edu.emory.mathcs.backport.java.util

Class PriorityQueue

Implemented Interfaces:
Collection, java.io.Serializable, Queue

public class PriorityQueue
extends AbstractQueue
implements java.io.Serializable, Queue

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).

Author:
Dawid Kurzyniec
Since:
1.5
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(Collection c)
Creates a PriorityQueue containing the elements in the specified collection.
PriorityQueue(Comparator comparator)
Creates a PriorityQueue with the specified initial capacity that orders its elements according to the specified comparator.
PriorityQueue(SortedSet s)
Creates a PriorityQueue containing the elements from the specified sorted set.
PriorityQueue(PriorityQueue q)
Creates a PriorityQueue containing the elements from the specified priority queue.
PriorityQueue(int initialCapacity)
Creates a PriorityQueue with the specified initial capacity that orders its elements according to their natural ordering.
PriorityQueue(int initialCapacity, Comparator comparator)

Method Summary

boolean
add(Object o)
Inserts the specified element into this priority queue.
void
clear()
Removes all of the elements from this queue.
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(Object o)
Returns true if this queue contains the specified element.
Object
element()
Retrieves, but does not remove, the head of this queue.
boolean
isEmpty()
Returns true if this queue contains no elements.
Iterator
iterator()
Returns an iterator over the elements in this queue.
boolean
offer(Object o)
Inserts the specified element into this priority queue.
Object
peek()
Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty.
Object
poll()
Retrieves and removes the head of this queue, or returns null if this queue is empty.
Object
remove()
Retrieves and removes the head of this queue.
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 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.

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

add, addAll, clear, element, remove

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

toArray, toArray

Constructor Details

PriorityQueue

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

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.

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.

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.

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.

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

PriorityQueue

public PriorityQueue(int initialCapacity,
                     Comparator comparator)

Method Details

add

public boolean add(Object o)
Inserts the specified element into this priority queue.
Specified by:
add in interface Queue
Overrides:
add in interface AbstractQueue
Parameters:
o - the element to add
Returns:
true (as per the spec for Collection.add)

clear

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

comparator

public 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.

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.
Specified by:
element in interface Queue
Overrides:
element in interface AbstractQueue
Returns:
the head of this queue

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.
Specified by:
offer in interface Queue
Parameters:
o - the element to add
Returns:
true (as per the spec for Queue.offer(Object))

peek

public 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 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

remove

public Object remove()
Retrieves and removes the head of this queue.
Specified by:
remove in interface Queue
Overrides:
remove in interface AbstractQueue
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.

Overrides:
toArray in interface AbstractCollection
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().
Overrides:
toArray in interface 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