java.util

Class PriorityQueue<E>

java.lang.Object

java.util.AbstractCollection<E>

java.util.AbstractQueue<E>

java.util.PriorityQueue<E>

All Implemented Interfaces:

Iterable<E>, Collection<E>, Queue<E>

public class PriorityQueue<E>
extends AbstractQueue<E>

A PriorityQueue holds elements on a priority heap, which orders the elements according to their natural order or according to the comparator specified at construction time. If the queue uses natural ordering, only elements that are comparable are permitted to be inserted into the queue.

The least element of the specified ordering is stored at the head of the queue and the greatest element is stored at the tail of the queue.

A PriorityQueue is not synchronized. If multiple threads will have to access it concurrently, use the PriorityBlockingQueue.

Constructor Summary

Constructors

Constructor and Description
PriorityQueue() Constructs a priority queue with an initial capacity of 11 and natural ordering.
PriorityQueue(Collection<? extends E> c) Constructs a priority queue that contains the elements of a collection.
PriorityQueue(int initialCapacity) Constructs a priority queue with the specified capacity and natural ordering.
PriorityQueue(int initialCapacity, Comparator<? super E> comparator) Constructs a priority queue with the specified capacity and comparator.
PriorityQueue(PriorityQueue<? extends E> c) Constructs a priority queue that contains the elements of another priority queue.
PriorityQueue(SortedSet<? extends E> c) Constructs a priority queue that contains the elements of a sorted set.

Method Summary

Modifier and Type	Method and Description
boolean	add(E o) Adds the specified object to the priority queue.
void	clear() Removes all the elements of the priority queue.
Comparator<? super E>	comparator() Gets the comparator of the priority queue.
boolean	contains(Object object) Answers if there is an element in this queue equals to the object.
Iterator<E>	iterator() Gets the iterator of the priority queue, which will not return elements in any specified ordering.
boolean	offer(E o) Inserts the element to the priority queue.
E	peek() Gets but does not remove the head of the queue.
E	poll() Gets and removes the head of the queue.
boolean	remove(Object o) Removes the specified object from the priority queue.
int	size() Gets the size of the priority queue.
Object[]	toArray() Returns all the elements in an array.
<T> T[]	toArray(T[] array) Returns all the elements in an array, and the type of the result array is the type of the argument array.

Methods inherited from class java.util.AbstractQueue

addAll, element, remove

Methods inherited from class java.util.AbstractCollection

containsAll, isEmpty, removeAll, retainAll, toString

Methods inherited from class java.lang.Object

clone, equals, getClass, hashCode, notify, notifyAll, wait, wait, wait

Methods inherited from interface java.util.Collection

containsAll, equals, hashCode, isEmpty, removeAll, retainAll

Constructor Detail

PriorityQueue

public PriorityQueue()

Constructs a priority queue with an initial capacity of 11 and natural ordering.

PriorityQueue

public PriorityQueue(int initialCapacity)

Constructs a priority queue with the specified capacity and natural ordering.

Parameters:

initialCapacity - the specified capacity.

Throws:

IllegalArgumentException - if the initialCapacity is less than 1.

PriorityQueue

public PriorityQueue(int initialCapacity,
                     Comparator<? super E> comparator)

Constructs a priority queue with the specified capacity and comparator.

Parameters:

initialCapacity - the specified capacity.

comparator - the specified comparator. If it is null, the natural ordering will be used.

Throws:

IllegalArgumentException - if the initialCapacity is less than 1.

PriorityQueue

public PriorityQueue(Collection<? extends E> c)

Constructs a priority queue that contains the elements of a collection. The constructed priority queue has the initial capacity of 110% of the size of the collection. The queue uses natural ordering to order its elements.

Parameters:

c - the collection whose elements will be added to the priority queue to be constructed.

Throws:

ClassCastException - if any of the elements in the collection are not comparable.

NullPointerException - if any of the elements in the collection are null.

PriorityQueue

public PriorityQueue(PriorityQueue<? extends E> c)

Constructs a priority queue that contains the elements of another priority queue. The constructed priority queue has the initial capacity of 110% of the specified one. Both priority queues have the same comparator.

Parameters:

c - the priority queue whose elements will be added to the priority queue to be constructed.

PriorityQueue

public PriorityQueue(SortedSet<? extends E> c)

Constructs a priority queue that contains the elements of a sorted set. The constructed priority queue has the initial capacity of 110% of the size of the sorted set. The priority queue will have the same comparator as the sorted set.

Parameters:

c - the sorted set whose elements will be added to the priority queue to be constructed.

Method Detail

iterator

public Iterator<E> iterator()

Gets the iterator of the priority queue, which will not return elements in any specified ordering.

Specified by:

iterator in interface Iterable<E>

Specified by:

iterator in interface Collection<E>

Specified by:

iterator in class AbstractCollection<E>

Returns:

the iterator of the priority queue.

size

public int size()

Gets the size of the priority queue. If the size of the queue is greater than the Integer.MAX, then it returns Integer.MAX.

Specified by:

size in interface Collection<E>

Specified by:

size in class AbstractCollection<E>

Returns:

the size of the priority queue.

clear

public void clear()

Removes all the elements of the priority queue.

Specified by:

clear in interface Collection<E>

Overrides:

clear in class AbstractQueue<E>

See Also:

AbstractCollection.iterator(), AbstractCollection.isEmpty(), AbstractCollection.size()

offer

public boolean offer(E o)

Inserts the element to the priority queue.

Parameters:

o - the element to add to the priority queue.

Returns:

always true

Throws:

ClassCastException - if the element cannot be compared with the elements in the priority queue using the ordering of the priority queue.

NullPointerException - if o is null.

poll

public E poll()

Gets and removes the head of the queue.

Returns:

the head of the queue or null if the queue is empty.

peek

public E peek()

Gets but does not remove the head of the queue.

Returns:

the head of the queue or null if the queue is empty.

comparator

public Comparator<? super E> comparator()

Gets the comparator of the priority queue.

Returns:

the comparator of the priority queue or null if the natural ordering is used.

remove

public boolean remove(Object o)

Removes the specified object from the priority queue.

Specified by:

remove in interface Collection<E>

Overrides:

remove in class AbstractCollection<E>

Parameters:

o - the object to be removed.

Returns:

true if the object was in the priority queue, false if the object was not in the priority queue.

add

public boolean add(E o)

Adds the specified object to the priority queue.

Specified by:

add in interface Collection<E>

Overrides:

add in class AbstractQueue<E>

Parameters:

o - the object to be added.

Returns:

always true.

Throws:

ClassCastException - if the element cannot be compared with the elements in the priority queue using the ordering of the priority queue.

NullPointerException - if o is null.

contains

public boolean contains(Object object)

Answers if there is an element in this queue equals to the object.

Specified by:

contains in interface Collection<E>

Overrides:

contains in class AbstractCollection<E>

Parameters:

object - the object to search for.

Returns:

true if object is an element of this Collection, false otherwise.

See Also:

AbstractCollection.contains(java.lang.Object)

toArray

public Object[] toArray()

Returns all the elements in an array. The result is a copy of all the elements.

Specified by:

toArray in interface Collection<E>

Overrides:

toArray in class AbstractCollection<E>

Returns:

the Array of all the elements

See Also:

AbstractCollection.toArray()

toArray

public <T> T[] toArray(T[] array)

Returns all the elements in an array, and the type of the result array is the type of the argument array. If the argument array is big enough, the elements from the queue will be stored in it(element immediately following the end of the queue is set to null, if any); otherwise, it will return a new array with the size of the argument array and size of the queue.

Specified by:

toArray in interface Collection<E>

Overrides:

toArray in class AbstractCollection<E>

Type Parameters:

T - the type of elements in the array

Parameters:

array - the array stores all the elements from the queue, if it has enough space; otherwise, a new array of the same type and the size of the queue will be used

Returns:

the Array of all the elements

Throws:

ArrayStoreException - if the type of the argument array is not compatible with every element in the queue

NullPointerException - if the argument array is null

See Also:

java.util.AbstractCollection#toArray(T[])