com.google.common.collect

Class MinMaxPriorityQueue<E>

java.lang.Object

java.util.AbstractCollection<E>

java.util.AbstractQueue<E>

com.google.common.collect.MinMaxPriorityQueue<E>

All Implemented Interfaces:

Iterable<E>, Collection<E>, Queue<E>

@Beta
 @GwtCompatible
public final class MinMaxPriorityQueue<E>
extends AbstractQueue<E>

A double-ended priority queue, which provides constant-time access to both its least element and its greatest element, as determined by the queue's specified comparator. If no comparator is given at creation time, the natural order of elements is used. If no maximum size is given at creation time, the queue is unbounded.

Usage example:

   

   MinMaxPriorityQueue<User> users = MinMaxPriorityQueue.orderedBy(userComparator)
       .maximumSize(1000)
       .create();

As a Queue it functions exactly as a PriorityQueue: its head element -- the implicit target of the methods peek(), poll() and AbstractQueue.remove() -- is defined as the least element in the queue according to the queue's comparator. But unlike a regular priority queue, the methods peekLast(), pollLast() and removeLast() are also provided, to act on the greatest element in the queue instead.

A min-max priority queue can be configured with a maximum size. If so, each time the size of the queue exceeds that value, the queue automatically removes its greatest element according to its comparator (which might be the element that was just added). This is different from conventional bounded queues, which either block or reject new elements when full.

This implementation is based on the min-max heap developed by Atkinson, et al. Unlike many other double-ended priority queues, it stores elements in a single array, as compact as the traditional heap data structure used in PriorityQueue.

This class is not thread-safe, and does not accept null elements.

Performance notes:

• If you only access one end of the queue, and do use a maximum size, this class will perform significantly worse than a PriorityQueue with manual eviction above the maximum size. In many cases Ordering.leastOf(java.lang.Iterable<E>, int) may work for your use case with significantly improved (and asymptotically superior) performance.
• The retrieval operations peek(), peekFirst(), peekLast(), AbstractQueue.element(), and size are constant-time.
• The enqueuing and dequeuing operations (offer(E), add(E), and all the forms of poll() and AbstractQueue.remove()) run in O(log n) time.
• The AbstractCollection.remove(Object) and AbstractCollection.contains(java.lang.Object) operations require linear (O(n)) time.
• If you only access one end of the queue, and don't use a maximum size, this class is functionally equivalent to PriorityQueue, but significantly slower.

Since:

8.0

Author:

Sverre Sundsdal, Torbjorn Gannholm

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
static class	MinMaxPriorityQueue.Builder<B> The builder class used in creation of min-max priority queues.

Method Summary

Modifier and Type	Method and Description
boolean	add(E element) Adds the given element to this queue.
boolean	addAll(Collection<? extends E> newElements) Adds all of the elements in the specified collection to this queue.
void	clear() Removes all of the elements from this queue.
Comparator<? super E>	comparator() Returns the comparator used to order the elements in this queue.
static <E extends Comparable<E>> MinMaxPriorityQueue<E>	create() Creates a new min-max priority queue with default settings: natural order, no maximum size, no initial contents, and an initial expected size of 11.
static <E extends Comparable<E>> MinMaxPriorityQueue<E>	create(Iterable<? extends E> initialContents) Creates a new min-max priority queue using natural order, no maximum size, and initially containing the given elements.
static MinMaxPriorityQueue.Builder<Comparable>	expectedSize(int expectedSize) Creates and returns a new builder, configured to build MinMaxPriorityQueue instances sized appropriately to hold expectedSize elements.
Iterator<E>	iterator() Returns an iterator over the elements contained in this collection, in no particular order.
static MinMaxPriorityQueue.Builder<Comparable>	maximumSize(int maximumSize) Creates and returns a new builder, configured to build MinMaxPriorityQueue instances that are limited to maximumSize elements.
boolean	offer(E element) Adds the given element to this queue.
static <B> MinMaxPriorityQueue.Builder<B>	orderedBy(Comparator<B> comparator) Creates and returns a new builder, configured to build MinMaxPriorityQueue instances that use comparator to determine the least and greatest elements.
E	peek() Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty.
E	peekFirst() Retrieves, but does not remove, the least element of this queue, or returns null if the queue is empty.
E	peekLast() Retrieves, but does not remove, the greatest element of this queue, or returns null if the queue is empty.
E	poll() Retrieves and removes the head of this queue, or returns null if this queue is empty.
E	pollFirst() Removes and returns the least element of this queue, or returns null if the queue is empty.
E	pollLast() Removes and returns the greatest element of this queue, or returns null if the queue is empty.
E	removeFirst() Removes and returns the least element of this queue.
E	removeLast() Removes and returns the greatest element of this queue.
int	size() Returns the number of elements in this collection.
Object[]	toArray() Returns an array containing all of the elements in this collection.

Methods inherited from class java.util.AbstractQueue

element, remove

Methods inherited from class java.util.AbstractCollection

contains, containsAll, isEmpty, remove, removeAll, retainAll, toArray, toString

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Methods inherited from interface java.util.Collection

contains, containsAll, equals, hashCode, isEmpty, parallelStream, remove, removeAll, removeIf, retainAll, spliterator, stream, toArray

Methods inherited from interface java.lang.Iterable

forEach

Method Detail

create

public static <E extends Comparable<E>> MinMaxPriorityQueue<E> create()

Creates a new min-max priority queue with default settings: natural order, no maximum size, no initial contents, and an initial expected size of 11.

create

public static <E extends Comparable<E>> MinMaxPriorityQueue<E> create(Iterable<? extends E> initialContents)

Creates a new min-max priority queue using natural order, no maximum size, and initially containing the given elements.

orderedBy

public static <B> MinMaxPriorityQueue.Builder<B> orderedBy(Comparator<B> comparator)

Creates and returns a new builder, configured to build MinMaxPriorityQueue instances that use comparator to determine the least and greatest elements.

expectedSize

public static MinMaxPriorityQueue.Builder<Comparable> expectedSize(int expectedSize)

Creates and returns a new builder, configured to build MinMaxPriorityQueue instances sized appropriately to hold expectedSize elements.

maximumSize

public static MinMaxPriorityQueue.Builder<Comparable> maximumSize(int maximumSize)

Creates and returns a new builder, configured to build MinMaxPriorityQueue instances that are limited to maximumSize elements. Each time a queue grows beyond this bound, it immediately removes its greatest element (according to its comparator), which might be the element that was just added.

size

public int size()

Description copied from interface: java.util.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

add

public boolean add(E element)

Adds the given element to this queue. If this queue has a maximum size, after adding element the queue will automatically evict its greatest element (according to its comparator), which may be element itself.

Specified by:

add in interface Collection<E>

Specified by:

add in interface Queue<E>

Overrides:

add in class AbstractQueue<E>

Parameters:

element - the element to add

Returns:

true always

addAll

public boolean addAll(Collection<? extends E> newElements)

Description copied from class: java.util.AbstractQueue

Adds all of the elements in the specified collection to this queue. Attempts to addAll of a queue to itself result in IllegalArgumentException. Further, the behavior of this operation is undefined if the specified collection is modified while the operation is in progress.

This implementation iterates over the specified collection, and adds each element returned by the iterator to this queue, in turn. A runtime exception encountered while trying to add an element (including, in particular, a null element) may result in only some of the elements having been successfully added when the associated exception is thrown.

Specified by:

addAll in interface Collection<E>

Overrides:

addAll in class AbstractQueue<E>

Parameters:

newElements - collection containing elements to be added to this queue

Returns:

true if this queue changed as a result of the call

See Also:

AbstractQueue.add(Object)

offer

public boolean offer(E element)

Adds the given element to this queue. If this queue has a maximum size, after adding element the queue will automatically evict its greatest element (according to its comparator), which may be element itself.

Parameters:

element - the element to add

Returns:

true if the element was added to this queue, else false

poll

public E poll()

Description copied from interface: java.util.Queue

Retrieves and removes the head of this queue, or returns null if this queue is empty.

Returns:

the head of this queue, or null if this queue is empty

peek

public E peek()

Description copied from interface: java.util.Queue

Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty.

Returns:

the head of this queue, or null if this queue is empty

pollFirst

public E pollFirst()

Removes and returns the least element of this queue, or returns null if the queue is empty.

removeFirst

public E removeFirst()

Removes and returns the least element of this queue.

Throws:

NoSuchElementException - if the queue is empty

peekFirst

public E peekFirst()

Retrieves, but does not remove, the least element of this queue, or returns null if the queue is empty.

pollLast

public E pollLast()

Removes and returns the greatest element of this queue, or returns null if the queue is empty.

removeLast

public E removeLast()

Removes and returns the greatest element of this queue.

Throws:

NoSuchElementException - if the queue is empty

peekLast

public E peekLast()

Retrieves, but does not remove, the greatest element of this queue, or returns null if the queue is empty.

iterator

public Iterator<E> iterator()

Returns an iterator over the elements contained in this collection, in no particular order.

The iterator is fail-fast: If the MinMaxPriorityQueue is modified at any time after the iterator is created, in any way except through the iterator's own remove method, the iterator will generally throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.

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 contained in this collection

clear

public void clear()

Description copied from class: java.util.AbstractQueue

Removes all of the elements from this queue. The queue will be empty after this call returns.

This implementation repeatedly invokes poll until it returns null.

Specified by:

clear in interface Collection<E>

Overrides:

clear in class AbstractQueue<E>

toArray

public Object[] toArray()

Description copied from class: java.util.AbstractCollection

Returns an array containing all of the elements in this collection. If this collection makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order.

The returned array will be "safe" in that no references to it are maintained by this collection. (In other words, this method must allocate a new array even if this collection is backed by an array). The caller is thus free to modify the returned array.

This method acts as bridge between array-based and collection-based APIs.

This implementation returns an array containing all the elements returned by this collection's iterator, in the same order, stored in consecutive elements of the array, starting with index 0. The length of the returned array is equal to the number of elements returned by the iterator, even if the size of this collection changes during iteration, as might happen if the collection permits concurrent modification during iteration. The size method is called only as an optimization hint; the correct result is returned even if the iterator returns a different number of elements.

This method is equivalent to:

 
 List<E> list = new ArrayList<E>(size());
 for (E e : this)
     list.add(e);
 return list.toArray();
 

Specified by:

toArray in interface Collection<E>

Overrides:

toArray in class AbstractCollection<E>

Returns:

an array containing all of the elements in this collection

comparator

public Comparator<? super E> comparator()

Returns the comparator used to order the elements in this queue. Obeys the general contract of PriorityQueue.comparator, but returns Ordering.natural() instead of null to indicate natural ordering.