PriorityQueue

public class PriorityQueue extends AbstractQueue<E>
implements Serializable

An unbounded priority {@linkplain Queue queue} based on a priority heap. The elements of the priority queue are ordered according to their {@linkplain Comparable 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.

Public Constructor Summary

	PriorityQueue() Creates a `PriorityQueue` with the default initial capacity (11) that orders its elements according to their {@linkplain Comparable natural ordering}.
	PriorityQueue(int initialCapacity) Creates a `PriorityQueue` with the specified initial capacity that orders its elements according to their {@linkplain Comparable 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 Method Summary

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 {@linkplain Comparable 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 Method Summary

From class java.util.AbstractQueue

boolean	add(E e) Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning `true` upon success and throwing an `IllegalStateException` if no space is currently available.
boolean	addAll(Collection<? extends E> c) Adds all of the elements in the specified collection to this queue.
void	clear() Removes all of the elements from this queue.
E	element() Retrieves, but does not remove, the head of this queue.
E	remove() Retrieves and removes the head of this queue.

From class java.util.AbstractCollection

boolean	add(E e) Ensures that this collection contains the specified element (optional operation). This implementation always throws an `UnsupportedOperationException`.
boolean	addAll(Collection<? extends E> c) Adds all of the elements in the specified collection to this collection (optional operation). This implementation iterates over the specified collection, and adds each object returned by the iterator to this collection, in turn.
void	clear() Removes all of the elements from this collection (optional operation). This implementation iterates over this collection, removing each element using the `Iterator.remove` operation.
boolean	contains(Object o) Returns `true` if this collection contains the specified element. This implementation iterates over the elements in the collection, checking each element in turn for equality with the specified element.
boolean	containsAll(Collection<?> c) Returns `true` if this collection contains all of the elements in the specified collection. This implementation iterates over the specified collection, checking each element returned by the iterator in turn to see if it's contained in this collection.
boolean	isEmpty() Returns `true` if this collection contains no elements. This implementation returns `size() == 0`.
abstract Iterator<E>	iterator() Returns an iterator over the elements contained in this collection.
boolean	remove(Object o) Removes a single instance of the specified element from this collection, if it is present (optional operation). This implementation iterates over the collection looking for the specified element.
boolean	removeAll(Collection<?> c) Removes all of this collection's elements that are also contained in the specified collection (optional operation). This implementation iterates over this collection, checking each element returned by the iterator in turn to see if it's contained in the specified collection.
boolean	retainAll(Collection<?> c) Retains only the elements in this collection that are contained in the specified collection (optional operation). This implementation iterates over this collection, checking each element returned by the iterator in turn to see if it's contained in the specified collection.
abstract int	size() Returns the number of elements in this collection.
<T> T[]	toArray(T[] contents) Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array. This implementation returns an array containing all the elements returned by this collection's iterator in the same order, stored in consecutive elements of the array, starting with index `0`.
Object[]	toArray() Returns an array containing all of the elements in this collection. This implementation returns an array containing all the elements returned by this collection's iterator, in the same order, stored in consecutive elements of the array, starting with index `0`.
String	toString() Returns a string representation of this collection.

From class java.lang.Object

Object	clone() Creates and returns a copy of this `Object`.
boolean	equals(Object obj) Compares this instance with the specified object and indicates if they are equal.
void	finalize() Invoked when the garbage collector has detected that this instance is no longer reachable.
final Class<?>	getClass() Returns the unique instance of `Class` that represents this object's class.
int	hashCode() Returns an integer hash code for this object.
final void	notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the `wait()` methods) to be woken up.
final void	notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the `wait()` methods) to be woken up.
String	toString() Returns a string containing a concise, human-readable description of this object.
final void	wait(long timeout, int nanos) Causes the calling thread to wait until another thread calls the `notify()` or `notifyAll()` method of this object or until the specified timeout expires.
final void	wait(long timeout) Causes the calling thread to wait until another thread calls the `notify()` or `notifyAll()` method of this object or until the specified timeout expires.
final void	wait() Causes the calling thread to wait until another thread calls the `notify()` or `notifyAll()` method of this object.

From interface java.util.Queue

abstract boolean	add(E e) Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning `true` upon success and throwing an `IllegalStateException` if no space is currently available.
abstract E	element() Retrieves, but does not remove, the head of this queue.
abstract boolean	offer(E e) Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions.
abstract E	peek() Retrieves, but does not remove, the head of this queue, or returns `null` if this queue is empty.
abstract E	poll() Retrieves and removes the head of this queue, or returns `null` if this queue is empty.
abstract E	remove() Retrieves and removes the head of this queue.

From interface java.util.Collection

abstract boolean	add(E e) Ensures that this collection contains the specified element (optional operation).
abstract boolean	addAll(Collection<? extends E> c) Adds all of the elements in the specified collection to this collection (optional operation).
abstract void	clear() Removes all of the elements from this collection (optional operation).
abstract boolean	contains(Object o) Returns `true` if this collection contains the specified element.
abstract boolean	containsAll(Collection<?> c) Returns `true` if this collection contains all of the elements in the specified collection.
abstract boolean	equals(Object o) Compares the specified object with this collection for equality.
abstract int	hashCode() Returns the hash code value for this collection.
abstract boolean	isEmpty() Returns `true` if this collection contains no elements.
abstract Iterator<E>	iterator() Returns an iterator over the elements in this collection.
Stream<E>	parallelStream() Returns a possibly parallel `Stream` with this collection as its source.
abstract boolean	remove(Object o) Removes a single instance of the specified element from this collection, if it is present (optional operation).
abstract 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.
abstract boolean	retainAll(Collection<?> c) Retains only the elements in this collection that are contained in the specified collection (optional operation).
abstract int	size() Returns the number of elements in this collection.
Spliterator<E>	spliterator() Creates a `Spliterator` over the elements in this collection.
Stream<E>	stream() Returns a sequential `Stream` with this collection as its source.
abstract <T> T[]	toArray(T[] a) Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array.
abstract Object[]	toArray() Returns an array containing all of the elements in this collection.

From interface java.lang.Iterable

void	forEach(Consumer<? super T> action) Performs the given action for each element of the `Iterable` until all elements have been processed or the action throws an exception.
abstract Iterator<E>	iterator() Returns an iterator over elements of type `T`.
Spliterator<E>	spliterator() Creates a `Spliterator` over the elements described by this `Iterable`.

Public Constructors

public PriorityQueue ()

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

public PriorityQueue (int initialCapacity)

Creates a PriorityQueue with the specified initial capacity that orders its elements according to their {@linkplain Comparable natural ordering}.

Parameters

initialCapacity	the initial capacity for this priority queue

Throws

IllegalArgumentException	if `initialCapacity` is less than 1

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	the comparator that will be used to order this priority queue. If `null`, the {@linkplain Comparable natural ordering} of the elements will be used.

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	the initial capacity for this priority queue
comparator	the comparator that will be used to order this priority queue. If `null`, the {@linkplain Comparable natural ordering} of the elements will be used.

Throws

IllegalArgumentException	if `initialCapacity` is less than 1

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 {@linkplain Comparable 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 ordering
NullPointerException	if the specified collection or any of its elements are null

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	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

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	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

public boolean add (E e)

Inserts the specified element into this priority queue.

Parameters

e	the element to add

Returns

true (as specified by Collection.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

public void clear ()

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

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 {@linkplain Comparable 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

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 to be checked for containment in this queue

Returns

true if this queue contains the specified element

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

an iterator over the elements in this queue

public boolean offer (E e)

Inserts the specified element into this priority queue.

Parameters

e

Returns

true (as specified by Queue.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

public E peek ()

public E poll ()

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	element to be removed from this queue, if present

Returns

true if this queue changed as a result of the call

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

the number of elements in this collection

public final Spliterator<E> spliterator ()

Creates a late-binding and fail-fast 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.

Returns

a Spliterator over the elements in this queue

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]);

Note that toArray(new Object[0]) is identical in function to toArray().

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 queue
NullPointerException	if the specified array is null

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

an array containing all of the elements in this queue