# Class ArrayBlockingQueue<E>

java.lang.Object
java.util.AbstractCollection<E>
java.util.AbstractQueue<E>
java.util.concurrent.ArrayBlockingQueue<E>

Type Parameters:
  `E` - the type of elements held in this queue

All Implemented Interfaces:
  `Serializable, Iterable<E>, Collection<E>, BlockingQueue<E>, Queue<E>`

public class ArrayBlockingQueue<E> extends AbstractQueue<E> implements BlockingQueue<E>, Serializable
A bounded blocking queue backed by an array. 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.

This is a classic "bounded buffer", in which a fixed-sized array holds elements inserted by producers and extracted by consumers. Once created, the capacity cannot be changed. Attempts to `put` an element into a full queue will result in the operation blocking; attempts to `take` an element from an empty queue will similarly block.

This class supports an optional fairness policy for ordering waiting producer and consumer threads. By default, this ordering is not guaranteed. However, a queue constructed with fairness set to `true` grants threads access in FIFO order. Fairness generally decreases throughput but reduces variability and avoids starvation.

This class and its iterator implement all of the optional methods of the `Collection` and `Iterator` interfaces.

This class is a member of the Java Collections Framework.

Since:
  1.5
See Also:
  - Serialized Form

## Constructor Summary

Constructors
Constructor
Description
`ArrayBlockingQueue(int capacity)`
Creates an `ArrayBlockingQueue` with the given (fixed) capacity and default access policy.
`ArrayBlockingQueue(int capacity, boolean fair)`
Creates an `ArrayBlockingQueue` with the given (fixed) capacity and the specified access policy.
`ArrayBlockingQueue(int capacity, boolean fair, Collection<? extends E> c)`
Creates an `ArrayBlockingQueue` with the given (fixed) capacity, the specified access policy and 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 if it is possible to do so immediately without exceeding the queue's capacity, returning `true` upon success and throwing an `IllegalStateException` if this queue is full.
`void`
`clear()`
Atomically removes all of the elements from this queue.
`boolean`
`contains(Object o)`
Returns `true` if this queue contains the specified element.
`int`
`drainTo(Collection<? super E> c)`
Removes all available elements from this queue and adds them to the given collection.
`int`
`drainTo(Collection<? super E> c, int maxElements)`
Removes at most the given number of available elements from this queue and adds them to the given collection.
`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.
`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 if it is possible to do so immediately without exceeding the queue's capacity, returning `true` upon success and `false` if this queue is full.
`boolean`
`offer(E e, long timeout, TimeUnit unit)`
Inserts the specified element at the tail of this queue, waiting up to the specified wait time for space to become available if the queue is full.
`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.
`E`
`poll(long timeout, TimeUnit unit)`
Retrieves and removes the head of this queue, waiting up to the specified wait time if necessary for an element to become available.
`void`
`put(E e)`
Inserts the specified element at the tail of this queue, waiting for space to become available if the queue is full.
`int`
`remainingCapacity()`
Returns the number of additional elements that this queue can ideally (in the absence of memory or resource constraints) accept without blocking.
`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 (optional operation).
`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.
`E`
`take()`
Retrieves and removes the head of this queue, waiting if necessary until an element becomes available.
`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 AbstractQueue

`addAll, element, remove`

### Methods declared in class AbstractCollection

`containsAll, isEmpty, toString`

### Methods declared in class Object

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

### Methods declared in interface Collection

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

### Methods declared in interface Queue

`element, remove`

## Constructor Details

### ArrayBlockingQueue

public ArrayBlockingQueue(int capacity)
Creates an `ArrayBlockingQueue` with the given (fixed) capacity and default access policy.

Parameters:
  `capacity` - the capacity of this queue
Throws:
  `IllegalArgumentException` - if `capacity < 1`

### ArrayBlockingQueue

public ArrayBlockingQueue(int capacity, boolean fair)
Creates an `ArrayBlockingQueue` with the given (fixed) capacity and the specified access policy.

Parameters:
  `capacity` - the capacity of this queue
  `fair` - if `true` then queue accesses for threads blocked on insertion or removal, are processed in FIFO order; if `false` the access order is unspecified.
Throws:
  `IllegalArgumentException` - if `capacity < 1`

### ArrayBlockingQueue

public ArrayBlockingQueue(int capacity, boolean fair, Collection<? extends E> c)
Creates an `ArrayBlockingQueue` with the given (fixed) capacity, the specified access policy and initially containing the elements of the given collection, added in traversal order of the collection's iterator.

Parameters:
  `capacity` - the capacity of this queue
  `fair` - if `true` then queue accesses for threads blocked on insertion or removal, are processed in FIFO order; if `false` the access order is unspecified.
  `c` - the collection of elements to initially contain
Throws:
  `IllegalArgumentException` - if `capacity` is less than `c.size()`, or less than 1.
  `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 if it is possible to do so immediately without exceeding the queue's capacity, returning `true` upon success and throwing an `IllegalStateException` if this queue is full.

Specified by:
  `add` in interface `BlockingQueue<E>`
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:
  `IllegalStateException` - if this queue is full
  `NullPointerException` - if the specified element is null

### offer

public boolean offer(E e)
Inserts the specified element at the tail of this queue if it is possible to do so immediately without exceeding the queue's capacity, returning `true` upon success and `false` if this queue is full. This method is generally preferable to method `add(E)`, which can fail to insert an element only by throwing an exception.

Specified by:
  `offer` in interface `BlockingQueue<E>`
Specified by:
  `offer` in interface `Queue<E>`
Parameters:
  `e` - the element to add
Returns:
  `true` if the element was added to this queue, else `false`
Throws:
  `NullPointerException` - if the specified element is null

### put

public void put(E e) throws InterruptedException
Inserts the specified element at the tail of this queue, waiting for space to become available if the queue is full.

Specified by:
  `put` in interface `BlockingQueue<E>`
Parameters:
  `e` - the element to add
Throws:
  `InterruptedException` - if interrupted while waiting
  `NullPointerException` - if the specified element is null

### offer

public boolean offer(E e, long timeout, TimeUnit unit) throws InterruptedException
Inserts the specified element at the tail of this queue, waiting up to the specified wait time for space to become available if the queue is full.

Specified by:
  `offer` in interface `BlockingQueue<E>`
Parameters:
  `e` - the element to add
  `timeout` - how long to wait before giving up, in units of `unit`
  `unit` - a `TimeUnit` determining how to interpret the `timeout` parameter
Returns:
  `true` if successful, or `false` if the specified waiting time elapses before space is available
Throws:
  `InterruptedException` - if interrupted while waiting
  `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

### take

public E take() throws InterruptedException
Description copied from interface: `BlockingQueue`
Retrieves and removes the head of this queue, waiting if necessary until an element becomes available.

Specified by:
  `take` in interface `BlockingQueue<E>`
Returns:
  the head of this queue
Throws:
  `InterruptedException` - if interrupted while waiting

### poll

public E poll(long timeout, TimeUnit unit) throws InterruptedException
Description copied from interface: `BlockingQueue`
Retrieves and removes the head of this queue, waiting up to the specified wait time if necessary for an element to become available.

Specified by:
  `poll` in interface `BlockingQueue<E>`
Parameters:
  `timeout` - how long to wait before giving up, in units of `unit`
  `unit` - a `TimeUnit` determining how to interpret the `timeout` parameter
Returns:
  the head of this queue, or `null` if the specified waiting time elapses before an element is available
Throws:
  `InterruptedException` - if interrupted while waiting

### 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

### size

public int size()
Returns the number of elements in this queue.

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

### remainingCapacity

public int remainingCapacity()
Returns the number of additional elements that this queue can ideally (in the absence of memory or resource constraints) accept without blocking. This is always equal to the initial capacity of this queue less the current `size` of this queue.

Note that you cannot always tell if an attempt to insert an element will succeed by inspecting `remainingCapacity` because it may be the case that another thread is about to insert or remove an element.

Specified by:
  `remainingCapacity` in interface `BlockingQueue<E>`
Returns:
  the remaining capacity

### 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).

Removal of interior elements in circular array based queues is an intrinsically slow and disruptive operation, so should be undertaken only in exceptional circumstances, ideally only when the queue is known not to be accessible by other threads.

Specified by:
  `remove` in interface `BlockingQueue<E>`
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

### 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 `BlockingQueue<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

### 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

### clear

public void clear()
Atomically removes all of the elements from this queue. The queue will be empty after this call returns.

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

### drainTo

public int drainTo(Collection<? super E> c)
Description copied from interface: `BlockingQueue`
Removes all available elements from this queue and adds them to the given collection. This operation may be more efficient than repeatedly polling this queue. A failure encountered while attempting to add elements to collection `c` may result in elements being in neither, either or both collections when the associated exception is thrown. Attempts to drain a queue to itself result in `IllegalArgumentException`. Further, the behavior of this operation is undefined if the specified collection is modified while the operation is in progress.

Specified by:
  `drainTo` in interface `BlockingQueue<E>`
Parameters:
  `c` - the collection to transfer elements into
Returns:
  the number of elements transferred
Throws:
  `UnsupportedOperationException` - if addition of elements is not supported by the specified collection
  `ClassCastException` - if the class of an element of this queue prevents it from being added to the specified collection
  `NullPointerException` - if the specified collection is null
  `IllegalArgumentException` - if the specified collection is this queue, or some property of an element of this queue prevents it from being added to the specified collection

### drainTo

public int drainTo(Collection<? super E> c, int maxElements)
Description copied from interface: `BlockingQueue`
Removes at most the given number of available elements from this queue and adds them to the given collection. A failure encountered while attempting to add elements to collection `c` may result in elements being in neither, either or both collections when the associated exception is thrown. Attempts to drain a queue to itself result in `IllegalArgumentException`. Further, the behavior of this operation is undefined if the specified collection is modified while the operation is in progress.

Specified by:
  `drainTo` in interface `BlockingQueue<E>`
Parameters:
  `c` - the collection to transfer elements into
  `maxElements` - the maximum number of elements to transfer
Returns:
  the number of elements transferred
Throws:
  `UnsupportedOperationException` - if addition of elements is not supported by the specified collection
  `ClassCastException` - if the class of an element of this queue prevents it from being added to the specified collection
  `NullPointerException` - if the specified collection is null
  `IllegalArgumentException` - if the specified collection is this queue, or some property of an element of this queue prevents it from being added to the specified collection

### 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

### 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

### 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 (optional operation). 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)`