Collection<?>))([Collection](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html)\<?\> c) Removes all of this collection's elements that are also contained in the specified collection (optional operation). |
| boolean | [removeIf](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#removeIf(java.util.function.Predicate<?%20super%20E>))([Predicate](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/function/Predicate.html)\<? super E\> filter) Removes all of the elements of this collection that satisfy the given predicate. |
| abstract boolean | [retainAll](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#retainAll(java.util.Collection<?>))([Collection](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html)\<?\> c) Retains only the elements in this collection that are contained in the specified collection (optional operation). |
| abstract int | [size](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#size())() Returns the number of elements in this collection. |
| [Spliterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Spliterator.html)\<E\> | [spliterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#spliterator())() Creates a `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Spliterator.html` over the elements in this collection. |
| [Stream](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/stream/Stream.html)\<E\> | [stream](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#stream())() Returns a sequential `Stream` with this collection as its source. |
| abstract \<T\> T\[\] | [toArray](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#toArray(T[]))(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. |
| abstract [Object\[\]](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) | [toArray](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#toArray())() Returns an array containing all of the elements in this collection. |

From interface [java.lang.Iterable](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Iterable.html)

|---|---|
| void | [forEach](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Iterable.html#forEach(java.util.function.Consumer<?%20super%20T>))([Consumer](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/function/Consumer.html)\<? super T\> action) Performs the given action for each element of the `Iterable` until all elements have been processed or the action throws an exception. |
| abstract [Iterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Iterator.html)\<E\> | [iterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Iterable.html#iterator())() Returns an iterator over elements of type `T`. |
| [Spliterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Spliterator.html)\<E\> | [spliterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Iterable.html#spliterator())() Creates a `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Spliterator.html` over the elements described by this `Iterable`. |

## Public Constructors

#### public
**ConcurrentLinkedQueue**
()

Creates a `ConcurrentLinkedQueue` that is initially empty.

#### public
**ConcurrentLinkedQueue**
([Collection](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html)\<? 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 |
|---|---|

##### Throws

| [NullPointerException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/NullPointerException.html) | if the specified collection or any of its elements are null |
|---|---|

## Public Methods

#### public boolean
**add**
(E e)

Inserts the specified element at the tail of this queue.
As the queue is unbounded, this method will never throw
`https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/IllegalStateException.html` or return `false`.

##### Parameters

| e | the element to add |
|---|---|

##### Returns

- `true` (as specified by `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#add(E)`)

##### Throws

| [NullPointerException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/NullPointerException.html) | if the specified element is null |
|---|---|

#### public boolean
**addAll**
([Collection](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html)\<? 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. Attempts to `addAll` of a queue to
itself result in `IllegalArgumentException`.

##### Parameters

| c | the elements to be inserted into this queue |
|---|---|

##### Returns

- `true` if this queue changed as a result of the call

##### Throws

| [NullPointerException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/NullPointerException.html) | if the specified collection or any of its elements are null |
| [IllegalArgumentException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/IllegalArgumentException.html) | if the collection is this queue |
|---|---|

#### public boolean
**contains**
([Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) 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](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Iterator.html)\<E\>
**iterator**
()

Returns 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*](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/package-summary#Weakly).

##### 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.
As the queue is unbounded, this method will never return `false`.

##### Parameters

| e | the element to add |
|---|---|

##### Returns

- `true` (as specified by `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Queue.html#offer(E)`)

##### Throws

| [NullPointerException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/NullPointerException.html) | if the specified element is null |
|---|---|

#### 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](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) 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.
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.

##### Returns

- the number of elements in this queue

#### public [Spliterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Spliterator.html)\<E\>
**spliterator**
()

Returns a `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Spliterator.html` over the elements in this queue.

The returned spliterator is
[*weakly consistent*](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/package-summary#Weakly).

The `Spliterator` reports `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Spliterator.html#CONCURRENT`,
`https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Spliterator.html#ORDERED`, and `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Spliterator.html#NONNULL`.

##### Returns

- a `Spliterator` over the 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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/ConcurrentLinkedQueue.html#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()`.

<br />

##### 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](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/ArrayStoreException.html) | if the runtime type of the specified array is not a supertype of the runtime type of every element in this queue |
| [NullPointerException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/NullPointerException.html) | if the specified array is null |
|---|---|

#### public [Object\[\]](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html)
**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

#### public [String](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/String.html)
**toString**
()

Returns a string representation of this collection. The string
representation consists of a list of the collection's elements in the
order they are returned by its iterator, enclosed in square brackets
("\[\]"). Adjacent elements are separated by the characters
", " (comma and space). Elements are converted to strings as
by `https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/String.html#valueOf(java.lang.Object)`.

##### Returns

- a string representation of this collection