- 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 aComparator
provided at queue construction time, depending on which constructor is used. A priority queue does not permitnull
elements. A priority queue relying on natural ordering also does not permit insertion of non-comparable objects (doing so may result inClassCastException
).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
, andelement
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
andIterator
interfaces. The Iterator provided in methoditerator()
is not guaranteed to traverse the elements of the priority queue in any particular order. If you need ordered traversal, consider usingArrays.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-safePriorityBlockingQueue
class.Implementation note: this implementation provides O(log(n)) time for the enqueing and dequeing methods (
offer
,poll
,remove()
andadd
); linear time for theremove(Object)
andcontains(Object)
methods; and constant time for the retrieval methods (peek
,element
, andsize
).This class is a member of the Java Collections Framework.
- Since:
- 1.5
- See Also:
- Serialized Form
-
-
Constructor Summary
Constructors Constructor and Description PriorityQueue()
Creates aPriorityQueue
with the default initial capacity (11) that orders its elements according to their natural ordering.PriorityQueue(Collection<? extends E> c)
Creates aPriorityQueue
containing the elements in the specified collection.PriorityQueue(int initialCapacity)
Creates aPriorityQueue
with the specified initial capacity that orders its elements according to their natural ordering.PriorityQueue(int initialCapacity, Comparator<? super E> comparator)
Creates aPriorityQueue
with the specified initial capacity that orders its elements according to the specified comparator.PriorityQueue(PriorityQueue<? extends E> c)
Creates aPriorityQueue
containing the elements in the specified priority queue.PriorityQueue(SortedSet<? extends E> c)
Creates aPriorityQueue
containing the elements in the specified sorted set.
-
Method Summary
Methods Modifier and Type Method and Description 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, ornull
if this queue is sorted according to the natural ordering of its elements.boolean
contains(Object o)
Returnstrue
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.-
Methods inherited from class java.util.AbstractCollection
containsAll, isEmpty, removeAll, retainAll, toString
-
-
-
-
Constructor Detail
-
PriorityQueue
public PriorityQueue()
Creates aPriorityQueue
with the default initial capacity (11) that orders its elements according to their natural ordering.
-
PriorityQueue
public PriorityQueue(int initialCapacity)
Creates aPriorityQueue
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
- ifinitialCapacity
is less than 1
-
PriorityQueue
public PriorityQueue(int initialCapacity, Comparator<? super E> comparator)
Creates aPriorityQueue
with the specified initial capacity that orders its elements according to the specified comparator.- Parameters:
initialCapacity
- the initial capacity for this priority queuecomparator
- the comparator that will be used to order this priority queue. Ifnull
, the natural ordering of the elements will be used.- Throws:
IllegalArgumentException
- ifinitialCapacity
is less than 1
-
PriorityQueue
public PriorityQueue(Collection<? extends E> c)
Creates aPriorityQueue
containing the elements in the specified collection. If the specified collection is an instance of aSortedSet
or is anotherPriorityQueue
, 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 orderingNullPointerException
- if the specified collection or any of its elements are null
-
PriorityQueue
public PriorityQueue(PriorityQueue<? extends E> c)
Creates aPriorityQueue
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 ofc
cannot be compared to one another according toc
's orderingNullPointerException
- if the specified priority queue or any of its elements are null
-
PriorityQueue
public PriorityQueue(SortedSet<? extends E> c)
Creates aPriorityQueue
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 orderingNullPointerException
- if the specified sorted set or any of its elements are null
-
-
Method Detail
-
add
public boolean add(E e)
Inserts the specified element into this priority queue.- Specified by:
add
in interfaceCollection<E>
- Specified by:
add
in interfaceQueue<E>
- Overrides:
add
in classAbstractQueue<E>
- Parameters:
e
- the element to add- Returns:
true
(as specified byCollection.add(E)
)- Throws:
ClassCastException
- if the specified element cannot be compared with elements currently in this priority queue according to the priority queue's orderingNullPointerException
- if the specified element is null
-
offer
public boolean offer(E e)
Inserts the specified element into this priority queue.- Specified by:
offer
in interfaceQueue<E>
- Parameters:
e
- the element to add- Returns:
true
(as specified byQueue.offer(E)
)- Throws:
ClassCastException
- if the specified element cannot be compared with elements currently in this priority queue according to the priority queue's orderingNullPointerException
- 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.
-
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 elemente
such thato.equals(e)
, if this queue contains one or more such elements. Returnstrue
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 interfaceCollection<E>
- Overrides:
remove
in classAbstractCollection<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)
Returnstrue
if this queue contains the specified element. More formally, returnstrue
if and only if this queue contains at least one elemente
such thato.equals(e)
.- Specified by:
contains
in interfaceCollection<E>
- Overrides:
contains
in classAbstractCollection<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 interfaceCollection<E>
- Overrides:
toArray
in classAbstractCollection<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 interfaceCollection<E>
- Overrides:
toArray
in classAbstractCollection<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 queueNullPointerException
- 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 interfaceIterable<E>
- Specified by:
iterator
in interfaceCollection<E>
- Specified by:
iterator
in classAbstractCollection<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 interfaceCollection<E>
- Specified by:
size
in classAbstractCollection<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 interfaceCollection<E>
- Overrides:
clear
in classAbstractQueue<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.
-
comparator
public Comparator<? super E> comparator()
Returns the comparator used to order the elements in this queue, ornull
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
-
-
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