PriorityBlockingQueue
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(int initialCapacity)
Creates a |
|
PriorityBlockingQueue(int initialCapacity, Comparator<? super E> comparator)
Creates a |
|
PriorityBlockingQueue(Collection<? extends E> c)
Creates a |
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 |
boolean
|
contains(Object o)
Returns |
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 |
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 |
void
|
put(E e)
Inserts the specified element into this priority queue. |
int
|
remainingCapacity()
Always returns |
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 |
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 | |
---|---|
Public constructors
PriorityBlockingQueue
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 | |
---|---|
UnsupportedOperationException |
|
ClassCastException |
|
NullPointerException |
|
IllegalArgumentException |
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 | |
---|---|
UnsupportedOperationException |
|
ClassCastException |
|
NullPointerException |
|
IllegalArgumentException |
forEach
public void forEach (Consumer<? super E> action)
Parameters | |
---|---|
action |
Consumer |
Throws | |
---|---|
NullPointerException |
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 |
spliterator
public Spliterator<E> spliterator ()
Returns a Spliterator
over the elements in this queue.
The spliterator does not traverse elements in any particular order
(the ORDERED
characteristic is not reported).
The returned spliterator is weakly consistent.
The Spliterator
reports Spliterator#SIZED
and
Spliterator#NONNULL
.
Implementation Note:
- The
Spliterator
additionally reportsSpliterator#SUBSIZED
.
Returns | |
---|---|
Spliterator<E> |
a Spliterator over the elements in this queue |
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]);
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 |