Java Reference
In-Depth Information
turns a special value (false or null) in the presence of failure. The methods that return
aspecialvalueareusefulinthecontextofcapacity-restricted
Queue
implementations
where failure is a normal occurrence.
Note
The
offer()
method is generally preferable to
add()
when using a
capacity-restricted queue because
offer()
does not throw
IllegalStateEx-
ception
.
Javasuppliesmany
Queue
implementationclasses,wheremostoftheseclassesare
members of the
java.util.concurrent
package:
LinkedBlockingQueue
,
LinkedTransferQueue
,and
SynchronousQueue
areexamples.Incontrast,the
java.util
package provides
LinkedList
and
PriorityQueue
as its
Queue
implementation classes.
Caution
Many
Queue
implementationclassesdonotallownullelementstobead-
ded.However,someclasses(suchas
LinkedList
)permitnullelements.Youshould
avoid adding a null element because null is used as a special return value by the
peek()
and
poll()
methods to indicate that a queue is empty.
PriorityQueue
The
PriorityQueue
classprovidesanimplementationofa
priority queue
,whichis
aqueuethatordersitselementsaccordingtotheirnaturalorderingorbyacomparator
provided when the queue is instantiated. Priority queues do not permit null elements,
and do not permit insertion of non-
Comparable
objects when relying on natural or-
dering.
Theelementattheheadofthepriorityqueueistheleastelementwithrespecttothe
specifiedordering.Ifmultipleelementsaretiedforleastelement,oneofthoseelements
isarbitrarilychosenastheleastelement.Similarly,theelementatthetailofthepriority
queue is the greatest element, which is arbitrarily chosen when there is a tie.
Priorityqueuesareunbounded,buthaveacapacitythatgovernsthesizeoftheintern-
alarraythatisusedtostorethepriorityqueue'selements.Thecapacityvalueisatleast
aslargeasthequeue'slength,andgrowsautomaticallyaselementsareaddedtothepri-
ority queue.
PriorityQueue
(whosegenerictypeis
PriorityQueue<E>
)suppliessixcon-
structors:
