- Type Parameters:
E
- the type of elements held in this queue
- All Implemented Interfaces:
Serializable
,Iterable<E>
,Collection<E>
,Queue<E>
public class PriorityQueue<E> extends AbstractQueue<E> implements Serializable
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.
- Since:
- 1.5
- See Also:
- Serialized Form
-
Constructor Summary
Constructors Constructor Description PriorityQueue()
Creates aPriorityQueue
with the default initial capacity (11) that orders its elements according to their natural ordering.PriorityQueue(int initialCapacity)
Creates aPriorityQueue
with the specified initial capacity that orders its elements according to their natural ordering.PriorityQueue(int initialCapacity, Comparator<? super E> comparator)
Creates aPriorityQueue
with the specified initial capacity that orders its elements according to the specified comparator.PriorityQueue(Collection<? extends E> c)
Creates aPriorityQueue
containing the elements in the specified collection.PriorityQueue(Comparator<? super E> comparator)
Creates aPriorityQueue
with the default initial capacity and whose elements are ordered according to the specified comparator.PriorityQueue(PriorityQueue<? extends E> c)
Creates aPriorityQueue
containing the elements in the specified priority queue.PriorityQueue(SortedSet<? extends E> c)
Creates aPriorityQueue
containing the elements in the specified sorted set. -
Method Summary
Modifier and Type Method Description 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, ornull
if this queue is sorted according to the natural ordering of its elements.boolean
contains(Object o)
Returnstrue
if this queue contains the specified element.void
forEach(Consumer<? super E> action)
Performs the given action for each element of theIterable
until all elements have been processed or the action throws an exception.Iterator<E>
iterator()
Returns an iterator over the elements in this queue.boolean
offer(E e)
Inserts the specified element into this priority queue.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).Spliterator<E>
spliterator()
Creates a late-binding and fail-fastSpliterator
over the elements in this queue.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, remove
Methods declared in class java.util.AbstractCollection
containsAll, isEmpty, toString
Methods declared in class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait
Methods declared in interface java.util.Collection
containsAll, equals, hashCode, isEmpty, parallelStream, size, stream, toArray
-
Constructor Details
-
PriorityQueue
public PriorityQueue()Creates aPriorityQueue
with the default initial capacity (11) that orders its elements according to their natural ordering. -
PriorityQueue
public PriorityQueue(int initialCapacity)Creates aPriorityQueue
with the specified initial capacity that orders its elements according to their natural ordering.- Parameters:
initialCapacity
- the initial capacity for this priority queue- Throws:
IllegalArgumentException
- ifinitialCapacity
is less than 1
-
PriorityQueue
Creates aPriorityQueue
with the default initial capacity and whose elements are ordered according to the specified comparator.- Parameters:
comparator
- the comparator that will be used to order this priority queue. Ifnull
, the natural ordering of the elements will be used.- Since:
- 1.8
-
PriorityQueue
Creates aPriorityQueue
with 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
- ifinitialCapacity
is less than 1
-
PriorityQueue
Creates aPriorityQueue
containing the elements in the specified collection. If the specified collection is an instance of aSortedSet
or is anotherPriorityQueue
, 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
-
PriorityQueue
Creates aPriorityQueue
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
- the priority queue whose elements are to be placed into this priority queue- Throws:
ClassCastException
- if elements ofc
cannot be compared to one another according toc
's orderingNullPointerException
- if the specified priority queue or any of its elements are null
-
PriorityQueue
Creates aPriorityQueue
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
- 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 orderingNullPointerException
- if the specified sorted set or any of its elements are null
-
-
Method Details
-
add
Inserts the specified element into this priority queue.- Specified by:
add
in interfaceCollection<E>
- Specified by:
add
in interfaceQueue<E>
- Overrides:
add
in 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 this priority queue according to the priority queue's orderingNullPointerException
- if the specified element is null
-
offer
Inserts the specified element into this priority queue.- Specified by:
offer
in 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 this priority queue according to the priority queue's orderingNullPointerException
- if the specified element is null
-
remove
Removes a single instance of the specified element from this queue, if it is present. More formally, removes an elemente
such thato.equals(e)
, if this queue contains one or more such elements. Returnstrue
if and only if this queue contained the specified element (or equivalently, if this queue changed as a result of the call).- Specified by:
remove
in interfaceCollection<E>
- Overrides:
remove
in classAbstractCollection<E>
- Parameters:
o
- element to be removed from this queue, if present- Returns:
true
if this queue changed as a result of the call
-
contains
Returnstrue
if this queue contains the specified element. More formally, returnstrue
if and only if this queue contains at least one elemente
such thato.equals(e)
.- Specified by:
contains
in interfaceCollection<E>
- Overrides:
contains
in classAbstractCollection<E>
- Parameters:
o
- object to be checked for containment in this queue- Returns:
true
if this queue contains the specified element
-
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.
- Specified by:
toArray
in interfaceCollection<E>
- Overrides:
toArray
in classAbstractCollection<E>
- Returns:
- an array containing all of the elements in this queue
-
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 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 ofString
:String[] y = x.toArray(new String[0]);
toArray(new Object[0])
is identical in function totoArray()
.- Specified by:
toArray
in interfaceCollection<E>
- Overrides:
toArray
in 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
-
iterator
Returns an iterator over the elements in this queue. The iterator does not return the elements in any particular order.- Specified by:
iterator
in interfaceCollection<E>
- Specified by:
iterator
in interfaceIterable<E>
- Specified by:
iterator
in classAbstractCollection<E>
- Returns:
- an iterator over the elements in this queue
-
clear
public void clear()Removes all of the elements from this priority queue. The queue will be empty after this call returns.- Specified by:
clear
in interfaceCollection<E>
- Overrides:
clear
in classAbstractQueue<E>
-
comparator
Returns the comparator used to order the elements in this queue, ornull
if this queue is sorted according to the natural ordering of its elements.- Returns:
- the comparator used to order this queue, or
null
if this queue is sorted according to the natural ordering of its elements
-
spliterator
Creates a late-binding and fail-fastSpliterator
over the elements in this queue. The spliterator does not traverse elements in any particular order (theORDERED
characteristic is not reported).The
Spliterator
reportsSpliterator.SIZED
,Spliterator.SUBSIZED
, andSpliterator.NONNULL
. Overriding implementations should document the reporting of additional characteristic values.- Specified by:
spliterator
in interfaceCollection<E>
- Specified by:
spliterator
in interfaceIterable<E>
- Returns:
- a
Spliterator
over the elements in this queue - Since:
- 1.8
-
removeIf
Description copied from interface:Collection
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.- Specified by:
removeIf
in interfaceCollection<E>
- Parameters:
filter
- a predicate which returnstrue
for elements to be removed- Returns:
true
if any elements were removed- Throws:
NullPointerException
- if the specified filter is null
-
removeAll
Description copied from class:AbstractCollection
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.- Specified by:
removeAll
in interfaceCollection<E>
- Overrides:
removeAll
in classAbstractCollection<E>
- Parameters:
c
- collection containing elements to be removed from this collection- Returns:
true
if 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:
AbstractCollection.remove(Object)
,AbstractCollection.contains(Object)
-
retainAll
Description copied from class:AbstractCollection
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.- Specified by:
retainAll
in interfaceCollection<E>
- Overrides:
retainAll
in classAbstractCollection<E>
- Parameters:
c
- collection containing elements to be retained in this collection- Returns:
true
if 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:
AbstractCollection.remove(Object)
,AbstractCollection.contains(Object)
-
forEach
Description copied from interface:Iterable
Performs the given action for each element of theIterable
until 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:
forEach
in interfaceIterable<E>
- Parameters:
action
- The action to be performed for each element- Throws:
NullPointerException
- if the specified action is null
-