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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() 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.

Summary

Public constructors

PriorityQueue()

Creates a PriorityQueue with the default initial capacity (11) that orders its elements according to their natural ordering.

PriorityQueue(int initialCapacity)

Creates a PriorityQueue with the specified initial capacity that orders its elements according to their natural ordering.

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, Comparator<? super E> comparator)

Creates a PriorityQueue with the specified initial capacity that orders its elements according to the specified comparator.

PriorityQueue(Collection<? extends E> c)

Creates a PriorityQueue containing the elements in the specified collection.

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.

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 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()
E poll()
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.

final Spliterator<E> spliterator()

Creates a late-binding and fail-fast Spliterator over 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.

Object[] toArray()

Returns an array containing all of the elements in this queue.

Inherited methods

From class java.util.AbstractQueue
From class java.util.AbstractCollection
From class java.lang.Object
From interface java.util.Queue
From interface java.util.Collection
From interface java.lang.Iterable

Public constructors

PriorityQueue

Added in API level 1
PriorityQueue ()

Creates a PriorityQueue with the default initial capacity (11) that orders its elements according to their natural ordering.

PriorityQueue

Added in API level 1
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

Added in API level 24
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

Added in API level 1
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

Added in API level 1
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

Added in API level 1
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

Added in API level 1
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

Added in API level 1
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 add(E))
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

Added in API level 1
void clear ()

Removes all of the elements from this priority queue. The queue will be empty after this call returns.

comparator

Added in API level 1
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

Added in API level 1
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

iterator

Added in API level 1
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

Added in API level 1
boolean offer (E e)

Inserts the specified element into this priority queue.

Parameters
e E
Returns
boolean true (as specified by offer(E))
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

peek

Added in API level 1
E peek ()

Returns
E

poll

Added in API level 1
E poll ()

Returns
E

remove

Added in API level 1
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

size

Added in API level 1
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

Added in API level 24
Spliterator<E> spliterator ()

Creates a late-binding and fail-fast Spliterator over the elements in this queue.

The Spliterator reports SIZED, SUBSIZED, and NONNULL. Overriding implementations should document the reporting of additional characteristic values.

Returns
Spliterator<E> a Spliterator over the elements in this queue

toArray

Added in API level 1
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]);
Note that 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

Added in API level 1
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
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