edu.emory.mathcs.backport.java.util

Class PriorityQueue

public class PriorityQueue extends AbstractQueue implements Serializable

An unbounded queue that supports element retrieval in the order of relative priority. The ordering can be defined via an explicit comparator; otherwise, the natural ordering of elements is used. Element at the head of the queue is always the smallest one according to the given ordering.

While this queue is logically unbounded, attempted additions may fail due to resource exhaustion (causing OutOfMemoryError). This class does not permit null elements. A priority queue relying on Comparable natural ordering also does not permit insertion of non-comparable objects (doing so results in ClassCastException).

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 PriorityQueue in any particular order. If you need ordered traversal, consider using Arrays.sort(pq.toArray()).

Operations on this class make no guarantees about the ordering of elements with equal priority. If you need to enforce an ordering, you can define custom classes or comparators that use a secondary key to break ties in primary priority values. See PriorityBlockingQueue for an example.

Implementation note: basic mutative methods (insert, offer, remove, poll etc) have complexity O(log(n)). Parameterless inspection methods (peek, element,isEmpty) have complexity O(1). Methods contains(Object) and remove(Object) have complexity O(n).

Since: 1.5

Author: Dawid Kurzyniec

Constructor Summary
PriorityQueue()
Creates a PriorityQueue with the default initial capacity (11) that orders its elements according to their Comparable natural ordering.
PriorityQueue(int initialCapacity)
Creates a PriorityQueue with the specified initial capacity that orders its elements according to their Comparable natural ordering.
PriorityQueue(Comparator comparator)
Creates a PriorityQueue with the specified initial capacity that orders its elements according to the specified comparator.
PriorityQueue(int initialCapacity, Comparator comparator)
PriorityQueue(PriorityQueue q)
Creates a PriorityQueue containing the elements from the specified priority queue.
PriorityQueue(SortedSet s)
Creates a PriorityQueue containing the elements from the specified sorted set.
PriorityQueue(Collection c)
Creates a PriorityQueue containing the elements in the specified collection.
Method Summary
booleanadd(Object o)
Inserts the specified element into this priority queue.
voidclear()
Removes all of the elements from this queue.
Comparatorcomparator()
Returns the comparator used to order the elements in this queue, or null if this queue uses the Comparable natural ordering of its elements.
booleancontains(Object o)
Returns true if this queue contains the specified element.
Objectelement()
Retrieves, but does not remove, the head of this queue.
booleanisEmpty()
Returns true if this queue contains no elements.
Iteratoriterator()
Returns an iterator over the elements in this queue.
booleanoffer(Object o)
Inserts the specified element into this priority queue.
Objectpeek()
Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty.
Objectpoll()
Retrieves and removes the head of this queue, or returns null if this queue is empty.
Objectremove()
Retrieves and removes the head of this queue.
booleanremove(Object o)
Removes a single instance of the specified element from this queue, if it is present.
intsize()
Returns the number of elements in this priority queue.
Object[]toArray()
Returns an array containing all of the elements in this queue.
Object[]toArray(Object[] 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.

Constructor Detail

PriorityQueue

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

PriorityQueue

public PriorityQueue(int initialCapacity)
Creates a PriorityQueue with the specified initial capacity that orders its elements according to their Comparable natural ordering.

Parameters: initialCapacity the initial capacity for this priority queue

Throws: IllegalArgumentException if initialCapacity is less than 1

PriorityQueue

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

Parameters: comparator the comparator used to order this priority queue. If null then the order depends on the elements' natural ordering.

Throws: IllegalArgumentException if initialCapacity is less than 1

PriorityQueue

public PriorityQueue(int initialCapacity, Comparator comparator)

PriorityQueue

public PriorityQueue(PriorityQueue q)
Creates a PriorityQueue containing the elements from the specified priority queue. This priority queue has an initial capacity of 110% of the size of the specified queue, and it is sorted according to the same comparator as the specified queue, or according to the natural ordering of its elements if the specified queue is sorted according to the natural ordering of its elements.

Parameters: q the queue whose elements are to be placed into this priority queue.

Throws: NullPointerException if the specified queue is null

PriorityQueue

public PriorityQueue(SortedSet s)
Creates a PriorityQueue containing the elements from the specified sorted set. This priority queue has an initial capacity of 110% of the size of the specified set, and it is sorted according to the same comparator as the specified set, or according to the natural ordering of its elements if the specified set is sorted according to the natural ordering of its elements.

Parameters: s the set whose elements are to be placed into this priority queue.

Throws: NullPointerException if the specified set or any of its elements are null

PriorityQueue

public PriorityQueue(Collection c)
Creates a PriorityQueue containing the elements in the specified collection. The priority queue has an initial capacity of 110% of the size of the specified collection. If the specified collection is a java.util.SortedSet or a PriorityQueue, this priority queue will be sorted according to the same comparator, or according to the natural ordering of its elements if the collection is sorted according to the natural ordering of its elements. Otherwise, this priority queue is 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 ordering. NullPointerException if the specified collection or any of its elements are null

Method Detail

add

public boolean add(Object o)
Inserts the specified element into this priority queue.

Parameters: o the element to add

Returns: true (as per the spec for Collection#add)

Throws: ClassCastException if the specified element cannot be compared with elements currently in the priority queue according to the priority queue's ordering NullPointerException if the specified element is null

clear

public void clear()
Removes all of the elements from this queue. The queue will be empty after this call returns.

comparator

public Comparator comparator()
Returns the comparator used to order the elements in this queue, or null if this queue uses the Comparable natural ordering of its elements.

Returns: the comparator used to order the elements in this queue, or null if this queue uses the natural ordering of its elements.

contains

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

element

public Object element()
Retrieves, but does not remove, the head of this queue.

Returns: the head of this queue

Throws: NoSuchElementException of the queue is empty

isEmpty

public boolean isEmpty()
Returns true if this queue contains no elements.

Returns: true if this queue contains no elements

iterator

public Iterator iterator()
Returns an iterator over the elements in this queue. The iterator does not return the elements in any particular order. The returned iterator is a thread-safe "fast-fail" iterator that will throw java.util.ConcurrentModificationException upon detected interference.

Returns: an iterator over the elements in this queue

offer

public boolean offer(Object o)
Inserts the specified element into this priority queue.

Parameters: o the element to add

Returns: true (as per the spec for Queue)

Throws: ClassCastException if the specified element cannot be compared with elements currently in the priority queue according to the priority queue's ordering NullPointerException if the specified element is null

peek

public Object peek()
Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty.

Returns: the head of this queue, or null if this queue is empty

poll

public Object poll()
Retrieves and removes the head of this queue, or returns null if this queue is empty.

Returns: the head of this queue, or null if this queue is empty

remove

public Object remove()
Retrieves and removes the head of this queue.

Returns: the head of this queue

remove

public boolean remove(Object o)
Removes a single instance of the specified element from this queue, if it is present. Returns true 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

size

public int size()
Returns the number of elements in this priority queue.

Returns: the number of elements in this priority queue

toArray

public Object[] toArray()
Returns an array containing all of the elements in this queue. The returned array 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

toArray

public Object[] toArray(Object[] 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 this queue fits in the specified array with room to spare (i.e., the array has more elements than this queue), the element in the array immediately following the end of the queue 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