PriorityQueue JDK 1.6)

Description

public class PriorityQueue

An unbounded priority queue based on a priority heap.

An unbounded priority queue based on a priority heap. The elements of the priority queue are ordered according to their 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 PriorityQueue.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 enqueing and dequeing 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.

See also:

Constructors

public PriorityQueue ()

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

public PriorityQueue (Collection<Object> c)

Creates a PriorityQueue containing the elements in the specified collection.

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
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 (int initialCapacity)

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

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:
- IllegalArgumentException - if {@code initialCapacity} is less than 1

public PriorityQueue (int initialCapacity, Comparator<Object> comparator)

Creates a PriorityQueue with the specified initial capacity that orders its elements according to the specified 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 {@code null}, the {@linkplain Comparable natural ordering} of the elements will be used.
Throws:
- IllegalArgumentException - if {@code initialCapacity} is less than 1

public PriorityQueue (PriorityQueue<Object> c)

Creates a PriorityQueue containing the elements in the specified priority queue.

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 {@code c} cannot be compared to one another according to {@code c}'s ordering
- NullPointerException - if the specified priority queue or any of its elements are null

public PriorityQueue (SortedSet<Object> c)

Creates a PriorityQueue containing the elements in the specified sorted set.

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

Methods

Hide/Show inherited methods

public boolean add ( E e) [Overrides AbstractQueue]

Inserts the specified element into this priority queue.

public boolean addAll (Collection<Object> c) [Inherited From AbstractQueue]

Adds all of the elements in the specified collection to this queue.

public void clear () [Overrides AbstractQueue]

Removes all of the elements from this priority queue.

public Comparator<Object> comparator ()

Returns the comparator used to order the elements in this queue, or null if this queue is sorted according to the natural ordering of its elements.

Returns the comparator used to order the elements in this
queue, or null if this queue is sorted according to
the natural ordering of its elements.
Returns: the comparator used to order this queue, or {@code null} if this queue is sorted according to the natural ordering of its elements

public boolean contains (Object o) [Overrides AbstractCollection]

Returns true if this queue contains the specified element.

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).
Returns: {@code true} if this queue contains the specified element
Parameters:
- o - object to be checked for containment in this queue

public boolean containsAll (Collection<Object> c) [Inherited From AbstractCollection]

Returns true if this collection contains all of the elements in the specified collection.

public E element () [Inherited From AbstractQueue]

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

public boolean isEmpty () [Inherited From AbstractCollection]

Returns true if this collection contains no elements.

public Iterator< E> iterator () [Specified in AbstractCollection]

Returns an iterator over the elements in this queue.

public boolean offer ( E e)

Inserts the specified element into this priority queue.

public E peek ()

public E poll ()

public E remove () [Inherited From AbstractQueue]

Retrieves and removes the head of this queue.

public boolean remove (Object o) [Overrides AbstractCollection]

Removes a single instance of the specified element from this queue, if it is present.

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).
Returns: {@code true} if this queue changed as a result of the call
Parameters:
- o - element to be removed from this queue, if present

public boolean removeAll (Collection<Object> c) [Inherited From AbstractCollection]

Removes all of this collection's elements that are also contained in the specified collection (optional operation).

pack-private boolean removeEq (Object o)

Version of remove using reference equality, not equals.

public boolean retainAll (Collection<Object> c) [Inherited From AbstractCollection]

Retains only the elements in this collection that are contained in the specified collection (optional operation).

public int size () [Specified in AbstractCollection]

public Object toArray () [Overrides AbstractCollection]

Returns an array containing all of the elements in this queue.

public T toArray ( T a) [Overrides AbstractCollection]

Returns an array containing all of the elements in this queue; the runtime type of the returned array is that of the specified array.

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 PriorityQueue.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().
Returns: an array containing all of the elements in this queue
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.
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 String toString () [Inherited From AbstractCollection]

Returns a string representation of this collection.