lucene HitCollector 的作用

HitCollector 的作用很简单，通过collect（）方法控制检索返回的结果，下面是lucene自带的一个例子----使用一个优先队
列，返回指定数目的Top n Doc。

 

package org.apache.lucene.search;

/**
 * Copyright 2004 The Apache Software Foundation
 *
 * Licensed 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 java.io.IOException;
import java.util.BitSet;

import org.apache.lucene.store.Directory;
import org.apache.lucene.document.Document;
import org.apache.lucene.index.IndexReader;
import org.apache.lucene.index.Term;
import org.apache.lucene.util.PriorityQueue;

/** A {@link HitCollector} implementation that collects the top-
scoring
 * documents, returning them as a {@link TopDocs}.  This is used by
{@link
 * IndexSearcher} to implement {@link TopDocs}-based search.
 *
 * <p>This may be extended, overriding the collect method to, e.g.,
 * conditionally invoke <code>super()</code> in order to filter which
 * documents are collected.
 **/
public class TopDocCollector extends HitCollector {
  private int numHits;
  private float minScore = 0.0f;

  int totalHits;
  PriorityQueue hq;

  /** Construct to collect a given number of hits.
   * @param numHits the maximum number of hits to collect
   */
  public TopDocCollector(int numHits) {
    this(numHits, new HitQueue(numHits));
  }

  TopDocCollector(int numHits, PriorityQueue hq) {
    this.numHits = numHits;
    this.hq = hq;
  }

  // javadoc inherited
  public void collect(int doc, float score) {
    if (score > 0.0f) {
      totalHits++;
      if (hq.size() < numHits || score >= minScore) {
        hq.insert(new ScoreDoc(doc, score));
        minScore = ((ScoreDoc)hq.top()).score; // maintain minScore
      }
    }
  }

  /** The total number of documents that matched this query. */
  public int getTotalHits() { return totalHits; }

  /** The top-scoring hits. */
  public TopDocs topDocs() {
    ScoreDoc[] scoreDocs = new ScoreDoc[hq.size()];
    for (int i = hq.size()-1; i >= 0; i--)      // put docs in array
      scoreDocs[i] = (ScoreDoc)hq.pop();

    float maxScore = (totalHits==0)
      ? Float.NEGATIVE_INFINITY
      : scoreDocs[0].score;

    return new TopDocs(totalHits, scoreDocs, maxScore);
  }
}