- Type Parameters:
E- the type of elements held in this queue
- All Implemented Interfaces:
Serializable,Iterable<E>,Collection<E>,BlockingQueue<E>,Queue<E>
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.
- Since:
- 1.5
- See Also:
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Constructor Summary
ConstructorsConstructorDescriptionCreates aPriorityBlockingQueuewith the default initial capacity (11) that orders its elements according to their natural ordering.PriorityBlockingQueue(int initialCapacity) Creates aPriorityBlockingQueuewith the specified initial capacity that orders its elements according to their natural ordering.PriorityBlockingQueue(int initialCapacity, Comparator<? super E> comparator) Creates aPriorityBlockingQueuewith the specified initial capacity that orders its elements according to the specified comparator.PriorityBlockingQueue(Collection<? extends E> c) Creates aPriorityBlockingQueuecontaining the elements in the specified collection. -
Method Summary
Modifier and TypeMethodDescriptionbooleanInserts the specified element into this priority queue.voidclear()Atomically removes all of the elements from this queue.Comparator<? super E> Returns the comparator used to order the elements in this queue, ornullif this queue uses the natural ordering of its elements.booleanReturnstrueif this queue contains the specified element.intdrainTo(Collection<? super E> c) Removes all available elements from this queue and adds them to the given collection.intdrainTo(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.voidPerforms the given action for each element of theIterableuntil all elements have been processed or the action throws an exception.iterator()Returns an iterator over the elements in this queue.booleanInserts the specified element into this priority queue.booleanInserts the specified element into this priority queue.peek()Retrieves, but does not remove, the head of this queue, or returnsnullif this queue is empty.poll()Retrieves and removes the head of this queue, or returnsnullif this queue is empty.Retrieves and removes the head of this queue, waiting up to the specified wait time if necessary for an element to become available.voidInserts the specified element into this priority queue.intAlways returnsInteger.MAX_VALUEbecause aPriorityBlockingQueueis not capacity constrained.booleanRemoves a single instance of the specified element from this queue, if it is present.booleanremoveAll(Collection<?> c) Removes all of this collection's elements that are also contained in the specified collection (optional operation).booleanRemoves all of the elements of this collection that satisfy the given predicate.booleanretainAll(Collection<?> c) Retains only the elements in this collection that are contained in the specified collection (optional operation).intsize()Returns the number of elements in this collection.Returns aSpliteratorover the elements in this queue.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.Methods declared in class java.util.AbstractQueue
addAll, element, removeMethods declared in class java.util.AbstractCollection
containsAll, isEmpty, toStringMethods declared in class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, waitMethods declared in interface java.util.Collection
addAll, containsAll, equals, hashCode, isEmpty, parallelStream, stream, toArray
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Constructor Details
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PriorityBlockingQueue
public PriorityBlockingQueue()Creates aPriorityBlockingQueuewith the default initial capacity (11) that orders its elements according to their natural ordering. -
PriorityBlockingQueue
public PriorityBlockingQueue(int initialCapacity) Creates aPriorityBlockingQueuewith the specified initial capacity that orders its elements according to their natural ordering.- Parameters:
initialCapacity- the initial capacity for this priority queue- Throws:
IllegalArgumentException- ifinitialCapacityis less than 1
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PriorityBlockingQueue
Creates aPriorityBlockingQueuewith the specified initial capacity that orders its elements according to the specified comparator.- Parameters:
initialCapacity- the initial capacity for this priority queuecomparator- the comparator that will be used to order this priority queue. Ifnull, the natural ordering of the elements will be used.- Throws:
IllegalArgumentException- ifinitialCapacityis less than 1
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PriorityBlockingQueue
Creates aPriorityBlockingQueuecontaining the elements in the specified collection. If the specified collection is aSortedSetor aPriorityBlockingQueue, 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 orderingNullPointerException- if the specified collection or any of its elements are null
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Method Details
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add
Inserts the specified element into this priority queue.- Specified by:
addin interfaceBlockingQueue<E>- Specified by:
addin interfaceCollection<E>- Specified by:
addin interfaceQueue<E>- Overrides:
addin classAbstractQueue<E>- Parameters:
e- the element to add- Returns:
true(as specified byCollection.add(E))- Throws:
ClassCastException- if the specified element cannot be compared with elements currently in the priority queue according to the priority queue's orderingNullPointerException- if the specified element is null
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offer
Inserts the specified element into this priority queue. As the queue is unbounded, this method will never returnfalse.- Specified by:
offerin interfaceBlockingQueue<E>- Specified by:
offerin interfaceQueue<E>- Parameters:
e- the element to add- Returns:
true(as specified byQueue.offer(E))- Throws:
ClassCastException- if the specified element cannot be compared with elements currently in the priority queue according to the priority queue's orderingNullPointerException- if the specified element is null
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put
Inserts the specified element into this priority queue. As the queue is unbounded, this method will never block.- Specified by:
putin interfaceBlockingQueue<E>- 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 orderingNullPointerException- if the specified element is null
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offer
Inserts the specified element into this priority queue. As the queue is unbounded, this method will never block or returnfalse.- Specified by:
offerin interfaceBlockingQueue<E>- Parameters:
e- the element to addtimeout- This parameter is ignored as the method never blocksunit- This parameter is ignored as the method never blocks- Returns:
true(as specified byBlockingQueue.offer)- Throws:
ClassCastException- if the specified element cannot be compared with elements currently in the priority queue according to the priority queue's orderingNullPointerException- if the specified element is null
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poll
Description copied from interface:QueueRetrieves and removes the head of this queue, or returnsnullif this queue is empty. -
take
Description copied from interface:BlockingQueueRetrieves and removes the head of this queue, waiting if necessary until an element becomes available.- Specified by:
takein interfaceBlockingQueue<E>- Returns:
- the head of this queue
- Throws:
InterruptedException- if interrupted while waiting
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poll
Description copied from interface:BlockingQueueRetrieves and removes the head of this queue, waiting up to the specified wait time if necessary for an element to become available.- Specified by:
pollin interfaceBlockingQueue<E>- Parameters:
timeout- how long to wait before giving up, in units ofunitunit- aTimeUnitdetermining how to interpret thetimeoutparameter- Returns:
- the head of this queue, or
nullif the specified waiting time elapses before an element is available - Throws:
InterruptedException- if interrupted while waiting
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peek
Description copied from interface:QueueRetrieves, but does not remove, the head of this queue, or returnsnullif this queue is empty. -
comparator
Returns the comparator used to order the elements in this queue, ornullif this queue uses the natural ordering of its elements.- Returns:
- the comparator used to order the elements in this queue,
or
nullif this queue uses the natural ordering of its elements
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size
public int size()Description copied from interface:CollectionReturns the number of elements in this collection. If this collection contains more thanInteger.MAX_VALUEelements, returnsInteger.MAX_VALUE.- Specified by:
sizein interfaceCollection<E>- Returns:
- the number of elements in this collection
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remainingCapacity
public int remainingCapacity()Always returnsInteger.MAX_VALUEbecause aPriorityBlockingQueueis not capacity constrained.- Specified by:
remainingCapacityin interfaceBlockingQueue<E>- Returns:
Integer.MAX_VALUEalways
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remove
Removes a single instance of the specified element from this queue, if it is present. More formally, removes an elementesuch thato.equals(e), if this queue contains one or more such elements. Returnstrueif and only if this queue contained the specified element (or equivalently, if this queue changed as a result of the call).- Specified by:
removein interfaceBlockingQueue<E>- Specified by:
removein interfaceCollection<E>- Overrides:
removein classAbstractCollection<E>- Parameters:
o- element to be removed from this queue, if present- Returns:
trueif this queue changed as a result of the call
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contains
Returnstrueif this queue contains the specified element. More formally, returnstrueif and only if this queue contains at least one elementesuch thato.equals(e).- Specified by:
containsin interfaceBlockingQueue<E>- Specified by:
containsin interfaceCollection<E>- Overrides:
containsin classAbstractCollection<E>- Parameters:
o- object to be checked for containment in this queue- Returns:
trueif this queue contains the specified element
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drainTo
Description copied from interface:BlockingQueueRemoves 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 collectioncmay result in elements being in neither, either or both collections when the associated exception is thrown. Attempts to drain a queue to itself result inIllegalArgumentException. Further, the behavior of this operation is undefined if the specified collection is modified while the operation is in progress.- Specified by:
drainToin interfaceBlockingQueue<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 collectionClassCastException- if the class of an element of this queue prevents it from being added to the specified collectionNullPointerException- if the specified collection is nullIllegalArgumentException- 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
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drainTo
Description copied from interface:BlockingQueueRemoves 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 collectioncmay result in elements being in neither, either or both collections when the associated exception is thrown. Attempts to drain a queue to itself result inIllegalArgumentException. Further, the behavior of this operation is undefined if the specified collection is modified while the operation is in progress.- Specified by:
drainToin interfaceBlockingQueue<E>- Parameters:
c- the collection to transfer elements intomaxElements- 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 collectionClassCastException- if the class of an element of this queue prevents it from being added to the specified collectionNullPointerException- if the specified collection is nullIllegalArgumentException- 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
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clear
public void clear()Atomically removes all of the elements from this queue. The queue will be empty after this call returns.- Specified by:
clearin interfaceCollection<E>- Overrides:
clearin classAbstractQueue<E>
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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.
- Specified by:
toArrayin interfaceCollection<E>- Overrides:
toArrayin classAbstractCollection<E>- Returns:
- an array containing all of the elements in this queue
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toArray
public <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. 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
xis a queue known to contain only strings. The following code can be used to dump the queue into a newly allocated array ofString:
Note thatString[] y = x.toArray(new String[0]);toArray(new Object[0])is identical in function totoArray().- Specified by:
toArrayin interfaceCollection<E>- Overrides:
toArrayin classAbstractCollection<E>- Type Parameters:
T- the component type of the array to contain the collection- 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 queueNullPointerException- if the specified array is null
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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.
- Specified by:
iteratorin interfaceCollection<E>- Specified by:
iteratorin interfaceIterable<E>- Specified by:
iteratorin classAbstractCollection<E>- Returns:
- an iterator over the elements in this queue
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spliterator
Returns aSpliteratorover the elements in this queue. The spliterator does not traverse elements in any particular order (theORDEREDcharacteristic is not reported).The returned spliterator is weakly consistent.
The
SpliteratorreportsSpliterator.SIZEDandSpliterator.NONNULL.- Specified by:
spliteratorin interfaceCollection<E>- Specified by:
spliteratorin interfaceIterable<E>- Implementation Note:
- The
Spliteratoradditionally reportsSpliterator.SUBSIZED. - Returns:
- a
Spliteratorover the elements in this queue - Since:
- 1.8
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removeIf
Description copied from interface:CollectionRemoves 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.- Specified by:
removeIfin interfaceCollection<E>- Parameters:
filter- a predicate which returnstruefor elements to be removed- Returns:
trueif any elements were removed- Throws:
NullPointerException- if the specified filter is null
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removeAll
Description copied from class:AbstractCollectionRemoves 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.- Specified by:
removeAllin interfaceCollection<E>- Overrides:
removeAllin classAbstractCollection<E>- Parameters:
c- collection containing elements to be removed from this collection- Returns:
trueif this collection changed as a result of the call- Throws:
NullPointerException- if this collection contains one or more null elements and the specified collection does not support null elements (optional) or if the specified collection is null- See Also:
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retainAll
Description copied from class:AbstractCollectionRetains 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.- Specified by:
retainAllin interfaceCollection<E>- Overrides:
retainAllin classAbstractCollection<E>- Parameters:
c- collection containing elements to be retained in this collection- Returns:
trueif this collection changed as a result of the call- Throws:
NullPointerException- if this collection contains one or more null elements and the specified collection does not permit null elements (optional) or if the specified collection is null- See Also:
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forEach
Description copied from interface:IterablePerforms the given action for each element of theIterableuntil all elements have been processed or the action throws an exception. Actions are performed in the order of iteration, if that order is specified. Exceptions thrown by the action are relayed to the caller.The behavior of this method is unspecified if the action performs side-effects that modify the underlying source of elements, unless an overriding class has specified a concurrent modification policy.
- Specified by:
forEachin interfaceIterable<E>- Parameters:
action- The action to be performed for each element- Throws:
NullPointerException- if the specified action is null
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