Lucene学习笔记——TopDocCollector类

package org.apache.lucene.search;

import org.apache.lucene.util.PriorityQueue;

public class TopDocCollector extends HitCollector {

  private ScoreDoc reusableSD;

  int totalHits;
  PriorityQueue hq;
    
  public TopDocCollector(int numHits) {
    this(numHits, new HitQueue(numHits));
  }

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

  // javadoc inherited
  public void collect(int doc, float score) {
    if (score > 0.0f) {
      totalHits++;
      if (reusableSD == null) {
        reusableSD = new ScoreDoc(doc, score);
      } else if (score >= reusableSD.score) {
        // reusableSD holds the last "rejected" entry, so, if
        // this new score is not better than that, there's no
        // need to try inserting it
        reusableSD.doc = doc;
        reusableSD.score = score;
      } else {
        return;
      }
      reusableSD = (ScoreDoc) hq.insertWithOverflow(reusableSD);
    }
  }

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

  // 获取topDocs，将hq中元素取出，放入scoreDocs数组，返回TopDocs对象。
  public TopDocs topDocs() {
    ScoreDoc[] scoreDocs = new ScoreDoc[hq.size()];
    for (int i = hq.size()-1; i >= 0; i--)      
      scoreDocs[i] = (ScoreDoc)hq.pop();
      
    float maxScore = (totalHits==0)
      ? Float.NEGATIVE_INFINITY
      : scoreDocs[0].score;
    
    return new TopDocs(totalHits, scoreDocs, maxScore);
  }
}

PriorityQueue默认情况下，类型为HitQueue（HitQueue为PriorityQueue子类，实现了lessThan方法，在执行insert时，对ScoreDoc对象进行比较，按score降序排序，score相同时，按doc的id降序排列）。