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Lucene example source code file (TopDocsCollector.java)

This example Lucene source code file (TopDocsCollector.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

collector, empty_topdocs, empty_topdocs, priorityqueue, scoredoc, scoredoc, topdocs, topdocs, topdocscollector, topdocscollector

The Lucene TopDocsCollector.java source code

package org.apache.lucene.search;

/**
 * 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.
 */


import org.apache.lucene.util.PriorityQueue;

/**
 * A base class for all collectors that return a {@link TopDocs} output. This
 * collector allows easy extension by providing a single constructor which
 * accepts a {@link PriorityQueue} as well as protected members for that
 * priority queue and a counter of the number of total hits.<br>
 * Extending classes can override any of the methods to provide their own
 * implementation, as well as avoid the use of the priority queue entirely by
 * passing null to {@link #TopDocsCollector(PriorityQueue)}. In that case
 * however, you might want to consider overriding all methods, in order to avoid
 * a NullPointerException.
 */
public abstract class TopDocsCollector<T extends ScoreDoc> extends Collector {

  // This is used in case topDocs() is called with illegal parameters, or there
  // simply aren't (enough) results.
  protected static final TopDocs EMPTY_TOPDOCS = new TopDocs(0, new ScoreDoc[0], Float.NaN);
  
  /**
   * The priority queue which holds the top documents. Note that different
   * implementations of PriorityQueue give different meaning to 'top documents'.
   * HitQueue for example aggregates the top scoring documents, while other PQ
   * implementations may hold documents sorted by other criteria.
   */
  protected PriorityQueue<T> pq;

  /** The total number of documents that the collector encountered. */
  protected int totalHits;
  
  protected TopDocsCollector(PriorityQueue<T> pq) {
    this.pq = pq;
  }
  
  /**
   * Populates the results array with the ScoreDoc instances. This can be
   * overridden in case a different ScoreDoc type should be returned.
   */
  protected void populateResults(ScoreDoc[] results, int howMany) {
    for (int i = howMany - 1; i >= 0; i--) { 
      results[i] = pq.pop();
    }
  }

  /**
   * Returns a {@link TopDocs} instance containing the given results. If
   * <code>results is null it means there are no results to return,
   * either because there were 0 calls to collect() or because the arguments to
   * topDocs were invalid.
   */
  protected TopDocs newTopDocs(ScoreDoc[] results, int start) {
    return results == null ? EMPTY_TOPDOCS : new TopDocs(totalHits, results);
  }
  
  /** The total number of documents that matched this query. */
  public int getTotalHits() {
    return totalHits;
  }
  
  /** Returns the top docs that were collected by this collector. */
  public TopDocs topDocs() {
    // In case pq was populated with sentinel values, there might be less
    // results than pq.size(). Therefore return all results until either
    // pq.size() or totalHits.
    return topDocs(0, totalHits < pq.size() ? totalHits : pq.size());
  }

  /**
   * Returns the documents in the rage [start .. pq.size()) that were collected
   * by this collector. Note that if start >= pq.size(), an empty TopDocs is
   * returned.<br>
   * This method is convenient to call if the application always asks for the
   * last results, starting from the last 'page'.<br>
   * <b>NOTE: you cannot call this method more than once for each search
   * execution. If you need to call it more than once, passing each time a
   * different <code>start, you should call {@link #topDocs()} and work
   * with the returned {@link TopDocs} object, which will contain all the
   * results this search execution collected.
   */
  public TopDocs topDocs(int start) {
    // In case pq was populated with sentinel values, there might be less
    // results than pq.size(). Therefore return all results until either
    // pq.size() or totalHits.
    return topDocs(start, totalHits < pq.size() ? totalHits : pq.size());
  }

  /**
   * Returns the documents in the rage [start .. start+howMany) that were
   * collected by this collector. Note that if start >= pq.size(), an empty
   * TopDocs is returned, and if pq.size() - start < howMany, then only the
   * available documents in [start .. pq.size()) are returned.<br>
   * This method is useful to call in case pagination of search results is
   * allowed by the search application, as well as it attempts to optimize the
   * memory used by allocating only as much as requested by howMany.<br>
   * <b>NOTE: you cannot call this method more than once for each search
   * execution. If you need to call it more than once, passing each time a
   * different range, you should call {@link #topDocs()} and work with the
   * returned {@link TopDocs} object, which will contain all the results this
   * search execution collected.
   */
  public TopDocs topDocs(int start, int howMany) {
    
    // In case pq was populated with sentinel values, there might be less
    // results than pq.size(). Therefore return all results until either
    // pq.size() or totalHits.
    int size = totalHits < pq.size() ? totalHits : pq.size();

    // Don't bother to throw an exception, just return an empty TopDocs in case
    // the parameters are invalid or out of range.
    if (start < 0 || start >= size || howMany <= 0) {
      return newTopDocs(null, start);
    }

    // We know that start < pqsize, so just fix howMany. 
    howMany = Math.min(size - start, howMany);
    ScoreDoc[] results = new ScoreDoc[howMany];

    // pq's pop() returns the 'least' element in the queue, therefore need
    // to discard the first ones, until we reach the requested range.
    // Note that this loop will usually not be executed, since the common usage
    // should be that the caller asks for the last howMany results. However it's
    // needed here for completeness.
    for (int i = pq.size() - start - howMany; i > 0; i--) { pq.pop(); }
    
    // Get the requested results from pq.
    populateResults(results, howMany);
    
    return newTopDocs(results, start);
  }

}

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