E - the type of objects stored in the queuepublic class BoundedPriorityQueue<E> extends AbstractSet<E> implements Queue<E>, SortedSet<E>
BoundedPriorityQueue implements a priority queue
with an upper bound on the number of elements. If the queue is not
full, added elements are always added. If the queue is full and
the added element is greater than the smallest element in the
queue, the smallest element is removed and the new element is
added. If the queue is full and the added element is not greater
than the smallest element in the queue, the new element is not
added.
Bounded priority queues are the ideal data structure with which
to implement n-best accumulators. A priority queue of bound
n can find the n-best elements of a
collection of m elements using O(n) space
and O(m n log n) time.
Bounded priority queues may also be used as the basis of a search implementation with the bound implementing heuristic n-best pruning.
Because bounded priority queues require a comparator and a
maximum size constraint, they do not comply with the recommendation
in the Collection interface in that they neither
implement a nullary constructor nor a constructor taking a single
collection. Instead, they are constructed with a comparator and
a maximum size bound.
Implementation Note: Priority queues are implemented on
top of TreeSet with element wrappers to adapt object
equality and the priority comparator. Because tree sets implement
balanced trees, the priority queue operations,
add(Object), pop() and
peek(), all require O(log n) time where
n is the size of the queue. A standard heap-based
implementation of a queue implements peeks in constant time and
adds and pops in O(log n) time. For our intended
applications, pops are more likely than peeks and we need access to
the worst element in the queue. An upside-down ordered heap
implementation of priority queues implements bounded adds most
efficiently, but requires up to O(n) for a pop or peek
and an O(n log n) sort before iteration.
| Constructor and Description |
|---|
BoundedPriorityQueue(Comparator<? super E> comparator,
int maxSize)
Construct a bounded priority queue which uses the specified
comparator to order elements and allows up to the specified
maximum number of elements.
|
| Modifier and Type | Method and Description |
|---|---|
boolean |
add(E o)
Throw an unsupported operation exception.
|
void |
clear()
Removes all elements from this queue.
|
Comparator<? super E> |
comparator()
Returns the comparator for this sorted set.
|
E |
element()
Returns the highest scoring object in this priority queue,
throwing an exception if the queue is empty.
|
E |
first()
Returns the highest scoring object in the priority queue.
|
SortedSet<E> |
headSet(E toElement)
Return the set of elements in this queue strictly less than the
specified element according to the comparator for this queue.
|
boolean |
isEmpty()
Returns
true if this bounded priority
queue has no elements. |
Iterator<E> |
iterator()
Returns an iterator over the elements in this bounded priority
queue.
|
E |
last()
Returns the lowest scoring object in the priority queue,
or
null if the queue is empty. |
int |
maxSize()
Returns the maximum size allowed for this queue.
|
boolean |
offer(E o)
Insert the specified element into the queue if possible.
|
E |
peek()
Returns the highest scoring object in the priority queue,
or
null if the queue is empty. |
E |
peekLast()
Returns the lowest scoring object in the priority queue,
or
null if the queue is empty. |
E |
poll()
Returns and removes the highest scoring element in this
queue, or
null if it is empty. |
E |
remove()
Returns and removes the highest scoring element in
this queue, throwing an exception if it is empty.
|
boolean |
remove(Object obj)
Removes the specified object from the priority queue.
|
boolean |
removeAll(Collection<?> c) |
void |
setMaxSize(int maxSize)
Sets the maximum size of this bounded priority queue to
the specified maximum size.
|
int |
size()
Returns the current number of elements in this
priority queue.
|
SortedSet<E> |
subSet(E fromElement,
E toElement)
Return the set of elements in this queue greater than or equal
to the specified lower bound and strictly less than the upper bound
element according to the comparator for this queue.
|
SortedSet<E> |
tailSet(E fromElement)
Return the set of elements in this queue grearer than or equal
to the specified element according to the comparator for this
queue.
|
equals, hashCodeaddAll, contains, containsAll, retainAll, toArray, toArray, toStringpublic BoundedPriorityQueue(Comparator<? super E> comparator, int maxSize)
comparator - Comparator to order elements.maxSize - Maximum number of elements in the queue.IllegalArgumentException - If the maximum size is less than 1.public E element()
element in interface Queue<E>NoSuchElementException - If the queue is empty.public E poll()
null if it is empty.
This method differs from remove() only
in that it returns null rather than throwing an
exception for an empty queue.
public boolean offer(E o)
public E remove()
This method differs from poll() only in
that it throws an exception for an empty queue
instead of returning null.
public boolean isEmpty()
true if this bounded priority
queue has no elements.isEmpty in interface Collection<E>isEmpty in interface Set<E>isEmpty in class AbstractCollection<E>true if this bounded priority
queue has no elements.public E peekLast()
null if the queue is empty.
The behavior with empty queues is what makes this method
different than the sorted set method last().
public E last()
null if the queue is empty.public SortedSet<E> headSet(E toElement)
In violation of the SortedSet interface
specification, the result of this method is not a view
onto this queue, but rather a static snapshot of the queue.
headSet in interface SortedSet<E>toElement - Exclusive upper bound on returned elements.ClassCastException - If the upper bound is not compatible with
this queue's comparator.NullPointerException - If the upper bound is null.public SortedSet<E> tailSet(E fromElement)
In violation of the SortedSet interface
specification, the result of this method is not a view
onto this queue, but rather a static snapshot of the queue.
tailSet in interface SortedSet<E>fromElement - Inclusive lower bound on returned elements.ClassCastException - If the lower bound is not compatible with
this queue's comparator.NullPointerException - If the lower bound is null.public SortedSet<E> subSet(E fromElement, E toElement)
In violation of the SortedSet interface
specification, the result of this method is not a view
onto this queue, but rather a static snapshot of the queue.
subSet in interface SortedSet<E>fromElement - Inclusive lower bound on returned elements.toElement - Exclusive upper bound on returned elements.ClassCastException - If the lower bound is not compatible with
this queue's comparator.NullPointerException - If the lower bound is null.IllegalArgumentException - If the lower bound is greater than the upper bound.public Comparator<? super E> comparator()
comparator in interface SortedSet<E>public E first()
first in interface SortedSet<E>NoSuchElementException - If the queue is empty.public E peek()
null if the queue is empty.
This method is different than the sorted set method
first(), which throws an exception if there is
an empty queue.
public boolean remove(Object obj)
remove in interface Collection<E>remove in interface Set<E>remove in class AbstractCollection<E>obj - Object to remove from priority queue.true if the object was removed.ClassCastException - If the specified object is not
compatible with this collection.public boolean removeAll(Collection<?> c)
removeAll in interface Collection<E>removeAll in interface Set<E>removeAll in class AbstractSet<E>public void setMaxSize(int maxSize)
Note that this operation is not thread safe and should not be called concurrently with any other operations on this queue.
maxSize - New maximum size for this queue.public boolean add(E o)
add in interface Collection<E>add in interface Queue<E>add in interface Set<E>add in class AbstractCollection<E>o - Object to add to queue (ignored).UnsupportedOperationException - Always.public void clear()
clear in interface Collection<E>clear in interface Set<E>clear in class AbstractCollection<E>public int size()
size in interface Collection<E>size in interface Set<E>size in class AbstractCollection<E>public int maxSize()
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