public void search(Query query, Collector results)

/*** <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 {@link Collector}* and {@link SimpleCollector}. 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 &lt;= 0.0</li>  * * </ul> * * @lucene.experimental */

public Map<String, Integer> groupBy(Query query, String field, int topCount) {Map<String, Integer> map = new HashMap<String, Integer>();long begin = System.currentTimeMillis();int topNGroups = topCount;int groupOffset = 0;int maxDocsPerGroup = 100;int withinGroupOffset = 0;try {FirstPassGroupingCollector c1 = new FirstPassGroupingCollector(field, Sort.RELEVANCE, topNGroups);boolean cacheScores = true; double maxCacheRAMMB = 4.0;CachingCollector cachedCollector = CachingCollector.create(c1, cacheScores, maxCacheRAMMB); indexSearcher.search(query, cachedCollector);Collection<SearchGroup<String>> topGroups = c1.getTopGroups(groupOffset, true);if (topGroups == null) { return null;} SecondPassGroupingCollector c2 = new SecondPassGroupingCollector(field, topGroups, Sort.RELEVANCE, Sort.RELEVANCE, maxDocsPerGroup, true, true, true);if (cachedCollector.isCached()) {// Cache fit within maxCacheRAMMB, so we can replay it:
            cachedCollector.replay(c2); } else {// Cache was too large; must re-execute query:
            indexSearcher.search(query, c2);}TopGroups<String> tg = c2.getTopGroups(withinGroupOffset);GroupDocs<String>[] gds = tg.groups;for(GroupDocs<String> gd : gds) {map.put(gd.groupValue, gd.totalHits);}} catch (IOException e) {e.printStackTrace();}long end = System.currentTimeMillis();System.out.println("group by time :" + (end - begin) + "ms");return map;}