java.util.concurrent
Class PriorityBlockingQueue<E>

java.lang.Object   
  java.util.AbstractCollection   <E>
      java.util.AbstractQueue   <E>
          java.util.concurrent.PriorityBlockingQueue<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 PriorityBlockingQueue<E>
extends AbstractQueue   <E>
implements BlockingQueue   <E>, Serializable   

An unbounded blocking queue that uses the same ordering rules as class PriorityQueue and supplies blocking retrieval operations. While this queue is logically unbounded, attempted additions may fail due to resource exhaustion (causing OutOfMemoryError). This class does not permit null elements. A priority queue relying on natural ordering also does not permit insertion of non-comparable objects (doing so results in ClassCastException).

This class and its iterator implement all of the optional methods of the Collection and Iterator interfaces. The Iterator provided in method iterator() is not guaranteed to traverse the elements of the PriorityBlockingQueue in any particular order. If you need ordered traversal, consider using Arrays.sort(pq.toArray()).

This class is a member of the Java Collections Framework.

Since:

1.5

See Also:

Serialized Form

Constructor Summary

PriorityBlockingQueue ()
Creates a PriorityBlockingQueue with the default initial capacity (11) that orders its elements according to their natural ordering (using Comparable).

PriorityBlockingQueue (Collection <? extends E> c)
Creates a PriorityBlockingQueue containing the elements in the specified collection.

PriorityBlockingQueue (int initialCapacity)
Creates a PriorityBlockingQueue with the specified initial capacity that orders its elements according to their natural ordering (using Comparable).

PriorityBlockingQueue (int initialCapacity, Comparator <? super E> comparator)
Creates a PriorityBlockingQueue with the specified initial capacity that orders its elements according to the specified comparator.

Method Summary

boolean	add (E o) Adds the specified element to this queue.
void	clear () Atomically removes all of the elements from this queue.
Comparator <? super E>	comparator () Returns the comparator used to order this collection, or null if this collection is sorted according to its elements natural ordering (using Comparable).
boolean	contains (Object o) Returns true if this collection contains the specified element.
int	drainTo (Collection <? super E> c) Removes all available elements from this queue and adds them into 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 into the given collection.
Iterator <E>	iterator () Returns an iterator over the elements in this queue.
boolean	offer (E o) Inserts the specified element into this priority queue.
boolean	offer (E o, long timeout, TimeUnit unit) Inserts the specified element into this priority queue.
E	peek () Retrieves, but does not remove, the head of this queue, returning null if this queue is empty.
E	poll () Retrieves and removes the head of this queue, or null if this queue is empty.
E	poll (long timeout, TimeUnit unit) Retrieves and removes the head of this queue, waiting if necessary up to the specified wait time if no elements are present on this queue.
void	put (E o) Adds the specified element to this priority queue.
int	remainingCapacity () Always returns Integer.MAX_VALUE because a PriorityBlockingQueue is not capacity constrained.
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 collection.
E	take () Retrieves and removes the head of this queue, waiting if no elements are present on this queue.
Object []	toArray () Returns an array containing all of the elements in this collection.
<T> T[]	toArray (T[] a) Returns an array containing all of the elements in this collection; 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

PriorityBlockingQueue

public PriorityBlockingQueue()

Creates a PriorityBlockingQueue with the default initial capacity (11) that orders its elements according to their natural ordering (using Comparable).

PriorityBlockingQueue

public PriorityBlockingQueue(int initialCapacity)

Creates a PriorityBlockingQueue with the specified initial capacity that orders its elements according to their natural ordering (using Comparable).

Parameters:

initialCapacity - the initial capacity for this priority queue.

Throws:

IllegalArgumentException - if initialCapacity is less than 1

PriorityBlockingQueue

public PriorityBlockingQueue(int initialCapacity,
                             Comparator   <? super E> comparator)

Creates a PriorityBlockingQueue with the specified initial capacity that orders its elements according to the specified comparator.

Parameters:

initialCapacity - the initial capacity for this priority queue.

comparator - the comparator used to order this priority queue. If null then the order depends on the elements' natural ordering.

Throws:

IllegalArgumentException - if initialCapacity is less than 1

PriorityBlockingQueue

public PriorityBlockingQueue(Collection   <? extends E> c)

Creates a PriorityBlockingQueue containing the elements in the specified collection. The priority queue has an initial capacity of 110% of the size of the specified collection. If the specified collection is a SortedSet or a PriorityQueue , this priority queue will be sorted according to the same comparator, or according to its elements' natural order if the collection is sorted according to its elements' natural order. Otherwise, this priority queue is ordered according to its elements' natural order.

Parameters:

c - the collection whose elements are to be placed into this priority queue.

Throws:

ClassCastException - if elements of the specified collection cannot be compared to one another according to the priority queue's ordering.

NullPointerException - if c or any element within it is null

Method Detail

add

public boolean add(E o)

Adds the specified element to this queue.

Specified by:

add in interface Collection <E>

Specified by:

add in interface BlockingQueue <E>

Overrides:

add in class AbstractQueue <E>

Parameters:

o - the element to add

Returns:

true (as per the general contract of Collection.add).

Throws:

NullPointerException - if the specified element is null.

ClassCastException - if the specified element cannot be compared with elements currently in the priority queue according to the priority queue's ordering.

comparator

public Comparator   <? super E> comparator()

Returns the comparator used to order this collection, or null if this collection is sorted according to its elements natural ordering (using Comparable).

Returns:

the comparator used to order this collection, or null if this collection is sorted according to its elements natural ordering.

offer

public boolean offer(E o)

Inserts the specified element into this priority queue.

Specified by:

offer in interface BlockingQueue <E>

Specified by:

offer in interface Queue <E>

Parameters:

o - the element to add

Returns:

true

Throws:

ClassCastException - if the specified element cannot be compared with elements currently in the priority queue according to the priority queue's ordering.

NullPointerException - if the specified element is null.

put

public void put(E o)

Adds the specified element to this priority queue. As the queue is unbounded this method will never block.

Specified by:

put in interface BlockingQueue <E>

Parameters:

o - the element to add

Throws:

ClassCastException - if the element cannot be compared with elements currently in the priority queue according to the priority queue's ordering.

NullPointerException - if the specified element is null.

offer

public boolean offer(E o,
                     long timeout,
                     TimeUnit    unit)

Inserts the specified element into this priority queue. As the queue is unbounded this method will never block.

Specified by:

offer in interface BlockingQueue <E>

Parameters:

o - the element to add

timeout - This parameter is ignored as the method never blocks

unit - This parameter is ignored as the method never blocks

Returns:

true

Throws:

ClassCastException - if the element cannot be compared with elements currently in the priority queue according to the priority queue's ordering.

NullPointerException - if the specified element is null.

take

public E take()
       throws InterruptedException   

Description copied from interface: BlockingQueue

Retrieves and removes the head of this queue, waiting if no elements are present on this queue.

Specified by:

take in interface BlockingQueue <E>

Returns:

the head of this queue

Throws:

InterruptedException - if interrupted while waiting.

poll

public E poll()

Description copied from interface: Queue

Retrieves and removes the head of this queue, or 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   

Description copied from interface: BlockingQueue

Retrieves and removes the head of this queue, waiting if necessary up to the specified wait time if no elements are present on this queue.

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 present.

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, returning 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()

Description copied from class: AbstractCollection

Returns the number of elements in this collection. If the collection contains more than Integer.MAX_VALUE elements, returns Integer.MAX_VALUE.

Specified by:

size in interface Collection <E>

Specified by:

size in class AbstractCollection <E>

Returns:

the number of elements in this collection.

remainingCapacity

public int remainingCapacity()

Always returns Integer.MAX_VALUE because a PriorityBlockingQueue is not capacity constrained.

Specified by:

remainingCapacity in interface BlockingQueue <E>

Returns:

Integer.MAX_VALUE

remove

public boolean remove(Object    o)

Removes a single instance of the specified element from this queue, if it is present.

Specified by:

remove in interface Collection <E>

Overrides:

remove in class AbstractCollection <E>

Parameters:

o - element to be removed from this collection, if present.

Returns:

true if the collection contained the specified element.

contains

public boolean contains(Object    o)

Description copied from class: AbstractCollection

Returns true if this collection contains the specified element. More formally, returns true if and only if this collection contains at least one element e such that (o==null ? e==null : o.equals(e)).

This implementation iterates over the elements in the collection, checking each element in turn for equality with the specified element.

Specified by:

contains in interface Collection <E>

Overrides:

contains in class AbstractCollection <E>

Parameters:

o - object to be checked for containment in this collection.

Returns:

true if this collection contains the specified element.

toArray

public Object   [] toArray()

Description copied from class: AbstractCollection

Returns an array containing all of the elements in this collection. If the collection makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order. The returned array will be "safe" in that no references to it are maintained by the collection. (In other words, this method must allocate a new array even if the collection is backed by an Array). The caller is thus free to modify the returned array.

This implementation allocates the array to be returned, and iterates over the elements in the collection, storing each object reference in the next consecutive element of the array, starting with element 0.

Specified by:

toArray in interface Collection <E>

Overrides:

toArray in class AbstractCollection <E>

Returns:

an array containing all of the elements in this collection.

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).

This implementation creates an empty string buffer, appends a left square bracket, and iterates over the collection appending the string representation of each element in turn. After appending each element except the last, the string ", " is appended. Finally a right bracket is appended. A string is obtained from the string buffer, and returned.

Overrides:

toString in class AbstractCollection <E>

Returns:

a string representation of this collection.

drainTo

public int drainTo(Collection   <? super E> c)

Description copied from interface: BlockingQueue

Removes all available elements from this queue and adds them into 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.

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 into 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.

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>

toArray

public <T> T[] toArray(T[] a)

Description copied from class: AbstractCollection

Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array. If the collection 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 collection.

If the collection fits in the specified array with room to spare (i.e., the array has more elements than the collection), the element in the array immediately following the end of the collection is set to null. This is useful in determining the length of the collection only if the caller knows that the collection does not contain any null elements.)

If this collection makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order.

This implementation checks if the array is large enough to contain the collection; if not, it allocates a new array of the correct size and type (using reflection). Then, it iterates over the collection, storing each object reference in the next consecutive element of the array, starting with element 0. If the array is larger than the collection, a null is stored in the first location after the end of the collection.

Specified by:

toArray in interface Collection <E>

Overrides:

toArray in class AbstractCollection <E>

Parameters:

a - the array into which the elements of the collection 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 the elements of the collection.

iterator

public Iterator   <E> iterator()

Returns an iterator over the elements in this queue. The iterator does not return the elements in any particular order. The returned iterator is a thread-safe "fast-fail" iterator that will throw ConcurrentModificationException upon detected interference.

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.