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Lucene example source code file (PriorityQueue.java)

This example Lucene source code file (PriorityQueue.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Java - Lucene tags/keywords

gc, gc, object, object, priorityqueue, priorityqueue, suppresswarnings, t, t

The Lucene PriorityQueue.java source code

package org.apache.lucene.util;

/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

/** 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.
 *
 * <p>NOTE: This class pre-allocates a full array of
 * length <code>maxSize+1, in {@link #initialize}.
 * 
 * @lucene.internal
*/
public abstract class PriorityQueue<T> {
  private int size;
  private int maxSize;
  private T[] heap;

  /** Determines the ordering of objects in this priority queue.  Subclasses
   *  must define this one method.
   *  @return <code>true iff parameter a is less than parameter b.
   */
  protected abstract boolean lessThan(T a, T b);

  /**
   * This method can be overridden by extending classes to return a sentinel
   * object which will be used by {@link #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.<br>
   * 
   * Those sentinel values should always compare worse than any non-sentinel
   * value (i.e., {@link #lessThan} should always favor the
   * non-sentinel values).<br>
   * 
   * 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.<br>
   * 
   * If this method is extended to return a non-null value, then the following
   * usage pattern is recommended:
   * 
   * <pre>
   * // extends getSentinelObject() to return a non-null value.
   * PriorityQueue<MyObject> 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();
   * </pre>
   * 
   * <b>NOTE: if this method returns a non-null value, it will be called by
   * {@link #initialize(int)} {@link #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.
   * 
   * @return the sentinel object to use to pre-populate the queue, or null if
   *         sentinel objects are not supported.
   */
  protected T getSentinelObject() {
    return null;
  }

  /** Subclass constructors must call this. */
  @SuppressWarnings("unchecked")
  protected final void initialize(int maxSize) {
    size = 0;
    int heapSize;
    if (0 == maxSize)
      // We allocate 1 extra to avoid if statement in top()
      heapSize = 2;
    else {
      if (maxSize == Integer.MAX_VALUE) {
        // Don't wrap heapSize to -1, in this case, which
        // causes a confusing NegativeArraySizeException.
        // Note that very likely this will simply then hit
        // an OOME, but at least that's more indicative to
        // caller that this values is too big.  We don't +1
        // in this case, but it's very unlikely in practice
        // one will actually insert this many objects into
        // the PQ:
        heapSize = Integer.MAX_VALUE;
      } else {
        // NOTE: we add +1 because all access to heap is
        // 1-based not 0-based.  heap[0] is unused.
        heapSize = maxSize + 1;
      }
    }
    heap = (T[]) new Object[heapSize]; // T is unbounded type, so this unchecked cast works always
    this.maxSize = maxSize;
    
    // If sentinel objects are supported, populate the queue with them
    T sentinel = getSentinelObject();
    if (sentinel != null) {
      heap[1] = sentinel;
      for (int i = 2; i < heap.length; i++) {
        heap[i] = getSentinelObject();
      }
      size = maxSize;
    }
  }

  /**
   * Adds an Object to a PriorityQueue in log(size) time. If one tries to add
   * more objects than maxSize from initialize an
   * {@link ArrayIndexOutOfBoundsException} is thrown.
   * 
   * @return the new 'top' element in the queue.
   */
  public final T add(T element) {
    size++;
    heap[size] = element;
    upHeap();
    return heap[1];
  }

  /**
   * 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 T insertWithOverflow(T element) {
    if (size < maxSize) {
      add(element);
      return null;
    } else if (size > 0 && !lessThan(element, heap[1])) {
      T ret = heap[1];
      heap[1] = element;
      updateTop();
      return ret;
    } else {
      return element;
    }
  }

  /** Returns the least element of the PriorityQueue in constant time. */
  public final T top() {
    // We don't need to check size here: if maxSize is 0,
    // then heap is length 2 array with both entries null.
    // If size is 0 then heap[1] is already null.
    return heap[1];
  }

  /** Removes and returns the least element of the PriorityQueue in log(size)
    time. */
  public final T pop() {
    if (size > 0) {
      T result = heap[1];			  // save first value
      heap[1] = heap[size];			  // move last to first
      heap[size] = null;			  // permit GC of objects
      size--;
      downHeap();				  // adjust heap
      return result;
    } else
      return null;
  }
  
  /**
   * Should be called when the Object at top changes values. Still log(n) worst
   * case, but it's at least twice as fast to
   * 
   * <pre>
   * pq.top().change();
   * pq.updateTop();
   * </pre>
   * 
   * instead of
   * 
   * <pre>
   * o = pq.pop();
   * o.change();
   * pq.push(o);
   * </pre>
   * 
   * @return the new 'top' element.
   */
  public final T updateTop() {
    downHeap();
    return heap[1];
  }

  /** Returns the number of elements currently stored in the PriorityQueue. */
  public final int size() {
    return size;
  }

  /** Removes all entries from the PriorityQueue. */
  public final void clear() {
    for (int i = 0; i <= size; i++) {
      heap[i] = null;
    }
    size = 0;
  }

  private final void upHeap() {
    int i = size;
    T node = heap[i];			  // save bottom node
    int j = i >>> 1;
    while (j > 0 && lessThan(node, heap[j])) {
      heap[i] = heap[j];			  // shift parents down
      i = j;
      j = j >>> 1;
    }
    heap[i] = node;				  // install saved node
  }

  private final void downHeap() {
    int i = 1;
    T node = heap[i];			  // save top node
    int j = i << 1;				  // find smaller child
    int k = j + 1;
    if (k <= size && lessThan(heap[k], heap[j])) {
      j = k;
    }
    while (j <= size && lessThan(heap[j], node)) {
      heap[i] = heap[j];			  // shift up child
      i = j;
      j = i << 1;
      k = j + 1;
      if (k <= size && lessThan(heap[k], heap[j])) {
        j = k;
      }
    }
    heap[i] = node;				  // install saved node
  }
  
  /** This method returns the internal heap array as Object[].
   * @lucene.internal
   */
  protected final Object[] getHeapArray() {
    return (Object[]) heap;
  }
}

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