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 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()) . Also, method drainTo can be used to remove some or all elements in priority order and place them in another collection.

Operations on this class make no guarantees about the ordering of elements with equal priority. If you need to enforce an ordering, you can define custom classes or comparators that use a secondary key to break ties in primary priority values. For example, here is a class that applies first-in-first-out tie-breaking to comparable elements. To use it, you would insert a new FIFOEntry(anEntry) instead of a plain entry object.

 
 class FIFOEntry<E extends Comparable<? super E>>
 implements Comparable<FIFOEntry<E>> {
 final static AtomicLong seq = new AtomicLong();
 final long seqNum;
 final E entry;
 public FIFOEntry(E entry) {
 seqNum = seq.getAndIncrement();
 this.entry = entry;
 }
 public E getEntry() { return entry; }
 public int compareTo(FIFOEntry<E> other) {
 int res = entry.compareTo(other.entry);
 if (res == 0 && other.entry != this.entry)
 res = (seqNum < other.seqNum ? -1 : 1);
 return res;
 }
 } 

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

Parameters:

initialCapacity - the initial capacity for this priority queue 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 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 the its elements' natural ordering of its elements order if the collection is sorted according to the its elements' natural ordering of its elements. order. Otherwise, this priority queue is ordered according to the its elements' natural ordering of its elements. 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 the specified collection or any of its elements are null - if c or any element within it is null

add

public boolean add(E e)  o) 

Inserts Adds the specified element into to this priority queue.

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 o - the element to add

Returns:

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

Throws:

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

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

offer comparator

public boolean 
public Comparator<? super E> offer comparator ( E e) () 

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

Specified by:

offer in interface BlockingQueue < E >

Specified by:

offer in interface Queue < E >

Parameters:

e - the element to add

Returns:

the comparator used to order this collection, or true null (as specified by Queue.offer(E) )

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 if this collection is sorted according to its elements natural ordering.

put offer

public void boolean put offer (E e)  o) 

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

Specified by:

put offer in interface BlockingQueue < E >

Specified by:

offer in interface Queue < E >

Parameters:

e 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 ordering.

NullPointerException - if the specified element is null .

offer put

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

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

Specified by:

offer put in interface BlockingQueue < E >

Parameters:

e - the element to add

timeout - This parameter is ignored as the method never blocks

unit - This parameter is ignored as the method never blocks 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 ordering.

NullPointerException - if the specified element is null .

poll offer

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

Description copied from interface: Queue

Retrieves and removes the head of this queue, or returns null if this queue is empty. Inserts the specified element into this priority queue. As the queue is unbounded this method will never block.

Specified by:

poll offer in interface Queue 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:

the head of this queue, or 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 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. 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 waiting.

poll

public Epoll(long timeout,
 TimeUnit unit)
 throws InterruptedException() 

Description copied from interface: BlockingQueue Queue

Retrieves and removes the head of this queue, waiting up to the specified wait time if necessary for an element to become available. Retrieves and removes the head of this queue, or null if this queue is empty.

Specified by:

poll poll in interface BlockingQueue Queue < 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 if this queue is empty.

peek poll

public Epeek poll () (long timeout,
 TimeUnit unit)
 throws InterruptedException

Description copied from interface: Queue BlockingQueue

Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty. 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:

peek poll in interface Queue 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 this queue is empty if the specified waiting time elapses before an element is present.

Throws:

InterruptedException - if interrupted while waiting.

comparator peek

public ComparatorE<? super E> comparator peek ()

Returns the comparator used to order the elements in this queue, or Description copied from interface: Queue

Retrieves, but does not remove, the head of this queue, returning null if this queue uses the natural ordering of its elements. if this queue is empty.

Specified by:

peek in interface Queue < E >

Returns:

the comparator used to order the elements in this queue, or the head of this queue, or null if this queue uses the natural ordering of its elements. if this queue is empty.

size

public int size()

Description copied from interface: Collection class: AbstractCollection

Returns the number of elements in this collection. If this 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 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. 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, collection, if present present.

Returns:

true if this queue changed as a result of the call if the collection contained the specified element.

contains

public boolean contains(Object o)

Description copied from class: AbstractCollection

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

Specified by:

contains in interface BlockingQueue < E >

Overrides:

contains in class AbstractCollection < E >

Parameters:

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

Returns:

true if this queue collection contains the specified element element.

toArray

public Object[] toArray()

Description copied from class: AbstractCollection

Returns an array containing all of the elements in this queue. The collection. If the collection makes any guarantees as to what order its elements are returned array by its iterator, this method must return the elements are in no particular the same order.

The returned array will be "safe" in that no references to it are maintained by this queue. the collection. (In other words, this method must allocate a new array). array even if the collection is backed by an Array). The caller is thus free to modify the returned array.

This method acts as bridge between array-based and collection-based APIs. 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 queue 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) 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 collection.

drainTo

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

Description copied from interface: BlockingQueue

Removes all available elements from this queue and adds them to 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 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 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 to into the given collection. A failure encountered while attempting to add elements to collection 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 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 queue; collection; the runtime type of the returned array is that of the specified array. The returned array elements are in no particular order. If the queue 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 queue. collection.

If this queue the collection fits in the specified array with room to spare (i.e., the array has more elements than this queue), the collection), the element in the array immediately following the end of the queue 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.)

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. 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.

Suppose 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 x null 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() . 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 queue collection are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purpose 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 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 queue.