public abstract class

PriorityQueue

extends Object
java.lang.Object
   ↳ org.apache.lucene.util.PriorityQueue<T>
Known Direct Subclasses

Class Overview

A PriorityQueue maintains a partial ordering of its elements such that the least element can always be found in constant time. Put()'s and pop()'s require log(size) time.

NOTE: This class pre-allocates a full array of length maxSize+1, in initialize(int).

Summary

Fields
protected T[] heap
Public Constructors
PriorityQueue()
Public Methods
final T add(T element)
Adds an Object to a PriorityQueue in log(size) time.
final void clear()
Removes all entries from the PriorityQueue.
T insertWithOverflow(T element)
Adds an Object to a PriorityQueue in log(size) time.
final T pop()
Removes and returns the least element of the PriorityQueue in log(size) time.
final int size()
Returns the number of elements currently stored in the PriorityQueue.
final T top()
Returns the least element of the PriorityQueue in constant time.
final T updateTop()
Should be called when the Object at top changes values.
Protected Methods
T getSentinelObject()
This method can be overridden by extending classes to return a sentinel object which will be used by initialize(int) to fill the queue, so that the code which uses that queue can always assume it's full and only change the top without attempting to insert any new object.
final void initialize(int maxSize)
Subclass constructors must call this.
abstract boolean lessThan(T a, T b)
Determines the ordering of objects in this priority queue.
[Expand]
Inherited Methods
From class java.lang.Object

Fields

protected T[] heap

Public Constructors

public PriorityQueue ()

Public Methods

public final T add (T element)

Adds an Object to a PriorityQueue in log(size) time. If one tries to add more objects than maxSize from initialize an ArrayIndexOutOfBoundsException is thrown.

Returns
  • the new 'top' element in the queue.

public final void clear ()

Removes all entries from the PriorityQueue.

public T insertWithOverflow (T element)

Adds an Object to a PriorityQueue in log(size) time. It returns the object (if any) that was dropped off the heap because it was full. This can be the given parameter (in case it is smaller than the full heap's minimum, and couldn't be added), or another object that was previously the smallest value in the heap and now has been replaced by a larger one, or null if the queue wasn't yet full with maxSize elements.

public final T pop ()

Removes and returns the least element of the PriorityQueue in log(size) time.

public final int size ()

Returns the number of elements currently stored in the PriorityQueue.

public final T top ()

Returns the least element of the PriorityQueue in constant time.

public final T updateTop ()

Should be called when the Object at top changes values. Still log(n) worst case, but it's at least twice as fast to 

 pq.top().change();
 pq.updateTop();
instead of 
 o = pq.pop();
 o.change();
 pq.push(o);

Returns
  • the new 'top' element.

Protected Methods

protected T getSentinelObject ()

This method can be overridden by extending classes to return a sentinel object which will be used by initialize(int) to fill the queue, so that the code which uses that queue can always assume it's full and only change the top without attempting to insert any new object.
Those sentinel values should always compare worse than any non-sentinel value (i.e., lessThan(T, T) should always favor the non-sentinel values).
By default, this method returns false, which means the queue will not be filled with sentinel values. Otherwise, the value returned will be used to pre-populate the queue. Adds sentinel values to the queue.
If this method is extended to return a non-null value, then the following usage pattern is recommended: 

 // extends getSentinelObject() to return a non-null value.
 PriorityQueue pq = new MyQueue(numHits);
 // save the 'top' element, which is guaranteed to not be null.
 MyObject pqTop = pq.top();
 <...>
 // now in order to add a new element, which is 'better' than top (after 
 // you've verified it is better), it is as simple as:
 pqTop.change().
 pqTop = pq.updateTop();
NOTE: if this method returns a non-null value, it will be called by initialize(int) size() times, relying on a new object to be returned and will not check if it's null again. Therefore you should ensure any call to this method creates a new instance and behaves consistently, e.g., it cannot return null if it previously returned non-null.

Returns
  • the sentinel object to use to pre-populate the queue, or null if sentinel objects are not supported.

protected final void initialize (int maxSize)

Subclass constructors must call this.

protected abstract boolean lessThan (T a, T b)

Determines the ordering of objects in this priority queue. Subclasses must define this one method.