java.util.concurrent

## Class ConcurrentLinkedQueue<E>

- java.lang.Object
- - java.util.AbstractCollection<E>
  - - java.util.AbstractQueue<E>
    - - java.util.concurrent.ConcurrentLinkedQueue<E>

- Type Parameters:
    `E` - the type of elements held in this collection
  All Implemented Interfaces:
    Serializable, Iterable<E>, Collection<E>, Queue<E>

      public class ConcurrentLinkedQueue<E>
      extends AbstractQueue<E>
      implements Queue<E>, Serializable

  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. Like most other concurrent collection implementations, this class does not permit the use of `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.

  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. 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 `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
  Author:
    Doug Lea
  See Also:
    Serialized Form

- - ### Constructor Summary

    Constructors
    Constructor and Description
    `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

    All Methods Instance Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `boolean` | `add(E e)` Inserts the specified element at the tail of this queue.
    `boolean` | `addAll(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.
    `boolean` | `contains(Object o)` Returns `true` if this queue contains the specified element.
    `boolean` | `isEmpty()` Returns `true` if this queue contains no elements.
    `Iterator<E>` | `iterator()` Returns an iterator over the elements in this queue in proper sequence.
    `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.
    `E` | `poll()` Retrieves and removes the head of this queue, or returns `null` if this queue is empty.
    `boolean` | `remove(Object o)` Removes a single instance of the specified element from this queue, if it is present.
    `int` | `size()` Returns the number of 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 inherited from class java.util.AbstractQueue

      `clear, element, remove`

    - ### Methods inherited from class java.util.AbstractCollection

      `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

      `element, remove`

    - ### Methods inherited from interface java.util.Collection

      `clear, containsAll, equals, hashCode, parallelStream, removeAll, removeIf, retainAll, spliterator, stream`

    - ### Methods inherited from interface java.lang.Iterable

      `forEach`

- - ### Constructor Detail

    - #### ConcurrentLinkedQueue

          public ConcurrentLinkedQueue()

      Creates a `ConcurrentLinkedQueue` that is initially empty.

    - #### ConcurrentLinkedQueue

          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
      Throws:
        `NullPointerException` - if the specified collection or any of its elements are null

  - ### Method Detail

    - #### add

          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 `IllegalStateException` or return `false`.
      Specified by:
        `add` in interface `Collection<E>`
      Specified by:
        `add` in interface `Queue<E>`
      Overrides:
        `add` in 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

    - #### offer

          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`.
      Specified by:
        `offer` in 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

    - #### poll

          public E poll()

      Description copied from interface: `Queue`
      Retrieves and removes the head of this queue, or returns `null` if this queue is empty.
      Specified by:
        `poll` in interface `Queue<E>`
      Returns:
        the head of this queue, or `null` if this queue is empty

    - #### peek

          public E peek()

      Description copied from interface: `Queue`
      Retrieves, but does not remove, the head of this queue, or returns `null` if this queue is empty.
      Specified by:
        `peek` in interface `Queue<E>`
      Returns:
        the head of this queue, or `null` if this queue is empty

    - #### isEmpty

          public boolean isEmpty()

      Returns `true` if this queue contains no elements.
      Specified by:
        `isEmpty` in interface `Collection<E>`
      Overrides:
        `isEmpty` in class `AbstractCollection<E>`
      Returns:
        `true` if this queue contains no elements

    - #### size

          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.

      Specified by:
        `size` in interface `Collection<E>`
      Specified by:
        `size` in class `AbstractCollection<E>`
      Returns:
        the number of elements in this queue

    - #### contains

          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)`.
      Specified by:
        `contains` in interface `Collection<E>`
      Overrides:
        `contains` in class `AbstractCollection<E>`
      Parameters:
        `o` - object to be checked for containment in this queue
      Returns:
        `true` if this queue contains the specified element

    - #### remove

          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).
      Specified by:
        `remove` in interface `Collection<E>`
      Overrides:
        `remove` in class `AbstractCollection<E>`
      Parameters:
        `o` - element to be removed from this queue, if present
      Returns:
        `true` if this queue changed as a result of the call

    - #### addAll

          public boolean addAll(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. Attempts to `addAll` of a queue to itself result in `IllegalArgumentException`.
      Specified by:
        `addAll` in interface `Collection<E>`
      Overrides:
        `addAll` in class `AbstractQueue<E>`
      Parameters:
        `c` - the elements to be inserted into this queue
      Returns:
        `true` if 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:
        `AbstractQueue.add(Object)`

    - #### toArray

          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.

      Specified by:
        `toArray` in interface `Collection<E>`
      Overrides:
        `toArray` in class `AbstractCollection<E>`
      Returns:
        an array containing all of the elements in this queue

    - #### toArray

          public <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 `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()`.
      Specified by:
        `toArray` in interface `Collection<E>`
      Overrides:
        `toArray` in class `AbstractCollection<E>`
      Type Parameters:
        `T` - the runtime 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

    - #### iterator

          public Iterator<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 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.

      Specified by:
        `iterator` in interface `Iterable<E>`
      Specified by:
        `iterator` in interface `Collection<E>`
      Specified by:
        `iterator` in class `AbstractCollection<E>`
      Returns:
        an iterator over the elements in this queue in proper sequence

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