lucene HitCollector 的作用

導讀：
　　HitCollector 的作用很簡單，通過collect（）方法控制檢索返回的結果，下面是lucene內建的一個例子----使用一個優先隊
　　列，返回指定數目的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.
　　*
　　*
　　This may be extended, overriding the collect method to, e.g.,
　　* conditionally invoke super()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() = 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);
　　}
　　}

本文轉自
http://blog.lough.com.cn/post/234/