Class Collector

  extended by
Direct Known Subclasses:
PositiveScoresOnlyCollector, TimeLimitingCollector, TopDocsCollector

public abstract class Collector
extends Object

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

Lucene's core collectors are derived from Collector. Likely your application can use one of these classes, or subclass TopDocsCollector, instead of implementing Collector directly:

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 setScorer( method, to hold onto the passed Scorer instance, and call 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 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());, 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 PositiveScoresOnlyCollector.

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


Constructor Summary
Method Summary
abstract  boolean acceptsDocsOutOfOrder()
          Return true if this collector does not require the matching docIDs to be delivered in int sort order (smallest to largest) to collect(int).
abstract  void collect(int doc)
          Called once for every document matching a query, with the unbased document number.
abstract  void setNextReader(IndexReader reader, int docBase)
          Called before collecting from each IndexReader.
abstract  void setScorer(Scorer scorer)
          Called before successive calls to collect(int).
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail


public Collector()
Method Detail


public abstract void setScorer(Scorer scorer)
                        throws IOException
Called before successive calls to collect(int). Implementations that need the score of the current document (passed-in to collect(int)), should save the passed-in Scorer and call scorer.score() when needed.



public abstract void collect(int doc)
                      throws IOException
Called once for every document matching a query, with the unbased document number.

Note: This is called in an inner search loop. For good search performance, implementations of this method should not call Searcher.doc(int) or IndexReader.document(int) on every hit. Doing so can slow searches by an order of magnitude or more.



public abstract void setNextReader(IndexReader reader,
                                   int docBase)
                            throws IOException
Called before collecting from each IndexReader. All doc ids in collect(int) will correspond to reader. Add docBase to the current IndexReaders internal document id to re-base ids in collect(int).

reader - next IndexReader
docBase -


public abstract boolean acceptsDocsOutOfOrder()
Return true if this collector does not require the matching docIDs to be delivered in int sort order (smallest to largest) to collect(int).

Most Lucene Query implementations will visit matching docIDs in order. However, some queries (currently limited to certain cases of BooleanQuery) can achieve faster searching if the Collector allows them to deliver the docIDs out of order.

Many collectors don't mind getting docIDs out of order, so it's important to return true here.

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