PriorityQueue (Java Platform SE 6) (original) (raw)
java.util
Class PriorityQueue
java.lang.Object
java.util.AbstractCollection
java.util.AbstractQueue
java.util.PriorityQueue
Type Parameters:
E - the type of elements held in this collection
All Implemented Interfaces:
Serializable, Iterable, Collection, Queue
public class PriorityQueue
extends AbstractQueue
implements Serializable
An unbounded priority queue based on a priority heap. The elements of the priority queue are ordered according to theirnatural 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 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, 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()).
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.
Since:
1.5
See Also:
| Constructor Summary |
|---|
| PriorityQueue() Creates a PriorityQueue with the default initial capacity (11) that orders its elements according to theirnatural ordering. |
| PriorityQueue(Collection<? extends E> c) 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 theirnatural ordering. |
| [PriorityQueue](../../java/util/PriorityQueue.html#PriorityQueue%28int, java.util.Comparator%29)(int initialCapacity,Comparator<? super E> comparator) Creates a PriorityQueue with the specified initial capacity that orders its elements according to the specified comparator. |
| PriorityQueue(PriorityQueue<? extends E> c) Creates a PriorityQueue containing the elements in the specified priority queue. |
| PriorityQueue(SortedSet<? extends E> c) 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 of its elements. |
| boolean | contains(Object o) 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[] | 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.AbstractQueue |
|---|
| addAll, element, remove |
| Methods inherited from class java.util.AbstractCollection |
|---|
| containsAll, isEmpty, removeAll, retainAll, toString |
| Methods inherited from class java.lang.Object |
|---|
| clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, [wait](../../java/lang/Object.html#wait%28long, int%29) |
| Methods inherited from interface java.util.Collection |
|---|
| containsAll, equals, hashCode, isEmpty, removeAll, retainAll |
| Constructor Detail |
|---|
PriorityQueue
public PriorityQueue()
Creates a PriorityQueue with the default initial capacity (11) that orders its elements according to theirnatural ordering.
PriorityQueue
public PriorityQueue(int initialCapacity)
Creates a PriorityQueue with the specified initial capacity that orders its elements according to theirnatural ordering.
Parameters:
initialCapacity - the initial capacity for this priority queue
Throws:
[IllegalArgumentException](../../java/lang/IllegalArgumentException.html "class in java.lang") - 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, the natural ordering of the elements will be used.
Throws:
[IllegalArgumentException](../../java/lang/IllegalArgumentException.html "class in java.lang") - if initialCapacity is less than 1
PriorityQueue
public PriorityQueue(Collection<? extends E> c)
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 thenatural ordering of its elements.
Parameters:
c - the collection whose elements are to be placed into this priority queue
Throws:
[ClassCastException](../../java/lang/ClassCastException.html "class in java.lang") - if elements of the specified collection cannot be compared to one another according to the priority queue's ordering
[NullPointerException](../../java/lang/NullPointerException.html "class in java.lang") - 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. 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](../../java/lang/ClassCastException.html "class in java.lang") - if elements of c cannot be compared to one another according to c's ordering
[NullPointerException](../../java/lang/NullPointerException.html "class in java.lang") - 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. 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](../../java/lang/ClassCastException.html "class in java.lang") - if elements of the specified sorted set cannot be compared to one another according to the sorted set's ordering
[NullPointerException](../../java/lang/NullPointerException.html "class in java.lang") - 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](../../java/util/Collection.html#add%28E%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/PriorityQueue.html "type parameter in PriorityQueue")>
Specified by:
[add](../../java/util/Queue.html#add%28E%29) in interface [Queue](../../java/util/Queue.html "interface in java.util")<[E](../../java/util/PriorityQueue.html "type parameter in PriorityQueue")>
Overrides:
[add](../../java/util/AbstractQueue.html#add%28E%29) in class [AbstractQueue](../../java/util/AbstractQueue.html "class in java.util")<[E](../../java/util/PriorityQueue.html "type parameter in PriorityQueue")>
Parameters:
e - the element to add
Returns:
true (as specified by Collection.add(E))
Throws:
[ClassCastException](../../java/lang/ClassCastException.html "class in java.lang") - if the specified element cannot be compared with elements currently in this priority queue according to the priority queue's ordering
[NullPointerException](../../java/lang/NullPointerException.html "class in java.lang") - if the specified element is null
offer
public boolean offer(E e)
Inserts the specified element into this priority queue.
Specified by:
[offer](../../java/util/Queue.html#offer%28E%29) in interface [Queue](../../java/util/Queue.html "interface in java.util")<[E](../../java/util/PriorityQueue.html "type parameter in PriorityQueue")>
Parameters:
e - the element to add
Returns:
true (as specified by Queue.offer(E))
Throws:
[ClassCastException](../../java/lang/ClassCastException.html "class in java.lang") - if the specified element cannot be compared with elements currently in this priority queue according to the priority queue's ordering
[NullPointerException](../../java/lang/NullPointerException.html "class in java.lang") - if the specified element is null
peek
public E peek()
Description copied from interface: [Queue](../../java/util/Queue.html#peek%28%29)
Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty.
Specified by:
[peek](../../java/util/Queue.html#peek%28%29) in interface [Queue](../../java/util/Queue.html "interface in java.util")<[E](../../java/util/PriorityQueue.html "type parameter in PriorityQueue")>
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), 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](../../java/util/Collection.html#remove%28java.lang.Object%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/PriorityQueue.html "type parameter in PriorityQueue")>
Overrides:
[remove](../../java/util/AbstractCollection.html#remove%28java.lang.Object%29) in class [AbstractCollection](../../java/util/AbstractCollection.html "class in java.util")<[E](../../java/util/PriorityQueue.html "type parameter in PriorityQueue")>
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).
Specified by:
[contains](../../java/util/Collection.html#contains%28java.lang.Object%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/PriorityQueue.html "type parameter in PriorityQueue")>
Overrides:
[contains](../../java/util/AbstractCollection.html#contains%28java.lang.Object%29) in class [AbstractCollection](../../java/util/AbstractCollection.html "class in java.util")<[E](../../java/util/PriorityQueue.html "type parameter in PriorityQueue")>
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](../../java/util/Collection.html#toArray%28%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/PriorityQueue.html "type parameter in PriorityQueue")>
Overrides:
[toArray](../../java/util/AbstractCollection.html#toArray%28%29) in class [AbstractCollection](../../java/util/AbstractCollection.html "class in java.util")<[E](../../java/util/PriorityQueue.html "type parameter in PriorityQueue")>
Returns:
an array containing all of the elements in this queue
toArray
public 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 tonull.
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 totoArray().
Specified by:
[toArray](../../java/util/Collection.html#toArray%28T[]%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/PriorityQueue.html "type parameter in PriorityQueue")>
Overrides:
[toArray](../../java/util/AbstractCollection.html#toArray%28T[]%29) in class [AbstractCollection](../../java/util/AbstractCollection.html "class in java.util")<[E](../../java/util/PriorityQueue.html "type parameter in PriorityQueue")>
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](../../java/lang/ArrayStoreException.html "class in java.lang") - if the runtime type of the specified array is not a supertype of the runtime type of every element in this queue
[NullPointerException](../../java/lang/NullPointerException.html "class in java.lang") - if the specified array is null
iterator
Returns an iterator over the elements in this queue. The iterator does not return the elements in any particular order.
Specified by:
[iterator](../../java/lang/Iterable.html#iterator%28%29) in interface [Iterable](../../java/lang/Iterable.html "interface in java.lang")<[E](../../java/util/PriorityQueue.html "type parameter in PriorityQueue")>
Specified by:
[iterator](../../java/util/Collection.html#iterator%28%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/PriorityQueue.html "type parameter in PriorityQueue")>
Specified by:
[iterator](../../java/util/AbstractCollection.html#iterator%28%29) in class [AbstractCollection](../../java/util/AbstractCollection.html "class in java.util")<[E](../../java/util/PriorityQueue.html "type parameter in PriorityQueue")>
Returns:
an iterator over the elements in this queue
size
public int size()
Description copied from interface: [Collection](../../java/util/Collection.html#size%28%29)
Returns the number of elements in this collection. If this collection contains more than Integer.MAX_VALUE elements, returnsInteger.MAX_VALUE.
Specified by:
[size](../../java/util/Collection.html#size%28%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/PriorityQueue.html "type parameter in PriorityQueue")>
Specified by:
[size](../../java/util/AbstractCollection.html#size%28%29) in class [AbstractCollection](../../java/util/AbstractCollection.html "class in java.util")<[E](../../java/util/PriorityQueue.html "type parameter in PriorityQueue")>
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](../../java/util/Collection.html#clear%28%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/PriorityQueue.html "type parameter in PriorityQueue")>
Overrides:
[clear](../../java/util/AbstractQueue.html#clear%28%29) in class [AbstractQueue](../../java/util/AbstractQueue.html "class in java.util")<[E](../../java/util/PriorityQueue.html "type parameter in PriorityQueue")>
poll
public E poll()
Description copied from interface: [Queue](../../java/util/Queue.html#poll%28%29)
Retrieves and removes the head of this queue, or returns null if this queue is empty.
Specified by:
[poll](../../java/util/Queue.html#poll%28%29) in interface [Queue](../../java/util/Queue.html "interface in java.util")<[E](../../java/util/PriorityQueue.html "type parameter in PriorityQueue")>
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, ornull if this queue is sorted according to the natural ordering of its elements
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