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java.util
Class PriorityQueue<E>
java.lang.Object
java.util.AbstractCollection<E>
java.util.AbstractQueue<E>
java.util.PriorityQueue<E>
-
-
Type Parameters:
-
E - the type of elements held in this collection
-
All Implemented Interfaces:
-
Serializable
,
Iterable
<E>,
Collection
<E>,
Queue
<E>
-
public class PriorityQueue<E>
- extends AbstractQueue<E>
- implements Serializable
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).
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,
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
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()).
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.
list structurally.
Instead, use the thread-safe
PriorityBlockingQueue
class.
Implementation note: this implementation provides O(log(n)) time for the
enqueing and dequeing
insertion
methods
(offer, poll,
(
offer
,
poll
,
remove()
and
add);
add
) methods;
linear time for the
remove(Object)
and
contains(Object)
methods; and constant time for the retrieval methods
(peek, element,
(
peek
,
element
,
and
size).
size
).
This class is a member of the
Java Collections Framework
Java Collections Framework
.
-
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
<? extends
E
Creates a
PriorityQueue
containing the elements in the specified collection. |
PriorityQueue
(int initialCapacity)
Creates a
PriorityQueue
with the specified initial capacity that orders its elements according to their
natural ordering
|
PriorityQueue
(int initialCapacity,
Comparator
<? super
E
Creates a
PriorityQueue
with the specified initial capacity that orders its elements according to the specified comparator. |
PriorityQueue
(
PriorityQueue
<? extends
E
Creates a
PriorityQueue
containing the elements in the specified priority queue. |
PriorityQueue
(
SortedSet
<? extends
E
Creates a
PriorityQueue
containing the elements in the specified sorted set. |
Method Summary
|
boolean |
add
(
E
e)
Inserts the specified element into this priority queue. |
void |
clear
()
Removes all of the elements from this priority queue. |
Comparator
<? super
E
|
comparator
()
Returns the comparator used to order the elements in this queue, or
null
if this queue is sorted according to the
natural ordering
|
boolean |
contains
(
Object
Returns
true
if this queue contains the specified element. |
Iterator
<
E
> |
iterator
()
Returns an iterator over the elements in this queue. |
boolean |
offer
(
E
e)
Inserts the specified element into this priority queue. |
E
|
peek
()
Retrieves, but does not remove, the head of this queue, or returns
null
if this queue is empty. |
E
|
poll
()
Retrieves and removes the head of this queue, or returns
null
if this queue is empty. |
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 collection. |
Object
|
toArray
()
Returns an array containing all of the elements in this queue.
|
<T> T[]
|
|
toArray
(T[] 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.
|
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:
-
IllegalArgumentException
- if
initialCapacity
is less than 1
PriorityQueue
public PriorityQueue(int initialCapacity,
Comparator<? super E> 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
null,
null
,
the
natural ordering
natural ordering
of the elements will be used. -
Throws:
-
IllegalArgumentException
- if
initialCapacity
is less than 1
PriorityQueue
public PriorityQueue(Collection<? extends E> c)
-
Creates a PriorityQueue containing the elements in the specified collection.
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 or 1 if the collection is empty.
If the specified collection is an instance of a
SortedSet
or is another
PriorityQueue, this
PriorityQueue
, the
priority queue will be ordered according to the same ordering. Otherwise, this priority queue will be ordered according to the
natural ordering
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
PriorityQueue
public PriorityQueue(PriorityQueue<? extends E> c)
-
Creates a PriorityQueue containing the elements in the specified priority queue.
Creates a
PriorityQueue
containing the elements in the specified priority queue. The priority queue has an initial capacity of 110% of the size of the specified priority queue or 1 if the priority queue is empty.
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
c
cannot be compared to one another according to
c's
c
's
ordering -
NullPointerException
- if the specified priority queue or any of its elements are null
PriorityQueue
public PriorityQueue(SortedSet<? extends E> c)
-
Creates a PriorityQueue containing the elements in the specified sorted set.
Creates a
PriorityQueue
containing the elements in the specified sorted set. The priority queue has an initial capacity of 110% of the size of the specified sorted set or 1 if the sorted set is empty.
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
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
add
public boolean add(E e)
-
Inserts the specified element into this priority queue.
-
-
Specified by:
-
add
in interface
Collection
<
E
>
-
Specified by:
-
add
in interface
Queue
<
E
>
-
Overrides:
-
add
in class
AbstractQueue
<
E
>
-
-
Parameters:
-
e - the element to add
-
Returns:
-
true
(as specified by
Collection.add(E)
)
-
Throws:
-
ClassCastException
- if the specified element cannot be compared with elements currently in this priority queue according to the priority queue's ordering
-
NullPointerException
- if the specified element is null
offer
public boolean offer(E e)
-
Inserts the specified element into this priority queue.
-
-
Specified by:
-
offer
in interface
Queue
<
E
>
-
-
Parameters:
-
e - the element to add
-
Returns:
-
true
(as specified by
Queue.offer(E)
)
-
Throws:
-
ClassCastException
- if the specified element cannot be compared with elements currently in this priority queue according to the priority queue's ordering
-
NullPointerException
- if the specified element is null
peek
public E peek()
-
Description copied from interface:
Queue
-
Retrieves, but does not remove, the head of this queue, or returns
null
if this queue is empty.
-
-
Specified by:
-
peek
in interface
Queue
<
E
>
-
-
Returns:
-
the head of this queue, or
null
if this queue is empty
remove
public boolean remove(Object o)
-
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),
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).
-
-
Specified by:
-
remove
in interface
Collection
<
E
>
-
Overrides:
-
remove
in class
AbstractCollection
<
E
>
-
-
Parameters:
-
o - element to be removed from this queue, if present
-
Returns:
-
true
if this queue changed as a result of the call
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).
o.equals(e)
.
-
-
Specified by:
-
contains
in interface
Collection
<
E
>
-
Overrides:
-
contains
in class
AbstractCollection
<
E
>
-
-
Parameters:
-
o - object to be checked for containment in this queue
-
Returns:
-
true
if this queue contains the specified element
toArray
public
Object
[]
toArray
()
-
Returns an array containing all of the elements in this queue. The 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
Collection
<
E
>
-
Overrides:
-
toArray
in class
AbstractCollection
<
E
>
-
-
Returns:
-
an array containing all of the elements in this queue
toArray
public <T> T[]
toArray
(T[] 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 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
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
Collection
<
E
>
-
Overrides:
-
toArray
in class
AbstractCollection
<
E
>
-
-
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
iterator
public Iterator<E> iterator()
-
Returns an iterator over the elements in this queue. The iterator does not return the elements in any particular order.
-
-
Specified by:
-
iterator
in interface
Iterable
<
E
>
-
Specified by:
-
iterator
in interface
Collection
<
E
>
-
Specified by:
-
iterator
in class
AbstractCollection
<
E
>
-
-
Returns:
-
an iterator over the elements in this queue
size
public int size()
-
Description copied from interface:
Collection
-
Returns the number of elements in this collection. If this collection contains more than
Integer.MAX_VALUE
elements, returns
Integer.MAX_VALUE
.
-
-
Specified by:
-
size
in interface
Collection
<
E
>
-
Specified by:
-
size
in class
AbstractCollection
<
E
>
-
-
Returns:
-
the number of elements in this collection
clear
public void clear()
-
Removes all of the elements from this priority queue. The queue will be empty after this call returns.
-
-
Specified by:
-
clear
in interface
Collection
<
E
>
-
Overrides:
-
clear
in class
AbstractQueue
<
E
>
-
poll
public E poll()
-
Description copied from interface:
Queue
-
Retrieves and removes the head of this queue, or returns
null
if this queue is empty.
-
-
Specified by:
-
poll
in interface
Queue
<
E
>
-
-
Returns:
-
the head of this queue, or
null
if this queue is empty
comparator
public Comparator<? super E> 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 this queue, or
null
if this queue is sorted according to the natural ordering of its
elements
elements.