Expert: Collectors are primarily meant to be used to gather raw results from a search, and implement sorting or custom result filtering, collation, etc.

As of 2.9, this class replaces the deprecated HitCollector, and offers an API for efficient collection of hits across sequential {@link IndexReader}s. {@link IndexSearcher} advances the collector through each of the sub readers, in an arbitrary order. This results in a higher performance means of collection.

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:

• {@link TopDocsCollector} is an abstract base class that assumes you will retrieve the top N docs, according to some criteria, after collection is done.
• {@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.
• {@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}.
• {@link TimeLimitingCollector}, which wraps any other Collector and aborts the search if it's taken too much time, will subclass Collector in 3.0 (presently it subclasses the deprecated HitCollector).
• {@link PositiveScoresOnlyCollector} wraps any other Collector and prevents collection of hits whose score is <= 0.0

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}.

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:

            Searcher searcher = new IndexSearcher(indexReader);
            final BitSet bits = new BitSet(indexReader.maxDoc());
            searcher.search(query, new Collector() {
            private int docBase;
            
            // ignore scorer
            public void setScorer(Scorer scorer) {
            }
            
            // 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;
            }
            });
            

Not all collectors will need to rebase the docID. For example, a collector that simply counts the total number of hits would skip it.

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}.

NOTE: This API is experimental and might change in incompatible ways in the next release.