java.util.concurrent

Class ArrayBlockingQueue<E>

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

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 ../../../../technotes/guides/collections/index.html"> 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, 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

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.
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(E> c)
int
drainTo(E> c, int maxElements)
Iterator
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)
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()
Object[]
toArray()
Returns an array containing all of the elements in this queue, in proper sequence.
String
toString()
Convert this Object to a human-readable String.

Methods inherited from class java.util.AbstractQueue<E>

add, addAll, clear, element, remove

Methods inherited from class java.util.AbstractCollection<E>

T[] toArray, add, addAll, clear, contains, containsAll, isEmpty, iterator, remove, removeAll, retainAll, size, toArray, toString

Methods inherited from class java.lang.Object

clone, equals, extends Object> getClass, finalize, hashCode, notify, notifyAll, toString, wait, wait, wait

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 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,
                          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

T[] toArray

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 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:
T[] toArray in interface Collection<E>
Overrides:
T[] toArray in interface 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

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>
add in interface Queue<E>
add in interface Collection<E>
Overrides:
add in interface 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

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 interface AbstractQueue<E>

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

drainTo

public int drainTo(E> c)
Specified by:
drainTo in interface BlockingQueue<E>

drainTo

public int drainTo(E> c,
                   int maxElements)
Specified by:
drainTo in interface BlockingQueue<E>

iterator

public Iterator 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 Collection<E>
iterator in interface Iterable<E>
Overrides:
iterator in interface AbstractCollection<E>
Returns:
an iterator over the elements in this queue in proper sequence

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>
offer in interface Queue<E>
Throws:
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>

peek

public E peek()
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

poll

public E poll()
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

poll

public E poll(long timeout,
              TimeUnit unit)
            throws InterruptedException
Specified by:
poll in interface BlockingQueue<E>

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>

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>

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 BlockingQueue<E>
remove in interface Collection<E>
Overrides:
remove in interface 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

size

public int size()
Returns the number of elements in this queue.
Specified by:
size in interface Collection<E>
Overrides:
size in interface AbstractCollection<E>
Returns:
the number of elements in this queue

take

public E take()
            throws InterruptedException
Specified by:
take in interface BlockingQueue<E>

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 interface AbstractCollection<E>
Returns:
an array containing all of the elements in this queue

toString

public String toString()
Convert this Object to a human-readable String. There are no limits placed on how long this String should be or what it should contain. We suggest you make it as intuitive as possible to be able to place it into System.out.println() and such.

It is typical, but not required, to ensure that this method never completes abruptly with a RuntimeException.

This method will be called when performing string concatenation with this object. If the result is null, string concatenation will instead use "null".

The default implementation returns getClass().getName() + "@" + Integer.toHexString(hashCode()).

Overrides:
toString in interface AbstractCollection<E>
Returns:
the String representing this Object, which may be null