PriorityBlockingQueue

PriorityBlockingQueue | API reference | Android Developers

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

java.lang.Object
↳	java.util.AbstractCollection<E>
	↳	java.util.AbstractQueue<E>
		↳	java.util.concurrent.PriorityBlockingQueue<E>

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() and the Spliterator provided in method spliterator() are 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();
   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.
`void`	`forEach(Consumer<? super E> action)`
`Iterator<E>`	`iterator()` Returns an iterator over the elements in this queue.
`boolean`	`offer(E e, long timeout, TimeUnit unit)` Inserts the specified element into this priority queue.
`boolean`	`offer(E e)` 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.
`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.
`boolean`	`retainAll(Collection<?> c)` Retains only the elements in this collection that are contained in the specified collection (optional operation).
`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.
`Object[]`	`toArray()` Returns an array containing all of the elements in this queue.
`<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.
`String`	`toString()` Returns a string representation of this collection.

Inherited methods

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).
`boolean`	`addAll(Collection<? extends E> c)` Adds all of the elements in the specified collection to this collection (optional operation).
`void`	`clear()` Removes all of the elements from this collection (optional operation).
`boolean`	`contains(Object o)` Returns `true` if this collection contains the specified element.
`boolean`	`containsAll(Collection<?> c)` Returns `true` if this collection contains all of the elements in the specified collection.
`boolean`	`isEmpty()` Returns `true` if this collection contains no elements.
`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).
`boolean`	`removeAll(Collection<?> c)` Removes all of this collection's elements that are also contained in the specified collection (optional operation).
`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.
`<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.
`Object[]`	`toArray()` Returns an array containing all of the elements in this collection.
`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)` Indicates whether some other object is "equal to" this one.
`void`	`finalize()` Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.
`final Class<?>`	`getClass()` Returns the runtime class of this `Object`.
`int`	`hashCode()` Returns a hash code value for the object.
`final void`	`notify()` Wakes up a single thread that is waiting on this object's monitor.
`final void`	`notifyAll()` Wakes up all threads that are waiting on this object's monitor.
`String`	`toString()` Returns a string representation of the object.
`final void`	`wait(long timeoutMillis, int nanos)` Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.
`final void`	`wait(long timeoutMillis)` Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.
`final void`	`wait()` Causes the current thread to wait until it is awakened, typically by being notified or interrupted.

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.
`default 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).
`default 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.
`default Spliterator<E>`	`spliterator()` Creates a `Spliterator` over the elements in this collection.
`default 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.
`default <T> T[]`	`toArray(IntFunction<T[]> generator)` Returns an array containing all of the elements in this collection, using the provided `generator` function to allocate the returned array.

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

`default 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`.
`default 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 natural ordering.

PriorityBlockingQueue

public PriorityBlockingQueue (int initialCapacity)

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

Parameters
`initialCapacity`	`int`: 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`	`int`: the initial capacity for this priority queue
`comparator`	`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

PriorityBlockingQueue

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 PriorityBlockingQueue, 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`	`Collection`: 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

add

public boolean add (E e)

Inserts the specified element into this priority queue.

Parameters
`e`	`E`: the element to add

Returns
`boolean`	`true` (as specified by `Collection.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

clear

public void clear ()

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

comparator

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
`Comparator<? super E>`	the comparator used to order the elements in this queue, or `null` if this queue uses the natural ordering of its elements

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).

Parameters
`o`	`Object`: object to be checked for containment in this queue

Returns
`boolean`	`true` if this queue contains the specified element

drainTo

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`	`Collection`: the collection to transfer elements into
`maxElements`	`int`: the maximum number of elements to transfer

Returns
`int`	the number of elements transferred

Throws
`ClassCastException`
`IllegalArgumentException`
`NullPointerException`
`UnsupportedOperationException`

drainTo

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`	`Collection`: the collection to transfer elements into

Returns
`int`	the number of elements transferred

Throws
`ClassCastException`
`IllegalArgumentException`
`NullPointerException`
`UnsupportedOperationException`

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 weakly consistent.

Returns
`Iterator<E>`	an iterator over the elements in this queue

offer

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`	`E`: the element to add
`timeout`	`long`: This parameter is ignored as the method never blocks
`unit`	`TimeUnit`: This parameter is ignored as the method never blocks

Returns
`boolean`	`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

offer

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`	`E`: the element to add

Returns
`boolean`	`true` (as specified by `Queue.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

peek

public E peek ()

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

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

poll

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`	`long`: how long to wait before giving up, in units of `unit`
`unit`	`TimeUnit`: a `TimeUnit` determining how to interpret the `timeout` parameter

Returns
`E`	the head of this queue, or `null` if the specified waiting time elapses before an element is available

Throws
`InterruptedException`

poll

public E poll ()

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

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

put

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`	`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

remainingCapacity

public int remainingCapacity ()

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

Returns
`int`	`Integer.MAX_VALUE` always

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

Parameters
`o`	`Object`: element to be removed from this queue, if present

Returns
`boolean`	`true` if this queue changed as a result of the call

removeAll

public boolean removeAll (Collection<?> c)

Removes all of this collection's elements that are also contained in the specified collection (optional operation). After this call returns, this collection will contain no elements in common with the specified collection.

Parameters
`c`	`Collection`: collection containing elements to be removed from this collection

Returns
`boolean`	`true` if this collection changed as a result of the call

Throws
`NullPointerException`

removeIf

public boolean removeIf (Predicate<? super E> filter)

Removes all of the elements of this collection that satisfy the given predicate. Errors or runtime exceptions thrown during iteration or by the predicate are relayed to the caller.

Parameters
`filter`	`Predicate`: a predicate which returns `true` for elements to be removed

Returns
`boolean`	`true` if any elements were removed

Throws
`NullPointerException`

retainAll

public boolean retainAll (Collection<?> c)

Retains only the elements in this collection that are contained in the specified collection (optional operation). In other words, removes from this collection all of its elements that are not contained in the specified collection.

Parameters
`c`	`Collection`: collection containing elements to be retained in this collection

Returns
`boolean`	`true` if this collection changed as a result of the call

Throws
`NullPointerException`

size

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
`int`	the number of elements in this collection

take

public E take ()

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

Returns
`E`	the head of this queue

Throws
`InterruptedException`

toArray

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
`Object[]`	an array containing all of the elements in this queue

toArray

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`	`T`: 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
`T[]`	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

toString

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
`String`	a string representation of this collection