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Lucene example source code file (Collector.java)

This example Lucene source code file (Collector.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, collector, io, ioexception, ioexception

The Lucene Collector.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,
 * See the License for the specific language governing permissions and
 * limitations under the License.

import java.io.IOException;

import org.apache.lucene.index.IndexReader;

 * <p>Expert: Collectors are primarily meant to be used to
 * gather raw results from a search, and implement sorting
 * or custom result filtering, collation, etc. </p>
 * <p>Lucene's core collectors are derived from Collector.
 * Likely your application can use one of these classes, or
 * subclass {@link TopDocsCollector}, instead of
 * implementing Collector directly:
 * <ul>
 *   <li>{@link TopDocsCollector} is an abstract base class
 *   that assumes you will retrieve the top N docs,
 *   according to some criteria, after collection is
 *   done.  </li>
 *   <li>{@link TopScoreDocCollector} is a concrete subclass
 *   {@link TopDocsCollector} and sorts according to score +
 *   docID.  This is used internally by the {@link
 *   IndexSearcher} search methods that do not take an
 *   explicit {@link Sort}. It is likely the most frequently
 *   used collector.</li>
 *   <li>{@link TopFieldCollector} subclasses {@link
 *   TopDocsCollector} and sorts according to a specified
 *   {@link Sort} object (sort by field).  This is used
 *   internally by the {@link IndexSearcher} search methods
 *   that take an explicit {@link Sort}.
 *   <li>{@link TimeLimitingCollector}, which wraps any other
 *   Collector and aborts the search if it's taken too much
 *   time.</li>
 *   <li>{@link PositiveScoresOnlyCollector} wraps any other
 *   Collector and prevents collection of hits whose score
 *   is <= 0.0</li>
 * </ul>
 * <p>Collector decouples the score from the collected doc:
 * the score computation is skipped entirely if it's not
 * needed.  Collectors that do need the score should
 * implement the {@link #setScorer} method, to hold onto the
 * passed {@link Scorer} instance, and call {@link
 * Scorer#score()} within the collect method to compute the
 * current hit's score.  If your collector may request the
 * score for a single hit multiple times, you should use
 * {@link ScoreCachingWrappingScorer}. </p>
 * <p>NOTE: The doc that is passed to the collect
 * method is relative to the current reader. If your
 * collector needs to resolve this to the docID space of the
 * Multi*Reader, you must re-base it by recording the
 * docBase from the most recent setNextReader call.  Here's
 * a simple example showing how to collect docIDs into a
 * BitSet:</p>
 * <pre>
 * Searcher searcher = new IndexSearcher(indexReader);
 * final BitSet bits = new BitSet(indexReader.maxDoc());
 * searcher.search(query, new Collector() {
 *   private int docBase;
 *   <em>// ignore scorer
 *   public void setScorer(Scorer scorer) {
 *   }
 *   <em>// accept docs out of order (for a BitSet it doesn't matter)
 *   public boolean acceptsDocsOutOfOrder() {
 *     return true;
 *   }
 *   public void collect(int doc) {
 *     bits.set(doc + docBase);
 *   }
 *   public void setNextReader(IndexReader reader, int docBase) {
 *     this.docBase = docBase;
 *   }
 * });
 * </pre>
 * <p>Not all collectors will need to rebase the docID.  For
 * example, a collector that simply counts the total number
 * of hits would skip it.</p>
 * <p>NOTE: Prior to 2.9, Lucene silently filtered
 * out hits with score <= 0.  As of 2.9, the core Collectors
 * no longer do that.  It's very unusual to have such hits
 * (a negative query boost, or function query returning
 * negative custom scores, could cause it to happen).  If
 * you need that behavior, use {@link
 * PositiveScoresOnlyCollector}.</p>
 * @lucene.experimental
 * @since 2.9
public abstract class Collector {
   * Called before successive calls to {@link #collect(int)}. Implementations
   * that need the score of the current document (passed-in to
   * {@link #collect(int)}), should save the passed-in Scorer and call
   * scorer.score() when needed.
  public abstract void setScorer(Scorer scorer) throws IOException;
   * Called once for every document matching a query, with the unbased document
   * number.
   * <p>
   * Note: This is called in an inner search loop. For good search performance,
   * implementations of this method should not call {@link Searcher#doc(int)} or
   * {@link org.apache.lucene.index.IndexReader#document(int)} on every hit.
   * Doing so can slow searches by an order of magnitude or more.
  public abstract void collect(int doc) throws IOException;

   * Called before collecting from each IndexReader. All doc ids in
   * {@link #collect(int)} will correspond to reader.
   * Add docBase to the current IndexReaders internal document id to re-base ids
   * in {@link #collect(int)}.
   * @param reader
   *          next IndexReader
   * @param docBase
  public abstract void setNextReader(IndexReader reader, int docBase) throws IOException;

   * Return <code>true if this collector does not
   * require the matching docIDs to be delivered in int sort
   * order (smallest to largest) to {@link #collect}.
   * <p> Most Lucene Query implementations will visit
   * matching docIDs in order.  However, some queries
   * (currently limited to certain cases of {@link
   * BooleanQuery}) can achieve faster searching if the
   * <code>Collector allows them to deliver the
   * docIDs out of order.</p>
   * <p> Many collectors don't mind getting docIDs out of
   * order, so it's important to return <code>true
   * here.
  public abstract boolean acceptsDocsOutOfOrder();

Other Lucene examples (source code examples)

Here is a short list of links related to this Lucene Collector.java source code file:

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