PriorityBlockingQueue

public class PriorityBlockingQueue extends AbstractQueue<E>
implements BlockingQueue<E> Serializable

An unbounded {@linkplain BlockingQueue 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 {@linkplain Comparable 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>> {
   static final AtomicLong seq = new AtomicLong(0);
   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;
   }
 }

Public Constructor Summary

	PriorityBlockingQueue() Creates a `PriorityBlockingQueue` with the default initial capacity (11) that orders its elements according to their {@linkplain Comparable natural ordering}.
	PriorityBlockingQueue(int initialCapacity) Creates a `PriorityBlockingQueue` with the specified initial capacity that orders its elements according to their {@linkplain Comparable 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 Method Summary

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 {@linkplain Comparable 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(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.
E	poll() Retrieves and removes the head of this queue, or returns `null` if this queue is empty.
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.
Spliterator<E>	spliterator() Returns a `Spliterator` over the elements in this queue.
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.

Inherited Method Summary

From class java.util.AbstractQueue

boolean	add(E e) Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning `true` upon success and throwing an `IllegalStateException` if no space is currently available.
boolean	addAll(Collection<? extends E> c) Adds all of the elements in the specified collection to this queue.
void	clear() Removes all of the elements from this queue.
E	element() Retrieves, but does not remove, the head of this queue.
E	remove() Retrieves and removes the head of this queue.

From class java.util.AbstractCollection

boolean	add(E e) Ensures that this collection contains the specified element (optional operation). This implementation always throws an `UnsupportedOperationException`.
boolean	addAll(Collection<? extends E> c) Adds all of the elements in the specified collection to this collection (optional operation). This implementation iterates over the specified collection, and adds each object returned by the iterator to this collection, in turn.
void	clear() Removes all of the elements from this collection (optional operation). This implementation iterates over this collection, removing each element using the `Iterator.remove` operation.
boolean	contains(Object o) Returns `true` if this collection contains the specified element. This implementation iterates over the elements in the collection, checking each element in turn for equality with the specified element.
boolean	containsAll(Collection<?> c) Returns `true` if this collection contains all of the elements in the specified collection. This implementation iterates over the specified collection, checking each element returned by the iterator in turn to see if it's contained in this collection.
boolean	isEmpty() Returns `true` if this collection contains no elements. This implementation returns `size() == 0`.
abstract Iterator<E>	iterator() Returns an iterator over the elements contained in this collection.
boolean	remove(Object o) Removes a single instance of the specified element from this collection, if it is present (optional operation). This implementation iterates over the collection looking for the specified element.
boolean	removeAll(Collection<?> c) Removes all of this collection's elements that are also contained in the specified collection (optional operation). This implementation iterates over this collection, checking each element returned by the iterator in turn to see if it's contained in the specified collection.
boolean	retainAll(Collection<?> c) Retains only the elements in this collection that are contained in the specified collection (optional operation). This implementation iterates over this collection, checking each element returned by the iterator in turn to see if it's contained in the specified collection.
abstract int	size() Returns the number of elements in this collection.
<T> T[]	toArray(T[] contents) Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array. This implementation returns an array containing all the elements returned by this collection's iterator in the same order, stored in consecutive elements of the array, starting with index `0`.
Object[]	toArray() Returns an array containing all of the elements in this collection. This implementation returns an array containing all the elements returned by this collection's iterator, in the same order, stored in consecutive elements of the array, starting with index `0`.
String	toString() Returns a string representation of this collection.

From class java.lang.Object

Object	clone() Creates and returns a copy of this `Object`.
boolean	equals(Object obj) Compares this instance with the specified object and indicates if they are equal.
void	finalize() Invoked when the garbage collector has detected that this instance is no longer reachable.
final Class<?>	getClass() Returns the unique instance of `Class` that represents this object's class.
int	hashCode() Returns an integer hash code for this object.
final void	notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the `wait()` methods) to be woken up.
final void	notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the `wait()` methods) to be woken up.
String	toString() Returns a string containing a concise, human-readable description of this object.
final void	wait(long timeout, int nanos) Causes the calling thread to wait until another thread calls the `notify()` or `notifyAll()` method of this object or until the specified timeout expires.
final void	wait(long timeout) Causes the calling thread to wait until another thread calls the `notify()` or `notifyAll()` method of this object or until the specified timeout expires.
final void	wait() Causes the calling thread to wait until another thread calls the `notify()` or `notifyAll()` method of this object.

From interface java.util.Queue

abstract boolean	add(E e) Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning `true` upon success and throwing an `IllegalStateException` if no space is currently available.
abstract E	element() Retrieves, but does not remove, the head of this queue.
abstract boolean	offer(E e) Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions.
abstract E	peek() Retrieves, but does not remove, the head of this queue, or returns `null` if this queue is empty.
abstract E	poll() Retrieves and removes the head of this queue, or returns `null` if this queue is empty.
abstract E	remove() Retrieves and removes the head of this queue.

From interface java.util.Collection

abstract boolean	add(E e) Ensures that this collection contains the specified element (optional operation).
abstract boolean	addAll(Collection<? extends E> c) Adds all of the elements in the specified collection to this collection (optional operation).
abstract void	clear() Removes all of the elements from this collection (optional operation).
abstract boolean	contains(Object o) Returns `true` if this collection contains the specified element.
abstract boolean	containsAll(Collection<?> c) Returns `true` if this collection contains all of the elements in the specified collection.
abstract boolean	equals(Object o) Compares the specified object with this collection for equality.
abstract int	hashCode() Returns the hash code value for this collection.
abstract boolean	isEmpty() Returns `true` if this collection contains no elements.
abstract Iterator<E>	iterator() Returns an iterator over the elements in this collection.
Stream<E>	parallelStream() Returns a possibly parallel `Stream` with this collection as its source.
abstract boolean	remove(Object o) Removes a single instance of the specified element from this collection, if it is present (optional operation).
abstract boolean	removeAll(Collection<?> c) Removes all of this collection's elements that are also contained in the specified collection (optional operation).
boolean	removeIf(Predicate<? super E> filter) Removes all of the elements of this collection that satisfy the given predicate.
abstract boolean	retainAll(Collection<?> c) Retains only the elements in this collection that are contained in the specified collection (optional operation).
abstract int	size() Returns the number of elements in this collection.
Spliterator<E>	spliterator() Creates a `Spliterator` over the elements in this collection.
Stream<E>	stream() Returns a sequential `Stream` with this collection as its source.
abstract <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.
abstract Object[]	toArray() Returns an array containing all of the elements in this collection.

From interface java.util.concurrent.BlockingQueue

abstract boolean	add(E e) Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning `true` upon success and throwing an `IllegalStateException` if no space is currently available.
abstract boolean	contains(Object o) Returns `true` if this queue contains the specified element.
abstract int	drainTo(Collection<? super E> c) Removes all available elements from this queue and adds them to the given collection.
abstract 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.
abstract boolean	offer(E e) Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning `true` upon success and `false` if no space is currently available.
abstract boolean	offer(E e, long timeout, TimeUnit unit) Inserts the specified element into this queue, waiting up to the specified wait time if necessary for space to become available.
abstract 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.
abstract void	put(E e) Inserts the specified element into this queue, waiting if necessary for space to become available.
abstract int	remainingCapacity() Returns the number of additional elements that this queue can ideally (in the absence of memory or resource constraints) accept without blocking, or `Integer.MAX_VALUE` if there is no intrinsic limit.
abstract boolean	remove(Object o) Removes a single instance of the specified element from this queue, if it is present.
abstract E	take() Retrieves and removes the head of this queue, waiting if necessary until an element becomes available.

From interface java.lang.Iterable

void	forEach(Consumer<? super T> action) Performs the given action for each element of the `Iterable` until all elements have been processed or the action throws an exception.
abstract Iterator<E>	iterator() Returns an iterator over elements of type `T`.
Spliterator<E>	spliterator() Creates a `Spliterator` over the elements described by this `Iterable`.

Public Constructors

public PriorityBlockingQueue ()

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

public PriorityBlockingQueue (int initialCapacity)

Creates a PriorityBlockingQueue with the specified initial capacity that orders its elements according to their {@linkplain Comparable 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 {@linkplain Comparable 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 {@linkplain Comparable 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

true (as specified by Collection.add(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

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 {@linkplain Comparable 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 weakly consistent.

Returns

an iterator over the elements in this queue

public boolean offer (E e)

Inserts the specified element into this priority queue. As the queue is unbounded, this method will never return false.

Parameters

e	the element to add

Returns

true (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

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 or return false.

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 (as specified by BlockingQueue.offer)

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 (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 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 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 always

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 Spliterator<E> spliterator ()

Returns a Spliterator over the elements in this queue.

The returned spliterator is weakly consistent.

The Spliterator reports Spliterator.SIZED and Spliterator.NONNULL.

Returns

a Spliterator over the elements in this queue

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 String.valueOf(Object).

Returns

a string representation of this collection