Java™ Platform
Standard Ed. 6

java.util.concurrent
Class ArrayBlockingQueue<E>

java.lang.Object
  extended by java.util.AbstractCollection<E>
      extended by java.util.AbstractQueue<E>
          extended by java.util.concurrent.ArrayBlockingQueue<E>
Type Parameters:
E - the type of elements held in this collection
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 increased. 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
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
 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.
 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.
 int size()
          Returns the number of 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.
 String toString()
          Returns a string representation of this collection.
 
Methods inherited from class java.util.AbstractQueue
addAll, element, remove
 
Methods inherited from class java.util.AbstractCollection
containsAll, isEmpty, removeAll, retainAll
 
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
addAll, containsAll, equals, hashCode, isEmpty, removeAll, retainAll
 

Constructor Detail

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 is less than 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 is less than 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 Detail

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 Collection<E>
Specified by:
add in interface BlockingQueue<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>
Specified by:
size in class AbstractCollection<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).

Specified by:
remove in interface Collection<E>
Specified by:
remove in interface BlockingQueue<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 Collection<E>
Specified by:
contains in interface BlockingQueue<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>
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

toString

public String toString()
Description copied from class: AbstractCollection
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 String.valueOf(Object).

Overrides:
toString in class AbstractCollection<E>
Returns:
a string representation of this collection

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 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

Java™ Platform
Standard Ed. 6

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