E - the type of elements held in this collectionpublic 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() is 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.
| Constructor and Description |
|---|
PriorityQueue()
Creates a
PriorityQueue with the default initial
capacity (11) that orders its elements according to their
natural ordering. |
PriorityQueue(Collection<? extends E> c)
Creates a
PriorityQueue containing the elements in the
specified collection. |
PriorityQueue(Comparator<? super E> comparator)
Creates a
PriorityQueue with the default initial capacity and
whose elements are ordered according to the specified comparator. |
PriorityQueue(int initialCapacity)
Creates a
PriorityQueue with the specified initial
capacity that orders its elements according to their
natural ordering. |
PriorityQueue(int initialCapacity,
Comparator<? super E> comparator)
Creates a
PriorityQueue with the specified initial capacity
that orders its elements according to the specified comparator. |
PriorityQueue(PriorityQueue<? extends E> c)
Creates a
PriorityQueue containing the elements in the
specified priority queue. |
PriorityQueue(SortedSet<? extends E> c)
Creates a
PriorityQueue containing the elements in the
specified sorted set. |
| Modifier and Type | Method and 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, or
null if this queue is sorted according to
the natural ordering of its elements. |
boolean |
contains(Object o)
Returns
true if this queue contains the specified element. |
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()
Retrieves, but does not remove, the head of this queue,
or returns
null if this queue is empty. |
E |
poll()
Retrieves and removes the head of this queue,
or returns
null if this queue is empty. |
boolean |
remove(Object o)
Removes a single instance of the specified element from this queue,
if it is present.
|
int |
size()
Returns the number of elements in this collection.
|
Spliterator<E> |
spliterator()
Creates a late-binding
and fail-fast
Spliterator 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.
|
addAll, element, removecontainsAll, isEmpty, removeAll, retainAll, toStringclone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, waitcontainsAll, equals, hashCode, isEmpty, parallelStream, removeAll, removeIf, retainAll, streampublic PriorityQueue()
PriorityQueue with the default initial
capacity (11) that orders its elements according to their
natural ordering.public PriorityQueue(int initialCapacity)
PriorityQueue with the specified initial
capacity that orders its elements according to their
natural ordering.initialCapacity - the initial capacity for this priority queueIllegalArgumentException - if initialCapacity is less
than 1public PriorityQueue(Comparator<? super E> comparator)
PriorityQueue with the default initial capacity and
whose elements are ordered according to the specified comparator.comparator - the comparator that will be used to order this
priority queue. If null, the natural ordering of the elements will be used.public PriorityQueue(int initialCapacity,
Comparator<? super E> comparator)
PriorityQueue with the specified initial capacity
that orders its elements according to the specified comparator.initialCapacity - the initial capacity for this priority queuecomparator - the comparator that will be used to order this
priority queue. If null, the natural ordering of the elements will be used.IllegalArgumentException - if initialCapacity is
less than 1public PriorityQueue(Collection<? extends E> c)
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.c - the collection whose elements are to be placed
into this priority queueClassCastException - 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 nullpublic PriorityQueue(PriorityQueue<? extends E> c)
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.c - the priority queue whose elements are to be placed
into this priority queueClassCastException - if elements of c cannot be
compared to one another according to c's
orderingNullPointerException - if the specified priority queue or any
of its elements are nullpublic PriorityQueue(SortedSet<? extends E> c)
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.c - the sorted set whose elements are to be placed
into this priority queueClassCastException - 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 nullpublic boolean add(E e)
add in interface Collection<E>add in interface Queue<E>add in class AbstractQueue<E>e - the element to addtrue (as specified by Collection.add(E))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 nullpublic boolean offer(E e)
offer in interface Queue<E>e - the element to addtrue (as specified by Queue.offer(E))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 nullpublic E peek()
Queuenull if this queue is empty.public boolean remove(Object o)
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).remove in interface Collection<E>remove in class AbstractCollection<E>o - element to be removed from this queue, if presenttrue if this queue changed as a result of the callpublic boolean contains(Object o)
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).contains in interface Collection<E>contains in class AbstractCollection<E>o - object to be checked for containment in this queuetrue if this queue contains the specified elementpublic Object[] toArray()
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.
toArray in interface Collection<E>toArray in class AbstractCollection<E>public <T> T[] toArray(T[] a)
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]);
Note that toArray(new Object[0]) is identical in function to
toArray().toArray in interface Collection<E>toArray in class AbstractCollection<E>T - the runtime type of the array to contain the collectiona - 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.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 nullpublic Iterator<E> iterator()
iterator in interface Iterable<E>iterator in interface Collection<E>iterator in class AbstractCollection<E>public int size()
Collectionsize in interface Collection<E>size in class AbstractCollection<E>public void clear()
clear in interface Collection<E>clear in class AbstractQueue<E>public E poll()
Queuenull if this queue is empty.public Comparator<? super E> comparator()
null if this queue is sorted according to
the natural ordering of its elements.null if this queue is sorted according to the
natural ordering of its elementspublic final Spliterator<E> spliterator()
Spliterator over the elements in this
queue.
The Spliterator reports Spliterator.SIZED,
Spliterator.SUBSIZED, and Spliterator.NONNULL.
Overriding implementations should document the reporting of additional
characteristic values.
spliterator in interface Iterable<E>spliterator in interface Collection<E>Spliterator over the elements in this queue Submit a bug or feature
For further API reference and developer documentation, see Java SE Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.
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