ConcurrentLinkedQueue (Java SE 21 & JDK 21)
Description: declaration: module: java.base, package: java.util.concurrent, class: ConcurrentLinkedQueue
Keywords: java.util.concurrent.ConcurrentLinkedQueue class

Module java.base
Package 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 queue

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 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:
  - Serialized Form

- ## Constructor Summary

  Constructors
  Constructor
  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

  Modifier and Type
  Method
  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.
  `void`
  `clear()`
  Removes all of the elements from this queue.
  `boolean`
  `contains(Object o)`
  Returns `true` if this queue contains the specified element.
  `void`
  `forEach(Consumer<? super E> action)`
  Performs the given action for each element of the `Iterable` until all elements have been processed or the action throws an exception.
  `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.
  `boolean`
  `removeAll(Collection<?> c)`
  Removes all of this collection's elements that are also contained in the specified collection (optional operation).
  `boolean`
  `removeIf(Predicate<? super E> filter)`
  Removes all of the elements of this collection that satisfy the given predicate.
  `boolean`
  `retainAll(Collection<?> c)`
  Retains only the elements in this collection that are contained in the specified collection (optional operation).
  `int`
  `size()`
  Returns the number of elements in this queue.
  `Spliterator<E>`
  `spliterator()`
  Returns a `Spliterator` over 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.AbstractQueue

  `clear, element, remove`

  ### Methods declared in class java.util.AbstractCollection

  `containsAll, toString`

  ### Methods declared in class java.lang.Object

  `clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait`

  ### Methods declared in interface java.util.Collection

  `containsAll, equals, hashCode, parallelStream, stream, toArray`

  ### Methods declared in interface java.util.Queue

  `element, remove`

- ## Constructor Details

  - ### 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 Details

  - ### 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>`
    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 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

  - ### 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 weakly consistent.

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

  - ### spliterator

    public Spliterator<E> spliterator()
    Returns a `Spliterator` over the elements in this queue.

    The returned spliterator is weakly consistent.

    The `Spliterator` reports `Spliterator.CONCURRENT`, `Spliterator.ORDERED`, and `Spliterator.NONNULL`.

    Specified by:
      `spliterator` in interface `Collection<E>`
    Specified by:
      `spliterator` in interface `Iterable<E>`
    Implementation Note:
      The `Spliterator` implements `trySplit` to permit limited parallelism.
    Returns:
      a `Spliterator` over the elements in this queue
    Since:
      1.8

  - ### removeIf

    public boolean removeIf(Predicate<? super E> filter)
    Description copied from interface: `Collection`
    Removes 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:
      `removeIf` in interface `Collection<E>`
    Parameters:
      `filter` - a predicate which returns `true` for elements to be removed
    Returns:
      `true` if any elements were removed
    Throws:
      `NullPointerException` - if the specified filter is null

  - ### removeAll

    public boolean removeAll(Collection<?> c)
    Description copied from class: `AbstractCollection`
    Removes 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:
      `removeAll` in interface `Collection<E>`
    Overrides:
      `removeAll` in class `AbstractCollection<E>`
    Parameters:
      `c` - collection containing elements to be removed from this collection
    Returns:
      `true` if 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:
      - `AbstractCollection.remove(Object)`
      - `AbstractCollection.contains(Object)`

  - ### retainAll

    public boolean retainAll(Collection<?> c)
    Description copied from class: `AbstractCollection`
    Retains 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:
      `retainAll` in interface `Collection<E>`
    Overrides:
      `retainAll` in class `AbstractCollection<E>`
    Parameters:
      `c` - collection containing elements to be retained in this collection
    Returns:
      `true` if 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:
      - `AbstractCollection.remove(Object)`
      - `AbstractCollection.contains(Object)`

  - ### clear

    public void clear()
    Description copied from class: `AbstractQueue`
    Removes all of the elements from this queue. The queue will be empty after this call returns.

    This implementation repeatedly invokes `poll` until it returns `null`.

    Specified by:
      `clear` in interface `Collection<E>`
    Overrides:
      `clear` in class `AbstractQueue<E>`

  - ### forEach

    public void forEach(Consumer<? super E> action)
    Description copied from interface: `Iterable`
    Performs the given action for each element of the `Iterable` until 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:
      `forEach` in interface `Iterable<E>`
    Parameters:
      `action` - The action to be performed for each element
    Throws:
      `NullPointerException` - if the specified action is null