edu.emory.mathcs.backport.java.util
Class PriorityQueue

java.lang.Object
  java.util.AbstractCollection
      edu.emory.mathcs.backport.java.util.AbstractCollection
          edu.emory.mathcs.backport.java.util.AbstractQueue
              edu.emory.mathcs.backport.java.util.PriorityQueue

All Implemented Interfaces:

java.util.Collection, Queue, java.io.Serializable

public class PriorityQueue

extends AbstractQueue

implements java.io.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 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

See Also:

Serialized Form

Constructor Summary

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

PriorityQueue(java.util.Collection c)
Creates a PriorityQueue containing the elements in the specified collection.

PriorityQueue(java.util.Comparator comparator)
Creates a PriorityQueue with the specified initial capacity that orders its elements 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, java.util.Comparator comparator)

PriorityQueue(PriorityQueue q)
Creates a PriorityQueue containing the elements from the specified priority queue.

PriorityQueue(java.util.SortedSet s)
Creates a PriorityQueue containing the elements from the specified sorted set.

Method Summary

boolean	add(java.lang.Object o) Inserts the specified element into this priority queue.
void	clear() Removes all of the elements from this queue.
java.util.Comparator	comparator() Returns the comparator used to order the elements in this queue, or null if this queue uses the natural ordering of its elements.
boolean	contains(java.lang.Object o) Returns true if this queue contains the specified element.
java.lang.Object	element() Retrieves, but does not remove, the head of this queue.
boolean	isEmpty() Returns true if this queue contains no elements.
java.util.Iterator	iterator() Returns an iterator over the elements in this queue.
boolean	offer(java.lang.Object o) Inserts the specified element into this priority queue.
java.lang.Object	peek() Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty.
java.lang.Object	poll() Retrieves and removes the head of this queue, or returns null if this queue is empty.
java.lang.Object	remove() Retrieves and removes the head of this queue.
boolean	remove(java.lang.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 priority queue.
java.lang.Object[]	toArray() Returns an array containing all of the elements in this queue.
java.lang.Object[]	toArray(java.lang.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.

Methods inherited from class edu.emory.mathcs.backport.java.util.AbstractQueue

addAll

Methods inherited from class java.util.AbstractCollection

containsAll, removeAll, retainAll, toString

Methods inherited from class java.lang.Object

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

Methods inherited from interface java.util.Collection

containsAll, equals, hashCode, removeAll, retainAll

Constructor Detail

PriorityQueue

public PriorityQueue()

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

PriorityQueue

public PriorityQueue(int initialCapacity)

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

Parameters:

initialCapacity - the initial capacity for this priority queue

Throws:

java.lang.IllegalArgumentException - if initialCapacity is less than 1

PriorityQueue

public PriorityQueue(java.util.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:

java.lang.IllegalArgumentException - if initialCapacity is less than 1

PriorityQueue

public PriorityQueue(int initialCapacity,
                     java.util.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:

java.lang.NullPointerException - if the specified queue is null

PriorityQueue

public PriorityQueue(java.util.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:

java.lang.NullPointerException - if the specified set or any of its elements are null

PriorityQueue

public PriorityQueue(java.util.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 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:

java.lang.ClassCastException - if elements of the specified collection cannot be compared to one another according to the priority queue's ordering.

java.lang.NullPointerException - if the specified collection or any of its elements are null

Method Detail

iterator

public java.util.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 ConcurrentModificationException upon detected interference.

Specified by:

iterator in interface java.util.Collection

Returns:

an iterator over the elements in this queue

comparator

public java.util.Comparator comparator()

Returns the comparator used to order the elements in this queue, or null if this queue uses the 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.

offer

public boolean offer(java.lang.Object o)

Inserts the specified element into this priority queue.

Specified by:

offer in interface Queue

Parameters:

o - the element to add

Returns:

true (as per the spec for Queue.offer(java.lang.Object))

Throws:

java.lang.ClassCastException - if the specified element cannot be compared with elements currently in the priority queue according to the priority queue's ordering

java.lang.NullPointerException - if the specified element is null

peek

public java.lang.Object peek()

Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty.

Specified by:

peek in interface Queue

Returns:

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

poll

public java.lang.Object poll()

Retrieves and removes the head of this queue, or returns null if this queue is empty.

Specified by:

poll in interface Queue

Returns:

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

size

public int size()

Returns the number of elements in this priority queue.

Specified by:

size in interface java.util.Collection

Returns:

the number of elements in this priority queue

add

public boolean add(java.lang.Object o)

Inserts the specified element into this priority queue.

Specified by:

add in interface Queue

Overrides:

add in class AbstractQueue

Parameters:

o - the element to add

Returns:

true (as per the spec for Collection.add(java.lang.Object))

Throws:

java.lang.ClassCastException - if the specified element cannot be compared with elements currently in the priority queue according to the priority queue's ordering

java.lang.NullPointerException - if the specified element is null

remove

public java.lang.Object remove()

Retrieves and removes the head of this queue.

Specified by:

remove in interface Queue

Overrides:

remove in class AbstractQueue

Returns:

the head of this queue

element

public java.lang.Object element()

Retrieves, but does not remove, the head of this queue.

Specified by:

element in interface Queue

Overrides:

element in class AbstractQueue

Returns:

the head of this queue

Throws:

java.util.NoSuchElementException - of the queue is empty

isEmpty

public boolean isEmpty()

Returns true if this queue contains no elements.

Specified by:

isEmpty in interface java.util.Collection

Returns:

true if this queue contains no elements

contains

public boolean contains(java.lang.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).

Specified by:

contains in interface java.util.Collection

Parameters:

o - object to be checked for containment in this queue

Returns:

true if this queue contains the specified element

toArray

public java.lang.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.

Specified by:

toArray in interface java.util.Collection

Overrides:

toArray in class AbstractCollection

Returns:

an array containing all of the elements in this queue

toArray

public java.lang.Object[] toArray(java.lang.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().

Specified by:

toArray in interface java.util.Collection

Overrides:

toArray in class AbstractCollection

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:

java.lang.ArrayStoreException - if the runtime type of the specified array is not a supertype of the runtime type of every element in this queue

java.lang.NullPointerException - if the specified array is null

remove

public boolean remove(java.lang.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).

Specified by:

remove in interface java.util.Collection

Parameters:

o - element to be removed from this queue, if present

Returns:

true if this queue changed as a result of the call

clear

public void clear()

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

Specified by:

clear in interface java.util.Collection

Overrides:

clear in class AbstractQueue