public class

PriorityBlockingQueue

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

Class Overview

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()). 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.

Summary

Public Constructors
PriorityBlockingQueue()
Creates a PriorityBlockingQueue with the default initial capacity (11) that orders its elements according to their natural ordering.
PriorityBlockingQueue(int initialCapacity)
Creates a PriorityBlockingQueue with the specified initial capacity that orders its elements according to their natural ordering.
PriorityBlockingQueue(int initialCapacity, Comparator<? super E> comparator)
Creates a PriorityBlockingQueue with the specified initial capacity that orders its elements according to the specified comparator.
PriorityBlockingQueue(Collection<? extends E> c)
Creates a PriorityBlockingQueue containing the elements in the specified collection.
Public Methods
boolean add(E e)
Inserts the specified element into 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, or null if this queue uses the natural ordering of its elements.
boolean contains(Object o)
Returns true if this queue contains the specified element.
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.
int drainTo(Collection<? super E> c)
Removes all available elements from this queue and adds them to the given collection.
Iterator<E> iterator()
Returns an iterator over the elements in this queue.
boolean offer(E e)
Inserts the specified element into this priority queue.
boolean offer(E e, long timeout, TimeUnit unit)
Inserts the specified element into this priority queue.
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 into 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.
<T> T[] toArray(T[] a)
Returns an array containing all of the elements in this queue; the runtime type of the returned array is that of the specified array.
Object[] toArray()
Returns an array containing all of the elements in this queue.
String toString()
Returns a string representation of this collection.
[Expand]
Inherited Methods
From class java.util.AbstractQueue
From class java.util.AbstractCollection
From class java.lang.Object
From interface java.lang.Iterable
From interface java.util.Collection
From interface java.util.Queue
From interface java.util.concurrent.BlockingQueue

Public Constructors

public PriorityBlockingQueue ()

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

public PriorityBlockingQueue (int initialCapacity)

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

Parameters
initialCapacity the initial capacity for this priority queue
Throws
IllegalArgumentException if initialCapacity is less than 1

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 that will be used to order this priority queue. If null, the natural ordering of the elements will be used.
Throws
IllegalArgumentException if initialCapacity is less than 1

public PriorityBlockingQueue (Collection<? extends E> c)

Creates a PriorityBlockingQueue containing the elements in the specified collection. If the specified collection is a SortedSet or a PriorityQueue, this priority queue will be ordered according to the same ordering. Otherwise, this priority queue will be ordered according to the natural ordering of its elements.

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

Public Methods

public boolean add (E e)

Inserts the specified element into this priority queue.

Parameters
e the element to add
Returns
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

public void clear ()

Atomically removes all of the elements from this queue. The queue will be empty after this call returns.

public Comparator<? super E> comparator ()

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

Returns
  • the comparator used to order the elements in this queue, or null if this queue uses the natural ordering of its elements

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).

Parameters
o object to be checked for containment in this queue
Returns
  • true if this queue contains the specified element

public 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. 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.

Parameters
c the collection to transfer elements into
maxElements the maximum number of elements to transfer
Returns
  • the number of elements transferred
Throws
UnsupportedOperationException
ClassCastException
NullPointerException
IllegalArgumentException

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

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.

Parameters
c the collection to transfer elements into
Returns
  • the number of elements transferred
Throws
UnsupportedOperationException
ClassCastException
NullPointerException
IllegalArgumentException

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 "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.

Returns
  • an iterator over the elements in this queue

public boolean offer (E e)

Inserts the specified element into this priority queue.

Parameters
e the element to add
Returns
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

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

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

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
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

public E peek ()

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

Returns
  • the head of this queue, or null if this queue is empty

public E poll ()

Retrieves and removes the head of this queue, or returns null if this queue is empty.

Returns
  • the head of this queue, or null if this queue is empty

public 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.

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

public void put (E e)

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

Parameters
e the element to add
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

public int remainingCapacity ()

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

Returns
  • Integer.MAX_VALUE

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 and only if this queue contained the specified element (or equivalently, if this queue changed as a result of the call).

Parameters
o element to be removed from this queue, if present
Returns
  • true if this queue changed as a result of the call

public int size ()

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

Returns
  • the number of elements in this collection

public E take ()

Retrieves and removes the head of this queue, waiting if necessary until an element becomes available.

Returns
  • the head of this queue
Throws
InterruptedException

public T[] toArray (T[] a)

Returns an array containing all of the elements in this queue; 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 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().

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

public Object[] toArray ()

Returns an array containing all of the elements in this queue. The returned array elements are in no particular order.

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.

Returns
  • an array containing all of the elements in this queue

public String toString ()

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 valueOf(Object).

Returns
  • a string representation of this collection