PriorityQueue
public
class
PriorityQueue
extends AbstractQueue<E>
implements
Serializable
java.lang.Object | |||
↳ | java.util.AbstractCollection<E> | ||
↳ | java.util.AbstractQueue<E> | ||
↳ | java.util.PriorityQueue<E> |
An unbounded priority queue based on a priority heap.
The elements of the priority queue are ordered according to their
natural ordering, or by a Comparator
provided at queue construction time, depending on which constructor is
used. A priority queue does not permit null
elements.
A priority queue relying on natural ordering also does not permit
insertion of non-comparable objects (doing so may result in
ClassCastException
).
The head of this queue is the least element
with respect to the specified ordering. If multiple elements are
tied for least value, the head is one of those elements -- ties are
broken arbitrarily. The queue retrieval operations poll
,
remove
, peek
, and element
access the
element at the head of the queue.
A priority queue is unbounded, but has an internal capacity governing the size of an array used to store the elements on the queue. It is always at least as large as the queue size. As elements are added to a priority queue, its capacity grows automatically. The details of the growth policy are not specified.
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 priority queue in any particular order. If you need ordered
traversal, consider using Arrays.sort(pq.toArray())
.
Note that this implementation is not synchronized.
Multiple threads should not access a PriorityQueue
instance concurrently if any of the threads modifies the queue.
Instead, use the thread-safe PriorityBlockingQueue
class.
Implementation note: this implementation provides
O(log(n)) time for the enqueuing and dequeuing methods
(offer
, poll
, remove()
and add
);
linear time for the remove(Object)
and contains(Object)
methods; and constant time for the retrieval methods
(peek
, element
, and size
).
This class is a member of the Java Collections Framework.
Summary
Public constructors | |
---|---|
PriorityQueue()
Creates a |
|
PriorityQueue(int initialCapacity)
Creates a |
|
PriorityQueue(Comparator<? super E> comparator)
Creates a |
|
PriorityQueue(int initialCapacity, Comparator<? super E> comparator)
Creates a |
|
PriorityQueue(Collection<? extends E> c)
Creates a |
|
PriorityQueue(PriorityQueue<? extends E> c)
Creates a |
|
PriorityQueue(SortedSet<? extends E> c)
Creates a |
Public methods | |
---|---|
boolean
|
add(E e)
Inserts the specified element into this priority queue. |
void
|
clear()
Removes all of the elements from this priority queue. |
Comparator<? super E>
|
comparator()
Returns the comparator used to order the elements in this
queue, or |
boolean
|
contains(Object o)
Returns |
void
|
forEach(Consumer<? super E> action)
|
Iterator<E>
|
iterator()
Returns an iterator over the elements in this queue. |
boolean
|
offer(E e)
Inserts the specified element into this priority queue. |
E
|
peek()
|
E
|
poll()
|
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)
|
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. |
final
Spliterator<E>
|
spliterator()
Creates a late-binding
and fail-fast |
<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. |
Inherited methods | |
---|---|
Public constructors
PriorityQueue
public PriorityQueue ()
Creates a PriorityQueue
with the default initial
capacity (11) that orders its elements according to their
natural ordering.
PriorityQueue
public PriorityQueue (int initialCapacity)
Creates a PriorityQueue
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 |
PriorityQueue
public PriorityQueue (Comparator<? super E> comparator)
Creates a PriorityQueue
with the default initial capacity and
whose elements are ordered according to the specified comparator.
Parameters | |
---|---|
comparator |
Comparator : the comparator that will be used to order this
priority queue. If null , the natural ordering of the elements will be used. |
PriorityQueue
public PriorityQueue (int initialCapacity, Comparator<? super E> comparator)
Creates a PriorityQueue
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 |
PriorityQueue
public PriorityQueue (Collection<? extends E> c)
Creates a PriorityQueue
containing the elements in the
specified collection. If the specified collection is an instance of
a SortedSet
or is another PriorityQueue
, 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 |
PriorityQueue
public PriorityQueue (PriorityQueue<? extends E> c)
Creates a PriorityQueue
containing the elements in the
specified priority queue. This priority queue will be
ordered according to the same ordering as the given priority
queue.
Parameters | |
---|---|
c |
PriorityQueue : the priority queue whose elements are to be placed
into this priority queue |
Throws | |
---|---|
ClassCastException |
if elements of c cannot be
compared to one another according to c 's
ordering |
NullPointerException |
if the specified priority queue or any of its elements are null |
PriorityQueue
public PriorityQueue (SortedSet<? extends E> c)
Creates a PriorityQueue
containing the elements in the
specified sorted set. This priority queue will be ordered
according to the same ordering as the given sorted set.
Parameters | |
---|---|
c |
SortedSet : the sorted set whose elements are to be placed
into this priority queue |
Throws | |
---|---|
ClassCastException |
if elements of the specified sorted set cannot be compared to one another according to the sorted set's ordering |
NullPointerException |
if the specified sorted set 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 this priority queue according to the priority queue's ordering |
NullPointerException |
if the specified element is null |
clear
public void clear ()
Removes all of the elements from this priority 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 is sorted according to
the natural ordering of its elements.
Returns | |
---|---|
Comparator<? super E> |
the comparator used to order this queue, or
null if this queue is sorted according to 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 |
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.
Returns | |
---|---|
Iterator<E> |
an iterator over the elements in this queue |
offer
public boolean offer (E e)
Inserts the specified element into this priority queue.
Parameters | |
---|---|
e |
E |
Returns | |
---|---|
boolean |
true (as specified by Queue#offer ) |
Throws | |
---|---|
ClassCastException |
if the specified element cannot be compared with elements currently in this priority queue according to the priority queue's ordering |
NullPointerException |
if the specified element is null |
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)
Parameters | |
---|---|
filter |
Predicate |
Returns | |
---|---|
boolean |
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 final Spliterator<E> spliterator ()
Creates a late-binding
and fail-fast Spliterator
over the elements in this
queue. The spliterator does not traverse elements in any particular order
(the ORDERED
characteristic is not reported).
The Spliterator
reports Spliterator#SIZED
,
Spliterator#SUBSIZED
, and Spliterator#NONNULL
.
Overriding implementations should document the reporting of additional
characteristic values.
Returns | |
---|---|
Spliterator<E> |
a Spliterator over 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 the queue fits in the specified array with room to spare
(i.e., the array has more elements than the queue), the element in
the array immediately following the end of the collection 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 |
toArray
public Object[] toArray ()
Returns an array containing all of the elements in this queue. The 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 |